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Car Audio Explained

This document was originally written in 2001 by one of our Directors, Julian Thompson, when he was involved in the design and installation of client systems on a daily basis. Audio was a passion for Julian but as the business moved further away from pure installation into internet commerce it became clear that his workshop skills would gradually be forgotten. For this reason he decided to write this to serve as a record of where he was at during that period. The document was first published on the internet in 2003 and was well received, even being adopted as the basis for a car electronics syllabus by an American collage. Julian has expanded and edited it to be more relevant to 2011, to include sections on Dual Voice Coil woofers and some other amendments. Some of the text is heavy going but the idea was to provide a technical resource and therefore it was felt necessary to retain those sections.

Understanding the basics about frequency response
Evaluating your car
Choosing speakers
DVC subwoofers and how to use them
Source units
Choosing amplifiers
Installing for a noise free system
Bass Boxes
Setting up your system
Crossovers and advanced setup
Sub Failure - Mechanical or Electrical??

Understanding the basics about stereo and the other formats

What makes a good sounding car audio system? Think about good home hi-fi. A good quality source; usually a CD player (but not always, especially with the advent of lossless MP3) Also good quality cables, a decent amplifier, and then this is played as sound by the speakers, which themselves will be made up of raw driver units and an enclosure, or cabinet. (There are other types of speaker but this is the norm).

There are only two speakers in stereo. Left and right. When you sit in the middle of the two speakers you are equidistant from each and in a perfect world they will play exactly what the microphones recorded in the studio and with the help of the acoustic environment you're in, the sound will be identical to the original performance. This is the theory and it is simple. There are some problems, however.Thompsons Ltd for Car audio

Firstly, before we move on you must understand that if you do not sit in the middle, the sound will not be good. So, more specifically; What makes good sound?

As stated above, in a perfect world you would get an accurate reproduction of what was played by the artist. But not just accurate in terms of tonal quality, or even accurate in terms of the timbre of the instruments; I mean accurate in terms of the easily discernable position of the various musical instruments across a virtual "stage" in your living room. What I am getting at is that unless you can sit in front of your home hi-fi equidistant from your speakers at roughly a 60 degree angle of attack and detect the fact that, for example, a violin or keyboard or whatever is on the right and a singer is in the middle and a drum kit is behind her, then you are not, quite honestly, a "good listener" yet, and you have some work to do yourself before you can be the objective judge of good stereo.

However, it is much easier to decide if the tonal quality of the music your stereo is playing is to your liking. This is subjective judgement and is the kind of judgement most people apply to listening - the bottom line is "Do I like the sound of that or not?"

Furthermore, since people profess to like different kinds of sound, it is clear that the original studio sound is not necessarily everyone's goal. We have, therefore, got a problem, since the whole ethos of "good sound" has now gone out of the window because we are all singing from a different song sheet, so to speak.

In car stereo

Transferring our "stereo" into "car stereo" is much more interesting still. Firstly, we are not, unless we happen to be the proud owner of a McLaren F1 (or have a penchant for sitting on gear sticks), going to be sitting in the middle.

This means that before we start, we have a massive disadvantage in terms of imaging, since the whole basis for this begins with the simple fact that if the guitar player was closer to the left microphone then he'll be played louder on the left speaker which will mean that he'll sound like he was on the left. This all goes legs up when the right speaker is blaring down your ear hole in the door panel and your car's left speaker is effectively hidden by some random bit of car or passenger. (Well done lots of car manufacturers)

In addition, our car is going to make noise; lots and lots of noise. Like listening to a home hifi with the vacuum cleaner turned on. Try it; I promise you it isn't fun.

Also, we have some more problems. Do you remember earlier when I said that your home stereo had "raw drivers" and "speaker enclosures"? These make a speaker system, obviously. You can't have a great speaker unit and a cardboard box "falling to bits" enclosure, obviously - that would be stupid. Obviously.

So, why do I get people in the car audio shop asking me if such and such "ExterminatorBASS338 £399 per pair" is the best speaker to use in their Vauxhall Corsa wobbly door card?

A RAW DRIVER IS NOT A "GOOD SPEAKER" - It is a good raw driver and a good starting point to build a good speaker but it, in itself, is not a good speaker without a little bit of time and effort and knowledge. You can make a very average speaker sound very convincing. Conversely you can make a very excellent driver sound very bad.

Car multimedia system

We are seeing more and more people wanting screens, PlayStation and digital TV installs. All this is lots and lots of fun but it is a totally separate arm of car audio and of course, because it is multi channel, it isn't car stereo. Just the same issues abound "in car" as they do "in home" - but fortunately there are processors available that enable you to have a full "dts" surround sound cinema system in the car which can easily be flicked into ordinary stereo mode for proper listening to a 2ch source

Multi channel stereo

The advent of multi channel surround sound in the home has done little to make understanding the situation easier. This format is exactly what it sounds like and the original recording was made using multiple microphones outputting to multiple channels on the media. In this case it would be valid to use more than 2 channels of output to mimic the additional microphones in the recording. At present this is used mainly for audio visual setups but with SACD and DVD Audio it will be interesting to see if "stereo" as a format is challenged!

The "Golden Rule"

Like anything worth doing, it is worth designing your system properly before you even take a look at the actual brands you are going to use. I have seen, and still continue to see so many people who have no idea where their car audio is going - they just carry on, buying one component after another with no regard for the rules. What are the rules? Well, my own Golden Rule is more of a definition - "A good car audio system is one that gets the job done with the smallest number of the highest quality components possible." There is no point at all in cramming the car full of components unless each one has a specific, defined role within the system as a whole. What do you want from YOUR system?

Remember earlier I talked about the definition of "good sound" and "good stereo"? Well, now is the time to decide if you subscribe to this definition. You'll probably be sitting there going "Yes, I want really clear sound that makes me feel as though I'm there with the artist." But be careful, because in reality many of those recording studios sound quite, well, flat. They are designed to be like this - everything is mixed to give a superb spectral balance and most of my customers don't actually want that, even though they say they do. What most people seem to want is a bit of extra treble to create a bit of "liveness" and a lot of extra bass to really make the sound punch out into something they feel, rather than hear. Some customers don't care about anything other than how loud the bass is, simple as that. Some customers just want the whole thing to make their ears bleed. I used to try to take customers under my wing to explain about "good sound" but now I realise this was a mistake. Who am I to tell someone that they shouldn't have three 12" woofers because this will make the sound unrealistically bassy? Exactly. What you need to do it to think about what you want it to sound like once it's finished. If you want a home hifi sound then great, or perhaps you want this but with a bit of extra "bite" - also great - but for goodness sake don't buy five 15" woofers and 3 component kits and then expect to win at sound quality in the IASCA finals! This might be a physical possibility but believe me you will be at a disadvantage!

You will also have to ask yourself what your attitude is to the way the system is going to be installed. Some people like a system installed out of the way in a car, so that it minimises the effect that the system has on the functionality of the vehicle. This is a very valid point of view, and of course will have implications as to which equipment proves to be the best for you. Many others like to spend time and effort making the installation either very aesthetically integrated or maybe just plain outrageous, with no regard to the functionality of the vehicle.

One thing I advise you not to do is to build a massive boot install until you have heard your system and got the sound just right! We do not have this luxury in our workshop most of the time because the customer wants his/her system designing in the store and then fitting in the workshop the next day. This is fine because we are talking about equipment that we recommend in the context of everyday business - we know it will work because we've done it all before. You most likely have not, and also, you can afford to be super critical when it is your own time you are spending doing the install. If you are contracting a shop (maybe us, even) to do the work for you there is merit in having a simple fitting job done and then living with the result for a few weeks to fine tune the sound to exactly what you want. There is nothing more frustrating than to spend hundreds of hours crafting a beautiful installation only to find out after everything is trimmed in Connolly leather that you need to find room for another amplifier because there isn't enough power to satisfy you!

Understanding the basics about frequency response

What is frequency in terms of car audio? You'll hear the word a lot and it just refers to how many times a second the sound wave oscillates, or put simply, how many times a second the speaker moves in and out. Thus a frequency of 40Hz (Hertz) means that your speaker will be going forwards and backwards 40 times a second. Similarly a frequency of 850Hz is 850 times a second and so on. 2Khz is "KiloHertz" and is just 2500Hz. Dead easy. In a nutshell:

  1. Sub bass is really the region from 25Hz up to around 100Hz.
  2. Mid Bass is 100Hz to 500Hz
  3. Mid Range is 500Hz to 5Khz
  4. Treble is from 5Khz upwards

(to the human threshold of hearing which varies from person to person but is on average about 15Khz for an adult. Harmonics exist in the audiable band that are created by the inaudiable band above 15Khz, so it is good to have speakers that will play these upper reaches.)

It is a simple fact derived from the laws of physics that smaller speaker will have a higher resonant frequency than a larger one - consequently and unsurprisingly a 15" woofer will be more comfortable playing the absolutely lowest notes at high volume than, say, an 8" one. Good bass box design can help redress this balance but this is your basic background. A 15" speaker is intrinsically NOT slower in response than an 8" one - it is just generally heavier and so requires hugely more power to create the same transient response.

And so; in the real world what speakers play what sounds?

We talked earlier about the stereo home setup. In an expensive home hifi the left and right speakers are designed to play music right down to the lowest bass note, which is known as "sub bass", because you tend to feel it as well as hear it. This is no different in a car system, of course, but again, as discussed before, we need to have rather more sub bass on tap in our car because it will be competing with all the tyre noise and wind noise once we're on the move. Plus the fact that, as we said, you might want to have more bass than technically necessary anyway, just for fun! Now, in an ideal world I would recommend the ACME MK1 SUPASPEEKER which plays from 25Hz right the way up to 20Khz and we'd all go home for tea and cakes in the conservatory. Ahem. Unfortunately there is no such speaker yet, and I can't see a viable way of fitting a pair of home audio tower speakers in my car. Yet. (!)

The solution is to use a subwoofer to play the low frequencies and the smaller speakers to play the higher ones. This sounds straightforward, and is, actually, but there are a great many facets to that simple statement. Before you can really choose a woofer size and design, you have got to inspect the car for its front speaker installation, or "full range" set up as it is known. This is because a car that is restricted to using 4" front speakers will run out of puff at a higher bass/midbass frequency than one that has room for 6.5" drivers and consequently will ideally employ a smaller diameter subwoofer which can take over from the full range speaker without a noticeable "gap" in the frequency response.

"Thompsons Tip" - Power ratings, a common misunderstanding

When you are researching car audio It is very important not to get hooked in to looking at the "wattage" on the box - recently we've seen these claimed wattages simply running away with themselves to the level of being plain ridiculous - to be fair the higher the on box "wattage" the more the stuff sells, but rest assured there is a tremendous amount of misinformation out there.

What you need to understand is that a 200 watt power HANDLING subwoofer will only reproduce 30 watts if you have a 30 watt amplifier. A 1000 watt subwoofer will produce, yes, only 30 watts from the 30 watt amplifier. A speaker with more power HANDLING will actually be heavier in the construction and thus likely actually LESS efficient than a smaller woofer - which means that you'll actually get MORE performance for your 30 watts of amp power from a 200 watt subwoofer than you will with a 1000 watt one.

Evaluating Your Car

This is a crucial, and almost always overlooked facet of car audio design. Once you know the kind of sound you want, you need to look at your car to decide what it will and will not be best at. I am referring specifically to the woofer installation and the full range speaker installation. As suggested above you should begin by evaluating the location, size and construction of the existing speaker apertures.

At the shop, we know how to tackle most cars to get what the customer wants, but as ever, it can sometimes be difficult to get this result within their budget or time constraints. For the DIY car stereo installer this isn't a problem, and of course you have a much greater time frame to examine your own car in. Do not rush into anything until you have thoroughly researched your plan.

Choosing Speaker Locations

Front speakers

This brings us smartly back round to that original problem we had at the beginning - we are not sat in the middle, so what can be done to get the best possible sound? As we have said, since the optimum is to be sat in the middle, we are looking to get as close to this ideal as possible by minimising the path length differences between the left hand speaker and your left ear and your right ear and the right speaker. Clearly this means that the car will "image" better from one side than the other and this is almost always the case. I generally set the car up to sound best for the person who's paid the bill (!) which is normally the driver! If you think about it, the best place to achieve this is with the speakers as far away from you and the passenger as possible - normally the kick wells under the dashboard. With speakers in the doors your right speaker gets close to your ear - even worse if the speakers are in the tops of the doors - witness the Ford Focus, or Peugeot 206! Awful! Note that this doesn't necessarily spoil the tonal quality of the system - it is quite possible to have a system that sounds beautiful tonally but which wouldn't know a good sound stage if it tripped over it! (This is very often how a factory system is in an expensive car)

2011 Edit - I recently listened to the factory systems in the Audi A8 and Jaguar XJ and although they have the speakers mounted in sub-optimal locations (and still are not quite right tonally) they solve the imaging problem impressively by locating the tweeters in the right place (base of the windscreen) and using some gentle time alignment. Pretty impressive and a long way forward from when the majority of this text was written in 2003.

One thing to mention here is the latest "time alignment" systems that delay the sending of audio information to specific speakers in the system to "dial out" the fact that we are not in the middle. This is clever stuff and does, if set up correctly, work. For a lot of reasons, however, such as phase disturbances, group delay and a whole host of furry-hooded-parker-anorak factoids, it is still fundamental to get the physical path-lengths as similar as possible, leaving your £6k's worth of Alpine computerisation as little sweat as possible! Proper DSP makes good stereo sound fantastic. It makes a dogs dinner sound like a dogs dinner.

Now I can see those Mercedes and Honda owners and lots of others of you jumping up and down at the back pointing at your C Classes and Civics and shouting that you just can't fit speakers in the kickwells on a right hand drive version of your car. And most of you would be right I'm sure - or even if you had the necessary two weeks of evenings to do the job perhaps you just can't bring yourself to chop up your car's interior. This is fine. You need to think carefully about what would be the NEXT best place that you could fit your speakers. Also, just because you've got speakers in the kick wells doesn't mean you're home and dry - they should be as "on axis" as possible, too, which means to say that the line drawn through the centre of the speaker should be pointing at your ear lobe and not the gearbox tunnel (BMW 3 series, anyone!). Actually, I find it works best if you aim the speakers at an imaginary point on the roof directly above the gearstick - IF POSSIBLE. It is all about compromise.

Now, there are 2 other very good reasons why doors are bad news unless you have to: Firstly, lots of cars have factory moulded plugs in the door jamb which just won't take aftermarket speaker cable. You can use the standard cable in this case but obviously there will be some (very, very, very) small losses in the connections. Much, much more importantly - crucially, even, is that the door is intrinsically not a very good speaker enclosure as it leaves the factory. Back to the home hifi again; imagine if I tried to market a home speaker that was made of thin sheet metal, with lots of bits of wire in it along with a large piece of glass that went up and down on a wobbly tin mechanism and a huge chunk of plastic trim panel clipped to it for good measure. Then make it all swing shut with a slam every time you went in your living room, not forgetting to swill water down the back of it whenever possible. Amusing but very real - if you have to use the doors (and often you must) you've got to make sure they are made into as good a speaker enclosure as possible. I just can't over emphasise how crucial to the sound it will be - and I don't mean just imaging and staging - I mean it will massively effect the sweetness of the vocals and the performance of the bass.

Rear fill / rear speaker

Once you've decided how big a front speaker you can fit in, you can look at rear speakers and make a decision on them. You should not, in theory, require any rear speakers, since there were never any rear microphones in the recording booth - it is stereo, remember. The addition of rear speakers on every car audio system except a DVD based Multimedia one is technically not right. However, since the rear walls of a concert hall or recording studio do create some reflections of sound (the concert hall a great deal more obviously) the rears are often used to reproduce these reflections. This has nothing whatsoever to do with whacking a set of 6x9 drivers into the parcel shelf and turning up the gain on your 1600W RMS amplifier! Since there are no dedicated rear left and rear right channels in a stereo world (don't confuse the fact that even a source unit that claims to have "front and rear RCA preouts" in reality only has 2 sets of LEFT AND RIGHT pre outs with their levels controlled by a fader) you need to be aware that if there is a gentle female vocal on the extreme left of the soundstage way deep in front of the ensemble, the left hand rear speaker will also try and play this which will somewhat spoil the effect (read "completely knacker it"!)

There are processors that do their best to create a phantom rear channel as reflections with varying degrees of success but in reality most car audio professionals either omit the rear completely or if it must be incorporated we tend to leave it short on treble response so that the most directional sounds only emanate from the front full range speaker set up. In every case where sound quality is the goal and imaging must be good I always have a system set up to sound nice first with just the fronts, and then crack open the gains on the back little by little to add a little bit of rear fill - rather like seasoning fine cooking. The reason it is necessary sometimes to use rear fill in a sound quality system can be threefold. A front only system can sometimes sound a bit "dry", especially if it is a big car. In addition, it is possible depending on how big your front speakers are, that you quite simply might not have the cone area in the front of the car to create a powerful enough midbass. I find this a particular problem in a car like, say a basic Clio or something where you've only got 4" dash speakers - I might then choose to add a pair of speakers to the back and craftily set up some crossovers to make them play only the midbass at a 6db/octave roll-off from 500Hz. Ahem. Sorry. Getting ahead of myself!

The third reason you might need some fill is a practical usage one that never really crops up in sound off competitions (remember that in a sound off, the car is stationary and so are the judges - hardly anyone uses rear fill in contests as it tends to degrade the imaging scores more than it impoves the tonal and spatial ones!) The issue is that as you drive your head moves, and as you turn corners you look through the side windows, which causes one ear or another to effectively point more towards the back of the car which is normally where the sub box is. This makes it damnably obvious that the sub is indeed in the back - and all of a sudden the integration goes. By carefully adding rear fill to the system you can make it much harder to detect that the woofer is in the rear of the car. Again this is less of an issue if you are using large front drivers since they will tend to drop lower than their smaller counterparts.

It is fair game to reflect your rear fill off a back windscreen since this spreads the sound and helps the effect. Do be wary of those cars where the rear speakers are in the back panels or doors firing across the vehicle. What happens here is that almost all the sound is lost into the back of the seat and then at different driver body positions your ears all of a sudden get a direct line of fire to the rear tweeter or something and everything goes weird. The worst culprit for this is something like a 4 door Golf, which has its rear tweeter up in the door handle and this is dreadful for spiking out the drivers right ear or the passenger's left.

There is of course nothing whatsoever to stop you from wanting a very, very, very loud car stereo, one that sounds a bit like a rock concert or club, where they have huge rear speakers and nobody cares about actual "sound quality" in its dictionary definition. This is the most modern set up and is the easiest to achieve, to be honest. The thing to do is to ensure that you get as much cone area as possible in and to be conservative with crossover frequencies - you want to make sure each speaker is driven hard so that you get that wonderfully exciting sound that happens just before a speaker drives into distortion but not so hard that you do over step the mark and cook your equipment. In this application a nice set of 6 x 9's will do you proud and it is of course very important again to ensure they are appropriately bolted down. Sounds obvious again but I won't embarrass any customers by telling you any stories

If you are using 6 x 9" type rears and you are after this kind of system then you can, if you wish, not worry as much about front speaker positioning. I would still pay attention to every other facet of their installation though to maximise output.


Some Truth About Amp and Sub Matching!-
When you have a 1000W sub and a 2000W amp your amp can easily control the sub cone, and you will be able to run the sub to its MECHANICAL limits, not being limited by the ELECTRICAL limits of the amp. When you are driving a sub to its MECHANICAL limits the sound is VERY BAD - you can instantly tell that you are overdriving it and back the power down, saving the subwoofer from destruction.

So now you know how large your fronts are going to be and where they are going to be installed. You also know if you're using rears and how big they will be.

The subwoofer system again is something that needs some thought. I would always advise that where space and budget permits, you choose a subwoofer setup that is more powerful than you Thompsons Ltd for Car audioactually need. This will enable you to alter the level of the sub to suit your taste without pushing it so hard that it becomes strained. How much is enough then, and what about the effect that different cars have on the bass?

I consider an "ordinary car" to be a standard hatchback - something like a Fiesta, or Golf. This will have a cardboard or fibreboard rear parcel shelf and a decent sized boot. In a car like this, you will find that a single subwoofer of whatever conventional size (10 or 12" or whatever) will be more than enough to provide a large amount of bass. Certainly there will be enough bass to do justice to any tracks at a sensible volume. Adding a second sub is literally twice the fun! But remember that this means two lots of frictional losses and two points of bass production which in theory could tinge the sound quality - in practice this never is a problem but there will be a slight loss of dynamics compared to a single woofer (assuming you share the same amp). Lack of bass, however, will not be a problem!

Again, clearly there is a customer base that just wants as much sub bass as possible and that isn't a problem either - fill the car with woofers and follow a few simple rules and bass will be yours!

In these cars we are essentially sat in the same compartment as the woofer, since the cardboard parcel shelf doesn't really count as far as the sub is concerned. The wavelength of a woofer running below 100Hz is very long and passes through fabric and low density materials like that shelf almost as though they didn't exist. In these cases you will most likely be installing your woofer in a bass box. The purpose of the box will be explained later but for now I will limit the explanation to saying that the box simply separates the front wave from the speaker from the back wave. If the box wasn't there the two waves would meet and you'd get so much cancellation that you'd basically not hear a thing!

In a car where you have a "boot" - like a BMW saloon, or even something like a Vectra saloon or Impreza or whatever, you have a different beast altogether. An inexperienced audio installer merely whacks a double 12 box in there and lets rip - and frankly it will sound like a decent 8" sub did in the hatchback. Not funny if you've spent a grand on bass. What to do then?

Well, it is all about using the car to your advantage. There are two issues to tackle here. The first is the fact that whenever there is vibration, unless it is either the air or the speaker vibrating, we are wasting energy. Put your hand on the boot of a 3 series with big woofers in the boot and it will normally get a free vibro massage. What is happening here is simple to understand. The woofer is in a totally separate enclosure in a saloon, and the enclosure is going to vibrate. You can minimise this with lots and lots of soundproofing and you certainly should but this alone will not be enough. The second issue is that the sound waves in the boot must be allowed out: they can't travel well through multiple layers of steel and plastic. In something like an Impreza the favourite trick is to make a sturdy bulkhead behind the (foam) back seats and construct the subwoofer enclosure into this bulkhead. The result is that the bass can't find its way around the sub box (the MDF has a very low resonant frequency once bolted in as a bulkhead) and has only one easy choice: forward into the cabin. There is less vibration and we get a very punchy bass.

If we go back to the dreaded 3 series BMW we have a problem with this in the fact that the rear seats are built like a WWII toilet. We have stripped a set down and painstakingly drilled out the rear then re trimmed, but to be honest it was a painful process, and a not altogether satisfactory idea.

Current thinking is to not use any rear speakers and to fire the woofers upwards at the rear deck - the holes for the rear speakers allow the boot to vent to the cabin and the result is lots of bass. It is worthwhile spending time making sealing panels that stop bass getting out of the sides and rear.

In sports cars, like the Toyota MR2 or MX5, for example, a bit more ingenuity is required. You will doubtless have seen the ones with a big 12" sub in the "boot" about 12 miles from the driver and behind an engine or whatever, but let me assure you that they do not sound good at all. One clown recently contracted a local competitor to install a pair of tens in the front boot of a Boxster - utter, utter madness (we got the job of sorting out - I wasn't sure whether to laugh or cry!). You've got to get the bass into the cabin and remember that because the cabin is so small you have a lot less air to move.

In an MR2 MKII we make bass boxes to house 8" woofers behind one or both seats. They fire straight into the occupants kidneys and are brilliant! In an MGF you simply make a small bass enclosure in the passenger footwell and install an 8" sub there (It does cost about 6" footroom, so you've got to ensure you've picked a short partner!!!!). I once had a Lotus Esprit and spent about 150 hours (yes I know, I know) making a glassfibre bassbox that replaced the glovebox and fired down onto the passenger's feet. The sound was just absolutely incredible but it was a total, total pig to do and if anyone asks me to do another I shall run off into the distance having first shoved two pencils up my nose and pulled some underpants over my head!

Choosing speakers


We have already discussed at length the fact that in shop demo rigs are neither consistent nor representative of your vehicle's enclosure characteristics. I'd also like to point out that the shop isn't representative of your vehicle's INTERIOR either, in terms of both size and the acoustic properties of the furnishings.

Furthermore, I would like to put it to you that the only time you are honestly and truly going to be able to say whether you truly love the sound of a loudspeaker system is once you've actually lived with it for a time. All is not lost, however, because there are some things you can tell about a driver just from its basic construction and perhaps by having a quick listen to it, even in a shop. Listening in a similar car to yours is a great idea if you can get lucky but frankly you have got to understand that as long as you don't buy a complete lemon you will be able to tune the vehicle's installation to tweak the sound fairly substantially, and that in honesty it is great fitting that makes a great sound to at least and probably more extent than a great raw driver.

The first thing I look at when assessing the quality of a speaker unit is absolutely not its price. Understand right now that there is more jiggery pokery going on in speaker pricing than in a politicians expense account. You have got to forget also how tasty the speaker actually looks - it's what is under the hood that counts, not the paint job. Do not confuse this statement with BUILD QUALITY, which is absolutely critical. You certainly do want a well made product but we don't care if it has a dull looking magnet and boring graphics.

Midrange speakers (also known as "Midbass" speakers) Thompsons Ltd for Car audio

(This can also be taken in context for the main driver part of a coaxial speaker)

Take the speaker in your hand and carefully flex the cone material between your thumb and first two fingers. You should be very careful not to damage the speaker (you don't want an invoice at this stage!) but a well made driver will exhibit great torsional strength in the cone. This is going to help sonic accuracy and the bass delivery. The cone will almost certainly flex, but if it feels like a wet newspaper you are dealing with a nasty speaker. The other thing to watch out for, and it is frankly impossible to tell, but the other evil with cones is weight. In an ideal world the cone would be as stiff as a board and as light as a feather - this is the compromise the manufacturer is trying to achieve. Excess weight in the cone means sonic overhang and poor transient response. Sony used a clever idea once where the cone was thicker near the centre and thinner near the edges. This put the strength supposedly where it was needed whilst controlling the weight of the cone. Other companies use a Kevlar honeycomb inside the cone to reinforce the structure. Whatever, the actual material isn't important but the result is.

The next thing to investigate is the surround edge. The function of the surround is to hold the speaker voice coil precisely vertical and allow free movement of the cone backwards and forwards. In an ideal world there would be no friction, a perfect seal and rigid lateral support. Life isn't perfect, however, and so the result is that speaker surrounds are, in the main, made of three basic types of material.

The very cheapest and nastiest speaker has a worryingly thin, cheap foam surround which will leak like a sieve under load. Since the cone will most likely have the rigidity of a tinned sardine this will probably make the sound no worse than it was going to be anyway!

Another variation on the cheap and nasty approach is a flimsy cloth, which is almost as bad.

Better drivers use a much thicker, non porous foam surround. This should be evenly glued all the way around the speaker and ought to have an almost rubbery quality about it. In fairness, a very many speakers have this so it will be easy to spot an inferior one after you've seen a few good ones.

The best type of surround is made from a thin but incredibly strong and light rubber. These are known as a "Nitrile" surround and are a feature on many, even fairly inexpensive drivers.

What about the basket? A cheap tin pressed basket isn't necessarily bad news; especially if everything else is good quality (the money had to be saved somewhere!) and the price is acceptable. The function of the basket is to hold everything together firmly. A cheap tin basket will tend to "ring" at certain frequencies (in theory - I challenge you to hear it!) whereas a more expensive cast basket is more rigid and has a lower resonant frequency. Modern injection moulded nylon baskets are good news too for the same reason, however these tend to be quite rare.

Now have a look at the voice coil. If you hold a speaker on its side and peer inside the basket you'll see an area where the cone converges - the distance across this constriction will be an almost perfect representation of the size of the voice coil. A big voice coil means more copper wire and more area for heat to dissipate into. This is good news. It will also mean slightly more weight which we know to be bad news but in this case although the sensitivity of the speaker with a large and heavy coil will be marginally less than one with a small coil the power handling will be better. On balance then, I like a nice big voice coil to give us plenty of power. Some of the early Rockford Fosgate Power series speakers had offensively large voice coils with vast magnets and Kevlar woven cones. They had ridiculous power handling as a result and were the speaker of choice for anyone wanting big volume to keep up with a brace of large subs! Subtle, however, they were not and the sound wasn't as sweet as the smaller and less powerful "Audiophile" range with its lighter Spruce paper cone and smaller coil/magnet assembly. Be wary if the box says "900 Watts Power Handling" and you're looking at a 4" driver with a 0.5" voice coil because although those might be great sounding speakers they sure won't be 900 Watts!

The magnet size is important too, but since there are obviously different strength magnets and also some very clever magnet technologies out there it is not recommended that you are too concerned about magnet size alone. I would recommend that if there is anything suspicious like, for example a tiny magnet acting on a huge voice coil, you ought to find evidence as to why this design has been adopted. For example, the original 1996 JBL Gti 400 and 500 series midrange speakers had simply enormous voice coils but very small magnets. To be honest you didn't have to be Sherlock Holmes to work out that these were very advanced speakers (the basket alone was a cast work of art) but it was nonetheless the case that JBL had used an advanced technique called "Stray Field Containment Geometry" - which focussed the power of the magnet on the voice coil rather than on all the screws and nails on your workbench! Needless to say, the speakers sounded fabulous.


(This can also be taken in context for the high frequency part of a coaxial speaker)

The material construction of a tweeter will play a great part in its sound. Generally, a hard metallic tweeter made of either titanium or more commonly aluminium will tend to have a sharp, hard edged metallic sound. A silk tweeter, conversely, will be smoother and less aggressive - also less detailed very often. Mylar (a hard plastic) is used too by some manufacturers and has its own, quite aggressive sound.

So that's it, is it? Er, no. Sorry. The only problem is with the above that although this is generally the case, very often you can find a tweeter that just doesn't do what you'd expect. Sometimes it is because the manufacturer has changed the impedance of the speaker (normally from 4ohms to 6ohms) to soften the sound - sometimes there will be a clever trick like a cunningly placed piece of plastic in front of the dome (JBL GTX25 if you've got a good memory!) to reduce sibilance (where the "S's" tend to rasssssp!) . Whatever there are a great many ideas some of which work well and some of which frankly are plain stupid. You should talk to people who have used the product or better still try and have a listen for ideas about the type of treble you can expect from a particular model.

Component kitsThompsons Ltd for Car audio

When you're buying a separate midrange and tweeter you will need to stop the midrange playing treble and the tweeter playing bass. You can do this with either an electronic crossover (known as an "active" setup since the processing is done before or in the amplifier) or a passive crossover ("passive" because it is done post-amplification). There are a great many things to know about crossovers and they merit a separate section of their own, but for now it is enough to say that more recently, there are a great many excellent "off the shelf" component kits that come with superb passive crossovers that are matched to their respective midrange and treble drivers. You have much less to get wrong if you go down this route and unless you know enough to clearly explain why you don't want to do this I would advise you to opt for this route.

Once you've decided you like the look of someone's complete kit have a look for a crossover. If you can't find one (and they'll normally be a large matchbox sized plastic jobbie) then have a look at the tweeter and midrange speakers and/or any supplied wiring looms. What happens on cheaper components (and is not necessarily a problem as long as the price reflects it) is that the tweeter simply has a capacitor in line with it that stops bass and midrange from entering into it. The bass unit will most likely have a small inductor on it too to stop the treble from sullying its midrange. If you can't see anything on the mid then what you are looking at is a speaker specifically designed to take advantage of the naturally occurring higher impedance as the frequency ramps up. This will by its very nature limit output around the treble region. Not ideal but on a budget kit which is otherwise good I wouldn't rule it out.

The crossover box, if you're lucky enough to have one, contains lots of complicated looking gizmo's which I will cover in the crossover section. I look for five desirable features in a crossover, the nicest of which is a built in attenuator on the tweeter circuit. This will normally say something like "0db, -3db, -6db" and then have a little jumper plug on the circuit board. This is good because it allows you to tone down the whole level of the tweeter - useful if they have been mounted, say, on the dashboard and are a touch too bright. Don't think you can get the same result by turning down the treble or "EQ"ing your way out of it either because in a passive system this will only reduce the volume of certain frequencies. If you see anything on a passive crossover that implies that you can "boost" the treble (some have +3db, 0db, -3db) (or similar) then realise that this is twoddle. A passive crossover can only reduce output - it can not increase it. What these settings would actually align to is that the "+3db" setting is actually "0db", the "0db" setting is actually "-3db" and so on.

The second nice feature is some kind of light bulb inside the crossover. I know this sounds odd but in practice this is a great little idea. Understand first that sound waves are AC (Alternating Current) voltage. When distortion occurs because some clown is overdriving his/her amplifier or head unit or preamp or whatever, the top and bottom of the waveform is squared off, or "clipped" (hence the term "clipping"). This makes the previously nice AC voltage turn into DC voltage. Now speakers hate DC - a little bit of DC will fry a £100 pair of tweeters faster than the bloke at the Chinese takeaway can rustle up a chow-mein. Cue seething email from distressed punter with a pair of melted tweeters mumbling about them being "faulty". Ahem. The bulbs, however, can help prevent this by simply lighting up! Bulbs love DC, and will prevent your speakers from flame grilling themselves. Yes, we do get the odd chap (or lass) who actually still manages to blow the bulbs, melt the resistors and every single track on the crossover board - don't let this be you!

The third nicety is a low cut option. If you've got a flash deck or preamp or amp then frankly you won't care but if you're running your new components off your head unit it is helpful to have a terminal for the positive of the mid driver that cuts out the really low bass. Kenwood provided this on their "DualMags" and it does help to maximise the power potential of a small amp, since no power is wasted trying to play notes the speakers can't really handle. Consequently the system can be played louder with no distortion.

Fourthly you might look out for a crossover that has a "Zobel" network and /or a series notch filter. Sounds complex and be assured, it is, but these techniques are there to keep the natural impedance anomalies that occur within a speaker at different frequencies under control. A Zobel or notch filter must be carefully designed for a specific driver and tweeter combination and is a very advanced task - this is why a high quality component kit can often sound much better using its own crossover rather than an (on paper) more technically advanced active network.

It is nice to have a phase switch too on a crossover, and we'll come to phasing later.

Finally, make sure the crossover is going to fit somewhere - if you've got a Smart Car you are no good with a pair of crossovers the size of a pizza box.

Choosing rear speakersThomsons Ltd, Car Audio

The technically best set of rear speakers for your system - if you're using any - are identical in voice to your fronts. We know, however, that since the speakers are unlikely to be housed in exactly the same size and type of mounting location between front and rear, it is very difficult to achieve this goal. We also have already stated that often we will restrict the frequency band that plays through the rear anyway. So the bottom line is simply this: If you can afford it and the mounting application and volume requirements of the system allow you to buy exactly the same speakers then this is the optimum. If not then don't worry one bit and just get something that will provide a similar sound.

Choosing a subwoofer

You will hear people refer to a 12 being "slow" and an 8 being "fast" - this is not actually true intrinsically. Yes, an 8 inch sub will generally be lighter in cone weight than a 12 inch sub, and so it comes as no surprise that for a given level of power the smaller one will be faster at responding and thus produce a tighter sound. However, if you want a twelve to be as fast as that eight, you've just got to up the power to weight ratio until it's the same; ie, you hook up a much bigger amp. Now you'll have the same transient response and much deeper bass to go with it.

However, it is true to say that generally the smaller the speaker the higher the resonant frequency, so yes, you'd tend to use a load of 10's if you wanted a really tight, punchy sound, or a 15" woofer or two if your preference was for a low, smooth rumble.

DVC subwoofers and how to use them

No section on subwoofers would be complete without a mention of the DVC (dual voice coil) products.

When you choose a woofer and an amp, it is CRITICAL that you match the two well together. You need, firstly, to look at the minimum bridged impedance is that your amp will deal with. So, in the case of, say, a two channel amp you might find that it says "minimum 4 ohm stereo" - which means if you connect two speakers to it (one to each channel) then you must ensure they are not less than 4 ohms each. This is straightforward, obviously, when you install a standard car speaker or even a pair of ordinary subwoofers which have only one pair of terminals.

Lets say then that you want to fit a subwoofer instead to that amp, in a bridged mode instead of stereo. Check the amp spec for "Bridged mode" - you might find that it allows 2 ohm, 1 ohm or even 0.5 ohm connection. This is important information which we will use in a moment. Most likely, though, on an ordinary amp you will only be allowed to connect a 4 ohm bridged load. If you don't take heed of this you will destroy your equipment, and pleading ignorant won't help you to get a warranty claim upheld. You'll be stuck with broken gear rather like reversing your new car into a post and expecting the dealer to fix it under warranty as if it were a faulty gearbox!

So - with that one sub we have - a 4 ohm one, and out 4 ohm amp, we are happy and you just wire it up. (then follow the setup instructions later on)

When you get to DVC woofers however (and many of ours are DVC) you now have 2 sets of terminals on the sub. So, you need to check the spec of the woofer to learn how many ohms per coil (set of terminals) that you have. Let's say you have a "dual 2 ohm" sub.

First of all, you absolutely CANNOT run only one side of a DVC sub. You will burn it out very quickly - again no warranty and it'll be dead obvious what you've done on the bench!

Second, you absolutely CANNOT run a STEREO signal into the two coils of a DVC sub. Think about this - if you have a stereo amp with a big bass note on the left channel and at that time no bass on the right channel then your amp will literally try to pull the voice coil in half! The result will be a weird sound, toasted amp and a toasted sub too. And no, warranty will be voice again!

So - the correct way to connect an amp to a DVC sub is to mono the inputs if you are using a stereo amp and then to adhere to the minimum load set out by the amp manufacturer.

Going back to our dual 2 ohm sub and 4 ohm stable amp we MUST wire the sub in series. A series wired coil with 2 ohms per coil will give a 4 ohm load - and your amp and sub will have a long and happy relationship. A parallel wired dual 2 ohm sub will have an impedence of around 1 ohm - which will send your amp into distortion and/or kill the power supply and/or melt the woofer coil too. Either way, it's warranty reject time again!

To wire a DVC sub in series this is the procedure
Take the + from the amp and connect that to the + on coil1 of the sub.
Take the - from coil 1 of the sub and connect that to + on coil2 of the sub.
Take the - from coil 2 of the sub back to the - on the amp.

Easy as pie.

To wire a DVC sub in parallel this is the procedure:
Take the + from the amp and connect it to the + on coil1 and coil2 of the sub.
Take the - from the amp and connect it to the - on coil1 and coil2 of the sub.

Again, easy as pie.

Here are the results of various wiring configurations:
2 ohm DVC sub in series = 4 ohms
2 ohm DVC sub in parallel = 1 ohm
4 ohm DVC sub in series = 8 ohms
4 ohm DVC sub in parallel = 2 ohms

You can see from this that a DVC sub gives you great flexibility in terms of your wiring, and that if you combine more than one sub you can combine the types of wiring - so if you had a pair of 4 ohm DVC subs you could wire them EACH in series to create an 8 ohm load PER sub, and then PARALLEL them together to make a 4 ohm load - this way you can see how to wire many woofers off one amp without damage. There are interesting calculators online to allow you to play with your configuration - my aim here was to introduce you to DVC and stop you from becoming that person every month that manages to break their new DVC sub through sheer ignorance.

Source units

Most modern units have about 4 x 50 watts of power on board. In reality it is no surprise that these claimed power outputs are achieved at levels of distortion (and by other means) that mean that it is not possible to listen at that level - the output breaks up way before. Truth is, however, that on board amps have come a long way in the past few years and do offer a reasonable level of performance when their power is used in a sensible way - more later. I personally wouldn't choose a unit because it was 4 x 50W over one that was 4 x 45w unless there were no other differences.

The next issue is a more complex one. It used to be fact that the optimum method for having a serious system was to have a CD player, then you had a line driver to boost the RCA pre out level, then it went through an equaliser in the boot to make it sound tonally right, then the frequencies were split to the various amps in a crossover. Finally your amps would get the signal and do their thing.

Nowadays some of this is still viable but advances in technology mean that with careful choice you can buy many top of the line units with built in processing, and I don't mean some foolish device that makes your car sound like a church, or stadium or something daft. What we are looking for are features like a variable high pass filter for the front and rear speakers - preferably separate and preferably one that can even be set up to work on the internal amplifier. We'd also like a low pass filter for the subwoofer with a few different crossover points and ideally a few different slope settings. A proper, easy to adjust equaliser with as many bands as possible would be a super thing to have and (I know I'm being greedy here!) I'd really like it to be parametrically adjustable with variable "Q" factor on as many bands as possible. Ahem. I know that might sound a bit complex but if you read the other sections of this guide you should be able to work it all out. I'm not sure if such a head unit is available yet (!) but the idea would, money no object, be to get as close as possible!

It would be very nice, if possible, to have a head unit with a big fat preamp voltage. Something like 4 (or even 5 now) volts would be super. Also, the impedance of the preamp stage in the deck is also important. Low impedance outputs mean that although you may have only 3 volts or whatever, you can hang many amps off of one pre-out without introducing noise into the system. A top of the line pre-amp doesn't guarantee a noise free car but it sure as hell helps if you car and gear do prove to be a problem!

Another desirable feature is a separate subwoofer output. This means you have very often got excellent control of the subwoofer from the head unit and sometimes there are additional benefits when it comes to troubleshooting noise.

While you are at it (and especially with some of the heavily graphically driven modern decks) it might be a nice idea to have a play with the menu systems to see if you can actually get along with operating the thing before you part with your money - especially crossover and equaliser adjustments, which want to be easy to do "on the fly".

Also, and especially if you intend to run any speakers off the internal amp, will the unit go a reasonable way up its volume scale without distorting its head off? I have found lots of modern units (Especially those with "Distruct-A-Bass" or whatever those stupid "Equaliser pattern" thingy's are called) that just won't drive beyond half volume without breaking up. The only reason I can think of for this is that the manufacturer wants you to buy the unit and then think your speakers are not "up to the job" because you have bus loads of distortion. Clever rascals. Anyway, we're too clever for them and know it is just bad design, don't we?

Kenwood have a fab control in their decks to avoid/minimise this called "Volume Offset". This allows you to set the volume up to 8 points down from where it really is - i.e. full volume now won't distort the speakers. This is damn clever and a real help in setting up properly.

Choosing amplifiers

A logical approach is needed to choose a car audio amp because there really is a massive range on offer. It is a fact of car audio that competitions have been won by cars running almost all the amplifier brands. It is agreed that it is probably a more difficult choice than head units because there are just so many more amps available than head units but nevertheless you can apply some simple logic.

You first need to understand that there are a lot of cheap amps out there that have poor quality components in them. They are rife in the UK because unlike head units, which age from year to year and also require their tuners to be set up for UK market use, an amp has no market boundaries. There is no excuse for not buying a pukka product now that prices have fallen so much over the past couple of years. US brand Lanzar amplifiers are a great example of top brand US product available at inexpensive prices.

The most difficult aspect when choosing an amp is understanding power figures. The simple answer is really that you'll never fully get to the bottom of just how powerful an amp is without bench running it yourself, but a discussion about RMS and Peak Power, and how they are rated will not go amiss.

This is the old chestnut and we've all seen it where a £100 amp is claimed to be 1000 watts and then some esoteric American company want £300 for a 40 watt tube amplifier. How can this be?

Power is a simple measure. No point getting into equations but basically in terms of amps there are the following important factors you need to take into account when talking about rated power.


Now read that again - it really is that simple. I'll come back to this in a moment.

1) Was the amp tested "All channels driven" when rated?
It is common practice to run an amp with one or more outputs disconnected to bump up the "per channel" rating. More about power sharing later but this is not really on and you'd hardly ever know they've pulled this stunt.

2) What voltage was the amp tested at?
Bearing in mind that the amplifier has to work twice as hard to produce the same level of power at 11V as it does at 12V, it isn't hard to work out that an amp tested for max power at 14.4V will kick out a hell of a lot more than one rated at 11.9V. This is another trick which is used to make an amp seem more powerful than it really is. In fairness, the rationale behind it is that the car will be running at or about 13 or so volts when you're driving so it is supposed to represent the sort of power you might expect. At least most manufacturers have the courtesy to tell you the voltage and if not, walk away. Do bear strongly in mind that an amp tested at the 11.9V standard will seem pathetically un-powerful compared to one tested at 14.4V but you've got to be comparing apples with apples.

3) What distortion limit was set for the test?
Again this is a classic stunt that the head unit manufacturers love! The average human ear can put up with listening to music with 1% distortion. Once the THD (Total Harmonic Distortion) figure climbs through 3%, however, we are very unhappy with things and reach for the volume knob to turn down the sound and bring the waveform back under control. Some amp manufacturers think it a good idea to test their products at these high levels of distortion and sure enough, at certain peak transients within the music at about 40% THD I am sure that the "Purple People Eater 2000W Super Model, £99.99" did indeed hit 2000W, for a fraction of a second. Before it exploded and took the lab and the technician with it. Not a useful measure for you and I though, and again, unless you are comparing spec with spec you can't be objective.

So - in summary, unless you can answer 1,2,3 above you CANNOT honestly tell what you are buying. I can't think of a single one of our products available that actually give you all this information - so yes, 1000W, 1900W, 4000W - it's all a bit "wooly information" and you've got to look at amp fuse ratings, cable sizes etc to help form a judgement. Thus, box power ratings are a guideline only.

Thompson Tip - Power, Power, Power - The Great Lie!

A simple check to see if your chosen manufacturer is getting involved in power hiking on the box is to check the fuse sizes.

Let's say an amp has a 30A fuse. Your car is 12 volts - 13-14 when the engine is running. 30 amps x 13 volts = 390 watts.

That's it - 390 watts - there is nothing on the planet that can make that amp produce a SINGLE watt more than that - impossible - and remember that amps are NOWHERE near 100% efficient. Expect 70% at most really - that's 270 watts from that. So if you're looking at the advert and it's telling you to expect 2100 watts "Peak" you're reading class A, 100% rubbish.

Don't think, though, that this makes the product bad. Look at Kicker amps - they have "honest" power ratings, they are great amps - amazing power, great reliability, but because they have been honest about the wattage nobody wants to buy them.

The result of this is that everyone is "out wattaging" everyone else - it's got ridiculous - so just look at the fuse ratings, do a simple bit of maths and go in with your eyes open. Many are great amps even if they don't make their "box" power!

Sound quality

This is magazine review nonesense. You should not be able to "hear" an amplifier at all, if you think about it! It should merely take in its very tiny waveform from the headunit and make it much bigger to send out to the speakers. End of story. As ever though in the real world you can hear differences in amps because nobody has managed to achieve the above yet! The differences are so subtle, however, that you've got no chance in a shop and the only way you'll be objective is to bribe your local dealer into lending you a couple of amps to hook up to a good home hifi and do some listening tests. Frankly it isn't worth it because there are just so many other things that are easier to achieve than this that make a bigger difference to the sound.

Multichannel, Stereo or Mono?

We touched on the subject of power sharing earlier. This happens because an amp only has one power supply. If you've got 4 channels in your amp, and 2 are driving a sub as a bridged mono load and the other two a stereo full range pair you will experience some loss of performance on, say, the full range pair when a massive bass note kicks in. Yes, you can minimise it with a big amp and /or a stiffening capacitor, but no, you can't eradicate it and two smaller amps will always be better. Also more expensive. And take up more room. I love multichannel amps, though, because they make for such tidy installs. Don't get obsessed by the power sharing issue as the best big 4 channel beasts today do remarkably well at minimising its effects. You just need to be aware that totting up wattages doesn't tell the full dynamics story even when you are talking proper watts.

A mono amp isn't an issue - but unless there is a good reason (like they only make the desired power rating or style I wanted in mono) then I'd always buy the stereo version because it is more versatile if you choose to expand the system later and need to pension the amp off to do something else. There is always the sale option though, and there are always people looking for decent mono sub amps.


All amplifiers are not the same. Despite manufacturers claims some amps are more susceptible to picking up interference than others. Also, some cheap amps have their audio grounds at the chassis. This means that the chance of introducing a ground loop into the system is much greater. If possible, ask the shop if the amplifiers concerned have audio circuit isolation greater than 200 ohms. To be fair, many shop staff might not know but their workshop staff might. Additionally, try asking the workshop staff which amps THEY'D choose. Remember they'll have to fit it and this means they'll know more about which products cause the noisy installations.

Installing for a noise free system

First of all, let me say that no matter how careful you are, the installation of a high fidelity audio system into a situation where you have got rectified AC - DC conversion as bad as a car alternator will only ever be 99% science. There is an element of luck there. You can make a good start by picking good quality components. But don't think that is the only factor. We continue to do installations with top of the line amplifiers and still are not immune to noise.

The most obvious form of extraneous noise is alternator whine. This high pitched squeal generally comes through your tweeters. It can vary from absolutely awful to virtually inaudible in a real world situation. Still, the severity isn't the issue; we are concerned here with ways to try and solve it. Some are cheap and easy and some are expensive and difficult! Every comment I make here assumes you have a noise problem; mostly you wouldn't necessarily have to resort to all these measures and of course you might have a system that should technically whine its head off and is in fact super quiet. These are all just ideas for you to try if you are tearing out your hair!

First, a candid description of the problem. What happens is that the alternator creates AC voltage. This is no good to the car, so the rectifier in the alternator converts it to DC. Now that should be great, but unfortunately the DC that the alternator makes is very poor quality. Sometimes, also, a car alternator can have rectifier problems that add to this "ripple" in the DC. Now most of the car's electronics are fine with all this, and even sensitive bits like airbags and ECU's etc have filtering in them to help smooth everything out. We are not fine, however, because we are trying to add a lot of highly powered amplification that will draw lots of current and then send any noise amplified to listening devices - speakers.

Firstly, we will deal with signal (RCA) transmission. It is a fact that twisted pair RCA leads and shielding do help to reduce radiated noise from the car's own wiring. They don't sound quite as good as parallel RCA leads (they function as a kind of low pass filter) but the difference is small and the trade off is worth it. You would, in an ideal world, run the RCA cables well away from the main power lead of the system as it winds is way down the car and also you should try and keep it away from the cars own wiring - in particular the fuel pump wire can cause its own problems. In the real world this is very difficult. - do your best. If you have a head unit with only one or two pre-outs and you have a requirement for more than that you will be using either a "pass through" on the amplifier(s) or "Y" leads in the boot. Be aware that you might want to try running 3 RCA's and splitting them at the head unit end as this lowers the current draw down each RCA and helps to reject noise more.

Secondly, the lower the source impedance of the head unit and the more voltage it's preamp can run cleanly the better. You should also be aware that different manufacturers treat head unit audio grounding in different ways which can also have an effect. It is not unusual to swap head units and cure the problem. In a nutshell there should only be one audio signal ground, and that should be at the head unit. What I mean by this is that the outer shield of your RCA leads should be effectively connected with a super low impedance to the chassis of the deck, which in turn should be grounded to one point and the actual ground wire of the deck should also be connected to this too. In an extreme world you can ground this to the car side of the firewall or even in the boot with the other amplifier grounds. Do make sure you use a fat cable (10AWG) if you are running a boot ground otherwise the exercise will cancel itself out!

The actual grounding point for the amps can also be a major issue. It goes without saying that the point of connection should be clean and paint free and ideally made with a bolt rather than a self tapper but additionally it can matter where the ground on the car is made. It is fair to say that various parts of a car body are attached to each other nowadays with all sorts of methods, some of which conduct well and some which do not. I would advise connection to a solid part of the boot area and don't be frightened to move this point if you don't get success first time. If you have the battery in the boot it is important to try a couple of locations. This is special because all of the ground return is along the chassis right past the amps and speaker wiring to a common point where the battery negative joins the chassis. Grounding back to the battery can sometimes therefore be counter productive since you pick up the epicentre of the noise. Try a ground somewhere else and remember that electricity doesn't take the most obvious path; a point a few inches from the battery may prove as good as somewhere three feet away. It is important also that all the car audio components see the same ground reference point; in an ideal world everything audio related would be referenced back to just one spot on the chassis.

Ground loop isolators and noise filters do have their place, and sometimes when you've tried everything and got nowhere it is a nice idea to try such a thing but be aware that most commercially available examples of these do and will reduce your sound quality; notably bass response and some midbass. It is therefore only recommended as a last resort. Of course there are instances where such an item won't help anyway (because you've not got a ground loop!).

One thing I have found helpful is to try muting the amplifer. If you short the amp's inputs (RCA inner shield to outer shield on all channels) and there is still noise then you have either a faulty amplifer or speaker cable that is picking up noise from other wires around it. Try substituting another make of amp. Often this will solve your problem. EMR (Electromagnetic Radiation) shielding tape is available from RS components etc and can help if you've got direct noise injection into a speaker wire. I have found that foil backed soundproofing works well as shielding too.

Another tip is to try moving the head unit into the boot and running it off the same type and length of RCA leads as you are using. If it is quiet when it uses the same ground and power feed as your amp with the RCA's just sat in the boot then clearly the cables are picking up noise en route or you have a bad ground/power feed to the head piece. If this is the case, also consider the quality of your RCA leads. This varies hugely and although just swapping brands of RCA is unlikely to solve a problem on its own a decent RCA is always a good investment. Some IXOS RCA's now use screw on connector bodies that actually clamp the outer shields onto the mating connector which is superb because often the RCA shields are a very loose fit - terrible when you think we are trying to achieve equal, low resistance grounding across our whole signal chain network.


Soundproofing your car is a critical factor in the performance of your audio system. Firstly, let us just once again reiterate the point that a more expensive, better built car will generally have a lower ambient noise and therefore will tend to sound better than a more basic car "straight out of the box". Soundproofing is done by the manufacturers to both a price and a weight (it is generally quite a heavy addition) and of course there are other factors which I am going to mention first.

The disadvantages of soundproofing your car (or things to watch out for) are really twofold. Firstly, by adding a tremendous level of soundproofing you will make your car a lot heavier. This is not necessarily a problem, but bear in mind always where you are adding weight and the effect this will have on performance, economy and handling. I try and achieve my goal of a "dead" car by adding material in areas that will make a big difference acoustically without piling too much in areas that will have minimal impact.

The second problem can be wind noise. On some cars there is a fair bit of wind whistle from around the window area at higher speed (motorway really) which the manufacturer knows about. Since wind noise is quite high frequency it is very irritating. The manufacturer ensures there is enough lower frequency rumble and tyre noise to cancel out this irritating whistle, but of course when you come along with wheelbarrows full of sound proofing you can end up with a car with much less road noise and lots of whistle since you can't soundproof the glass. No. You can't.

This wind whistle is only a problem on a very small number of vehicles, generally the older ones with separate frames around the glass and it is only an issue when you've absolutely gone mad with deadening!

So, with those two proviso's out of the way you need to consider WHY we are soundproofing. There are 4 reasons.

1) We are soundproofing the area around the speakers themselves to make the door or panel that the speaker is fitted in a really solid base from which the driver can perform. This means you can use glassfibre resin and cloth to stiffen mdf or steel, perhaps some braces to stiffen further any flimsy bits - whatever it takes the energy you spend will be handsomely rewarded with killer midbass. You can use multiple layers of soundproofing material around the speakers and behind them, soundproof all the door cards to stop vibrations. Every detail will pay dividends.

2) We soundproof other bits of the car to stop those bits vibrating with the sound waves inside the car. This improves the sound because acoustic energy is no longer having to move bits of vehicle instead of air. This work is very important but it is where the most mistakes are normally made by beginners. If you have an OE steel wheel arch panel made from thick steel - heavily curved and with ribs moulded into it, you don't need to pile on a stack of soundproofing. Similarly if you have a flimsy panel under your spare wheel you must go to town on it because it will be a much worse source of vibration than that wheelarch - just because you can't get to it as easily doesn't mean don't do it!

3) Sound travels through air slower than plastic or metal, so it is important to deaden any panel that can receive direct vibration from the speaker cone or frame. If you've got door speakers in your car go outside to your car now and turn up your audio system. Put your hand on your door card. Free vibro massage? Thought so. This means your speaker is working very hard to move your door card and you are probably listening to your window switch or door handle rather than your speaker. Unsurprisingly this kills the sonic imaging and makes for a very "all around you" sound rather than a focussed and musical performance. If you soundproof those doorcards really well (say 3 layers) you will kill most of that vibration and your speakers (whatever make they are - even the standard ones) will sound much, much better. To completely kill the vibration you've got to apply so much deadening the card won't fit the car anymore and will weigh a ton, so I like to use glass fibre resin and cloth followed by 2 layers of really good soundproofing.

4) We are soundproofing also to lower the ambient noise in the car which unsurprisingly makes a given volume of audio sound effectively louder. This in turn means you need to use less power and again that brings dividends in dynamic quality. Additionally it sounds less like a music concert played inside a vacuum cleaner (!)

You can spend ages tracking every source of vibration and movement and this is what you need to do to compete at the highest level of sound off competition, but for most people a careful prioritisation of the most rattly areas of the car will create enough of an improvement to be satisfied.

A word about materials. We sell some material called "SoundX" which is a very sticky, inexpensive soundproofing. It is not quite as good as Dynamat but is less than half the price! The other great thing about it is that it sticks amazingly well and this is a bonus if you need to do your headlining; some of the cheaper brands don't normally stick well enough upside down and on a hot day come unstuck with the consequences being a saggy headlining. Not nice.

Bass Boxes

The science of box design is complex and there is a great deal of information available in many books. It isn't my intention therefore to write a complete manual - rather I aim to fill you in on some basics and the "tricks of the trade" as it were.

The manufacturers of the better woofers will generally give you the enclosure size that they consider optimal. You would think, therefore, that there is no rocket science involved at all and you'd be right. However, it should occur to you that the speaker box will not just "stop working" if you build it differently from how they suggest - it is back to that old chestnut again called "subjective" sound quality.

What the manufacturer thinks is the best size of box might result in a sound that you don't like. You can, and should if you're interested, use a bass box design program to fiddle with enclosure sizes but you won't get a feel for how any changes SOUND unless you do them and listen to them. I will, therefore, give you a brief rundown on my opinions based on listening to hundreds of bass boxes over the years. I think I might have to do a bit of theory as well and it is tough to stop it getting a bit "heavy"!

What does a bass box do?

Why do you need a bass box? Well, you don't, to be precise. Since we are trying to stop the frontwave and backwave from the speaker colliding we can use a baffle board to achieve this. This is known as "Infinite Baffle" or "Free Air". When a speaker is in a bass box, the air pressure inside the box acts as "acoustic suspension" and helps support the speaker's cone. In a free air application this can't happen so you must ensure you buy a speaker rated for free air usage otherwise you could easily overdrive it and cause damage. It is technically possible to build a bass enclosure so big that it is classed as "Infinite Baffle" - when the box reaches such a large size that the speakers' own mechanical suspension becomes a stronger resistance than the actual air pressure in the box. Generally a free air rated sub will have more mechanical suspension than one recommended for use with assistance from a bass box. I would advise that you use free air only when you can't use a bass box, because although they can be very good, you will suffer from poorer power handling. I did one recently, for example, with a pair of Kenwood 10's in a Saab convertible. A baffle board was made across the bulkhead behind the rear seats and the seat backrest had its mountings moved forward by an inch or so to allow a small air gap to appear at the top behind the headrests. The very action of firing the free air woofers into the solid metal seatback just a couple of inches away is a similar effect to "corner loading" a subwoofer in a room and resulted in bass reinforcement and an increase in power handling. I also made damn sure I over powered the subwoofers hugely to make sure I had enough energy to control the cones and stop them wobbling around on the low notes. Remember it is distortion that kills speakers. Not power - unless you go really daft. The result was that the backwave was completely contained in the huge boot and we had some big bass from that sucker. I simply had no room for a box and we used the car to our best advantage. That's what it is all about.

I wouldn't recommend you kit out a normal hatchback with a free air sub because the parcel shelf supports will collapse and squeak and the shelf will come forwards (subwoofer still attached) and cleanly slice off your head in an accident (which must be considered inconvenient). Also, because the rear seats are flimsy and the backwave will penetrate them like it does when you've got a bass box in there, it will have some cancellation, and therefore sound less good than a bass box.

Sealed or Ported?

In a nutshell a ported box is more efficient than a sealed box, and will provide better bass extension at high volume. It will be approximately 3db louder (quite a lot, since db is a logarithmic scale) than an equivalent sealed box. On the downside, below the optimum tuned frequency the output will be much less than a sealed box (a sealed box approximates to 12db/octave rolloff and a ported one approximates to 24db/octave - explanation in the crossover section!) In addition, the sound from a ported box can be very "one note bass" if you are not very careful, so as a beginner I would start with a sealed enclosure. It is also a lot easier to make a sealed box in to a ported one than the other way around!

Sealed enclosures

We have discussed free air installation - what happens when you install the woofer in a box is that, obviously, you completely remove any chance of cancellation caused by the front wave and back wave colliding. Which is very good.

It is common sense that installing the woofer in a small box will result in a smaller amount of air for the speaker to compress during operation. This provides more "acoustic suspension" than in a larger box and hence the result that your woofer will tend to have a higher power handling in a smaller enclosure. Thus you now understand why I fall about laughing when people crow on about their "800 Watt" woofer. In what box? With how much distortion? At what frequency? And so on and so on.

Box size

Everything has a frequency at which it "tunes" - witness the tuning fork to test hearing in the doc's surgery, or the frequency at which your rear number plate turns into a speaker with bass from a subwoofer inside the car! A raw subwoofer has a resonant frequency too, which is denoted by the "Fs" figure in the "Thiele-Small" Parameters supplied with it. (A list of technical information about the driver intended to provide the speaker builder with a full map of the characteristics of that woofer.)

Big boxes make low bass. The problem is, you see, that the power handling drops as the woofer box gets bigger as a result of acoustic suspension being less effective. So, although our trailer sized big box will provide a lot of low bass for a given input, the amount of input we can actually give it will be severely limited.

At the other end of the spectrum, a tiny box with loads of power driven into it will be loud and will handle a lot of abuse. It will be a punchy sound because the resonant frequency of the smaller box will be higher. However, it just won't do low bass.

From this you can see that the Fs of the speaker system will vary with box size, i.e. the combined Fs of the woofer and the box is adjustable.

It bought to be apparent that we need a compromise; a bass box big enough to give the desired low bass yet small enough to retain power handling. The manufacturer of a speaker will generally give a recommended box size which they feel will give a good balance between <cynical old git mode on> good sound and warranty returns <cynical old git mode off> !!!!!!! No, seriously, they'll normally get a good balance, but you can "have a fiddle" - or what if you don't know the "correct" volume or anything about a speaker? Well, it is difficult to "guess" a box volume out of the air nowadays because there are just so many permutations of speaker. More recently we've seen a tremendous shift towards woofers specifically designed to work with very space saving enclosures, so it is a real tough call now to suggest the "old" standard values that we all were used to working with for a given woofer size. Nevertheless, to give you an idea, an average sized box for a 10 would be about 1cuft, a 12 would be about 1.5cuft and a 15 somewhere in the region of 2.5cuft. Think yourself lucky - not so long ago we'd need 4cuft for that 15! (So no grocery shopping, then!)

If you've got a manufacturers recommended size of say, 1.5cuft, and you have already made (or bought) an enclosure, it can be interesting to make the box slightly smaller by putting a solid object of known displacement inside the box to "use up" some room. You can then listen to the more punchy sound of the smaller enclosure without actually making it.

Conversely, if you feel that your box is a mite on the small side you can get some loft insulation and wad the inside of the box with it. Initially fill the box totally, but don't "pack" it full - a loose fill will do. This will trick the speaker into thinking you've got a bigger box than you really have and a warm, bassy sound will be the result. You can reduce the amount of fill to moderate the level of effect.

Ported boxes

A ported box is a much more complicated proposition, because it has a very important size/port length relationship. The basic principle is that at "port tuning" frequency, the acoustic output of the speaker goes up dramatically and the cone travel (excursion) of the speaker DROPS. This, in English, means that at a very narrow frequency band you will actually be listening to the port, and that the bass box has massive power handling, big efficiency and lots and lots of output. Which is nice. The problem is that below this "miracle" frequency, the cone "unloads" and the power handling goes through the floor because the cone excursion goes mad. It is, therefore, very important that the tuning frequency be set at the correct level. There are many port length calculators available on the web to play with - one interesting thing is that when you insert your data into the equations you'll no doubt come across the fact that you can use a smaller diameter of port and get away with a much shorter length for a given frequency. You haven't stumbled across a new discovery that puts you up there with Einstein, however, because there is a problem. Once you get down much below a 3" pipe your "port noise" is going to become a real issue. Basically what happens is that you end up with a lot of air in a small tube and it makes intrusive noise. You can use an "Aero port" (where the ends of the port are shaped like a bell mouth) to minimise the noise and they do work, but don't skimp on port diameter and you'll not have a problem.

You can work out Vb by simply measuring the sides of the box (length x width x height) then dividing by the amount of cubic inches in a cubic foot (1728). Remember to use internal dimensions.

One other thing to remember is that the Vb (volume of box) figure you input ought to be the internal volume of the box minus the displacement of the port as if it was a cylinder closed in at both ends.

So that covers the easy bit. Now you've actually got to decide what frequency you want the box to tune at. We can choose a boomy frequency like, say 50Hz if we want lots of hair raising output, or maybe 70Hz if you want a really "slappy" bass. Choosing, say 35Hz will give very low rumbling bass, but the speaker has its limits, however, so remember that as you choose a frequency. If you go too low the output above Fb (the tuning frequency) will not be loud enough and if you go too high then you must ensure the amplifier has a crossover set up to limit output below Fb (known as a subsonic filter and recommended in every ported box application) Subsonic filters are found on many quality bass amps - they reduce the output of the amp below port tuning otherwise the cones slap around and can cause damage. Or if you set the gain to create tolerance there you end up with less output nominally than you would have had if you could have used a subsonic cut-off.

Bandpass boxes

A bandpass box is simply a bass box with another bass box in front of the subwoofer. There are many types of bandpass box, and their design merits a book of its own, so suffice to say that they act as an acoustical crossover in their own right. The bandpass box is very efficient and plays only a selected "BAND" of frequencies. I wouldn't recommend that you try and design one without software and a good working knowledge of what you're trying to achieve but the ones in the shops are generally very punchy, with tremendous extension. Their disadvantage is in the amount of power needed to make the given performance.

Choosing a tuning frequency - Transfer function

This sounds super complex but in reality is just the way that the car itself affects the bass. So far we have acknowledged that this is an issue, but we've not discussed how to actually make any measurements. This phenomenon is how sometimes you can have one guy with a £150 sub and amp that blows the doors off your £800 worth which just happens to tune right in a frequency "black hole" of your car!


Don't be. Bottom line is that you can buy an off the shelf bass box and whack it in you boot. It will work and there will be bass. We have to make subjective recommendations every day to customers and I am not going to lie and say I have performed transfer function analysis of every car on the market because I haven't. I have done enough of them to know approximately what will be going on, however, and I have listened to enough woofers and boxes in enough cars to advise people of the result.

When someone comes in shopping for a woofer and amp can you imagine what their response would be if I offered to acoustically measure their car first, at a cost of two hours labour (£60)?

Exactly. They'd probably think I was a nutter and leg it off to "MotorChain" or wherever to buy that "Destruct-A-Bass Pro" for £100.

Nevertheless, it is a fact that if you want to do it properly; if you want "the edge", then you've got to attend to the details. Frankly, this is an art that is dying, as I said before, and I'm afraid that most shops wouldn't know a transfer function from a wobbly plastic spoiler held on with blue-tack. Sigh.

So. If you won't pay to get a TFA (Transfer function analysis), then how do you do it yourself? Well, it can be done in a few ways, but an easy and cheap way is this.

Firstly you'll need a CD or MP3 track that has on it sine waves at 1Hz increments starting at about 10Hz and going up to 100HZ. Secondly, you will need a sound level meter, which can be bought from any electronics retailer. As ever you get what you pay for but even the cheap ones are okay nowadays.

This method is my preferred one because although not as accurate as a converted power (method) which derives its free field response from speaker voltages, it does take everything into account - i.e. standing waves, boundary effects etc. Also it is easy to do!

1) Drive into a large, empty field in a very quiet area (I know, you are going to look an total arse but it is worth it and you've only got to do this bit once!) Rig up the car so that your woofer box is on a 20 foot cable. Point it away from the car and secure the speaker to a pair of stepladders, so that the woofer is well off the ground. It must be firmly strapped to the ladders so it can't vibrate.

2) Position your sound meter the exact distance from the speaker as it will be in the car from the speaker to the drivers head. The meter should face the speaker cone head on, however, even if the woofer is to point backwards in the car.

3) Turn on the CD player and play the lowest note. Set the volume to the highest level you can without over driving the woofer. Remember this setting. The woofer will move a lot at these very low notes so be careful.

4) Record the SPL reading for each frequency in turn, all the way up to 100HZ or so.

5) If you've got time, repeat the exercise again to verify accuracy, averaging out any anomalies.

You have just done a DIY version of a free-field response test. Now you can put the woofer in the car, and with the exact same volume setting, repeat the test with the mic in the drivers head position. If you now subtract the free field response figures from the in car figures you will have the transfer function of the car itself.

Measuring the resonant frequency of a bass box

To measure this, simply wire a 50ohm, 20Watt resistor across the + and - terminals of your speaker and play the frequencies again in order like you did above, but measuring the voltage across the terminals as you go, rather than the SPL. The tuning frequency of a sealed box is derived from the combined interaction of the Fs of the speaker and the woofer box and the voltage peak you get during your sweep equates to your tuning frequency. On a ported box, you'll find that you get TWO voltage peaks, and in fact you might like to draw a graph. One peak is created by the speaker itself and the other by the port. The tuning of the ported box approximates to the middle of the DIP between the two peaks.

Using the information

The theory is a simple one. Let's say that your car provides 30db (and although this is huge, trust me it is absolutely realistic!) of bass boost at 50Hz. Let's also assume that at 65Hz the car starts to rattle and vibrate and a lot of energy is used up and so the car has -10db of boost at this frequency.

Now, depending on what you want to achieve you might do one of two things. You could set your bass box to Fb at 50Hz and the result would be a huge output at this "sweet spot" - because you've tuned the CAR's resonant frequency with the bass box's resonant frequency. You would, of course, have a "one note" bass, but boy, it would be LOUD !!!

The other option would be to set your bass system up to peak at the -10db point of the car, so it "evens out" the response. Not as loud, but you are "EQ" ing the car acoustically at the engineering phase and the result will sound more natural. Since less ELECTRICAL EQ will be needed later the phase disturbances will be less and the resultant sound will be technically superior. This is what a successful sound off competitor is doing even before he's touched a neon light or a jigsaw!

If you have a little drive up the motorway with your new sound meter and flick it on at 70mph, you will see that your woofer has got rather a lot of work to do just to overcome road noise. With a cheap meter you won't see which frequencies are being created by the car, but rest assured it is all bass. This is why a car set up to win sound quality (car static with engine off, remember), with a realistic bass response, maybe even a box design done to take advantage of the car's transfer function, will generally not be bassy enough once you're on the move. That is why it is important to decide what you want from your system before you begin.

Building a bass box.

The ideal bass box would be acoustically dead and perfectly sealed apart from any designed ports or channels. This means that those ridiculous things you see in "MotorChain" or wherever with the foghorn things in them and 2 12 inch cardboard speakers are not ideal since they are made from pieces of old rabbit hutch. Worse still they are held together entirely by staples and chewing gum. What we need is a well made "Marine Ply" or MDF box (ideally 18mm minimum) that is carefully glued and screwed together and then silicone sealed around its inside edges.

For big subs (powerful 12's and 15's) you might want to go up to 2 laminated layers of 15mm MDF or even 2 of 18mm but obviously your weight is going to be a possible factor. It does no harm whatsoever to soundproof the inside of the box too but remember to account for the volume of the soundproofing when you design the bassbox as it will make a fair difference.

A good way to test the seal on a double bass box is to press down on one sub. The other will rise and you should hold the sub down and carefully watch. If there are any leaks the other sub will gradually sink. Start again!

Remember when you are trimming a box don't screw the sub in over carpet since it is porous and will create leakage. Instead trim round the sub with it trial fitted on a couple of screws then remove and peel off the ring of carpet. Now use some non porous rubber sealant tape or silicone to properly seal the woofer in. The same applies to any "binding post" (terminal blocks) you might be using.

You can use other materials for the construction of a bass box - fibreglass is excellent - but you've got to make sure you use enough layers to make it really strong. This is important because if you don't the box will tend to add colouration to the sound and "ring" with the music. As a rule of thumb I like 5 layers of glassfibre as a minimum and it is helpful too to bond in some reinforcing wooden splints into the flatter areas of the box.

Setting up your system for reliable, high performance operation

Setting up the gain structure of a system is straightforward, but from the outset you need to learn that you alone are responsible for the well-being of your equipment. The wrong settings or misuse will lead to damage that will not be covered under warranty. For this reason you need to take a couple of hours out to learn a few basics and put them in to practice at setup time and during operation.

In an ideal world you would have an oscilloscope and a test disc to hand - but to be fair most amateur installers won't have either so this description should help you to get the most from whatever products you have.

What are we trying to do?

The purpose of gain controls on car audio components is to allow the matching of different products in an appropriate manner. Gain controls are NOT volume controls!!

If you glance at the specification for your head unit, you will most likely notice it mentions the "pre-out voltage" - something like 4v, 3v, or 5v. This refers to the kind of level the amps can expect to receive when you crank the volume knob up all the way. Basically we want our source unit (CD, iPod, Mp3) to be running as high a voltage as possible for most of our listening. This way, we get maximum voltage in to the amps, which means they have to "step up" the voltage to the speakers less which, in turn, gives us better signal to noise ratio and again, better dynamics. Additionally, we get more immunity from noise down the cables which has got to be a good thing.

Where to start, then?

Turn all amp and processor (EQ etc) levels down to minimum. Turn off loudness on the deck, turn off any built in EQ's or DSP's or "Distruct-a-bass" controls on the deck too.

Have to hand a few of your favourite discs, MD's or MP3's (or tapes? - Yikes). Ideally you want a range because they are all recorded at different levels - and we want to build some "headroom" into the system so that even if you put in a disc recorded at a low level you can still extract some volume out of it.

Now, turn up the head unit as far as you can before you begin to notice distortion in any of the speakers. You might be lucky and be able to wind the deck to maximum with no distortion at all, in which case great but it is most likely that you'll hear everything go pear shaped about a couple of notches below maximum. (less if you are running any speakers off the head unit). Try your selection of discs and decide on a maximum safe volume setting. In this case we'll pretend maximum safe setting is 33 out of 35 maximum.

We will build in a couple of clicks of "headroom" - so from now on when I tell you to turn your deck all the way to maximum I mean 31, not 35, okay (33 safe minus 2 headroom = 31).

Now, with the deck at maximum (31) and all the speakers except the fronts your amp is driving more power than the disconnected, start to turn up the amplifier's gain control.(If you have a processor in line between the deck and the amp you would start with gaining that and then do the amp)

It is unlikely that you will get much beyond half way up before the front speakers start to complain - they may make a "cracking" sound (normally indicates that speakers will take) or maybe the bass will just start to distort badly (either poorly fitted speakers or poor speakers or maybe bad crossover points) Or perhaps everything will just distort (you've found the maximum power point of your amplifier before it starts to toast your speakers with a festival of distortion!!). In any event you'll want to turn the gain down again until just before this happens.

You've now successfully set your fronts up. Do watch out if you are using a 4 channel amp with the fronts on as well as, say, the sub, because as you turn up the sub you will reduce the amount of oomph available for the fronts and hence you will have to drop the gain a bit on both front and sub once you've finished otherwise you will have distortion.

Also, if you are setting up an equaliser before your amplifier, I would recommend you put on say 4 to 6db of boost to all sliders so that you allow for adjustments later (if you max everything out with the sliders flat you will get distortion when you have to swing a slider into the positive)

Now you can repeat the process for the sub, and then the rears, but bear in mind that you will almost certainly want only a bit of rear fill if you are going for imaging so in this respect it is okay to treat the gain control like a volume knob! Also with the sub you might find (depending on how daft your woofer setup is!) that your internal organs get rearranged before you find the limit so again, feel free to undergain it if you wish!

Conclusion and WARNING!

Remember that the best sounding systems are the non-stressed systems - there is no point making your equipment work close to its failure point. If you are setting up your system you and you alone must take responsibility for your actions and understand that you can toast equipment by setting things up too heavily gained.

As I mentioned in "Source Units", the Kenwood head units have a "volume offset" control which is very useful for gaining a system. This allows you to literally "offset" the real volume level by up to 8 points. This means you can have the head unit turned up all the way but really the unit will be sending out up to 8 points less voltage - and no distortion!

The most common gaining "misconception" is that it is somehow the fault of the speakers that they "can't take it" when you turn the volume up beyond half way - when in fact we know it is most likely just poor gain structure!

Crossovers and advanced setup

A crossover is required in car audio (and most other audio) because, as we said earlier, there is no speaker that can play every required frequency. It is therefore necessary to use a subwoofer for the low notes, mids for the medium notes and tweeters for the high notes. It goes without saying that you must filter out the unwanted frequencies before the sound can be sent to the speaker. For example, you wouldn't want bass to go to a tweeter because this will result in the tweeter diaphragm from pulling itself to pieces - so as well as good sound when done right, bad crossover setup has the potential to break things.

There are essentially 2 types of crossover (there is acoustical crossover too but we're not going in to that!) and these are passive and active.

Passive Crossovers

These either single inductors or capacitors that go in series in the speaker line or properly designed items with multiple components like you'd get in a front speaker kit. In any event they are called "passive" because they are not powered by anything other than the amplifier and they always act after the amp's power stage. Passive crossovers are not used as much nowadays in car audio except in the front speaker kits we discussed earlier. The reason they are used in that application is that there can be many other components in the crossover that help with impedence correction, notch smoothing and other complex things. Don't assume that those passive's are bin fodder then, is the message, because often a carefully designed passive setup that has been done in the lab is better than any old active deck you can fit.

Active Crossovers

Literally this means a crossover that is engineered into the signal processing somewhere before the speaker outputs of the amp. It might be in the head unit, an in line processor or often on the amp itself. The major benefit of an active crossover is its adjustability. Do remember that if your amplifier has frequency selection markings on it next to a potentiometer you must be aware that there is no guarantee the screen printing will perfectly correspond to the actual knob! Thus you must actually use your ears to confirm that sensible things are happening as you are tuning. (you can use an RTA line in directly but you probably won't have an RTA) (RTA is "Real Time Analyser" and it is a nice bit of kit we use in the workshop for setting up)

Crossover points

Crossovers are a subject that literally could fill a book, so there is no way I can do it proper justice here. I will try to give an overview, therefore.

I would suggest firstly that you begin by setting your equipment to give 100Hz low pass in to your subwoofer. Set up for 100Hz high pass for the front speakers. If you have an active setup on the fronts then you must speak to your supplier about where to set the mid/tweeter point. If you can't get any information I would start at 5000Hz. Be wary of coming down lower than this because although there can be imaging and staging benefits you can also cook the tweeters if you are not careful. 4500Hz would be a minimum for most 1" domes and if you really like to cane your system you could go for 6500Hz to really protect them.

For the rear speakers, if you use 6x9's, you might want to try them full range if they are loud enough. If you want more volume and are experiencing distortion and so have had to curb your gain setting you could try a 50Hz or even 100Hz high pass. This will allow you to run more gain in any event.


With the front speakers, again, it is possible to experiment with different points and gain adjustments. You might, for example, with a 6.5" front kit, want to bring down the high-pass point to 80Hz - you'll get a lovely rich bass (as long as it is fantastically fitted) but will drop your power handling a bit. Conversely if you want to make your ears bleed you could set up for a 150Hz front high-pass and crank the gain to the fronts a bit.

If you are going for pure sound quality I like an 80Hz high-pass on the fronts (or lower if they will take it but this is rare and generally restricted to custom floor pods or kick pod installations) and an 80Hz low-pass on the sub.

On rear fill I like to come in again at 80Hz and fade out at around 1Khz (but this will take some experimentation and changes from car to car and system to system)

I've got no amps other than a sub amp and my system sounds pants!! Help!

Simply turn down the bass control on your head unit all the way and whack up the gain on your subwoofer. You also want to run the sub to be crossed over as high as possible without unloading the cone at high volumes (a sort of banging and crashing, uncontrolled flabby sound caused by too high a frequency being played too loud by the sub!). Try 120Hz. You'll get a nice sound doing this and in fact this is a good way to start the upgrade path.

Crossover slopes

This is complicated. Basically, the steeper the crossover slope the more quickly the sound will be attenuated as the frequency moves away from that crossover zone. A steeper slope cuts sound off faster, making punchier, in your face type of system. Also, this can help to make a subwoofer less obviously in the rear of the car, or allow you to run more aggressive crossover points on mids or tweeters.

As a rule of thumb, a single component like an inductor on a sub or a bass blocker on a mid will cross over at only 6db/octave. An ordinary amplifier will tend to have a 12db/octave slope in its active deck and some processors amps and head units have facility to switch to 18 or even 24db/octave slopes.

The higher the number the steeper the slope, obviously - you've got to listen to the system in order to decide which slope is best - in most cases you can't adjust it anyway but if you do get such a control I would advise starting at 12db/octave and try switching 18 or 24 firstly on the subwoofer to see if you get an improvement. Note that the actual crossover point itself may have to be moved a touch to compensate for the more aggressive attenuation you have switched in.

There are two types of phasing. The first is more common and certainly is not subjective; electrical phasing. This refers to the positive terminals of the amplifier actually being connected to the positive terminals of the speaker and similar care being taken to ensure the negatives are with the negatives. This will ensure that all the speakers move backwards and forwards simultaneously - or "in phase."

Simple, but what would happen if you got this wrong? Well, in a situation where the "backwave" off the rear of one speaker shares the same airspace as the "backwave" off the other speaker (ie 6x9 shelf speakers) what will happen is that the bass response will cancel itself out as one speaker will be moving forwards as the other is moving backwards. Easy enough to understand, then, and it is a good idea to get all the speakers in phase to begin with.

However, in the case of a car that does not "image" perfectly (ie 99%of cars), there are some tricks we can play with the electrical phasing. "Imaging" and "staging" are common terms in car audio and refer to how well the audio system sets up an imaginary "stage" in front of the listener and how realistic the instruments sound in terms of both their location on that imaginary stage and also the way in which the sonic ambience around them is both consistent and believable. Because you are not sat in the middle (here we go again!) and because you are most likely not equidistant from the left and right hand speakers, the arrival times of the sound waves will be slightly offset and so the net result of this will be that although you may well have the front speakers electrically in phase the resultant sound will be ACOUSTICALLY "out of phase"!

You can play with this easily on your home hifi. Just set up the 2 speakers in front of you, and stand in the middle. Play a nice open female vocal or something and listen to the sound image - she'll sound dead center between the speakers and almost like she is stood right in front of you. Cost of the hifi is unimportant for this test and every hifi will do it. Next, move to the right and slightly forwards as you listen carefully. Notice how she diffuses and it becomes difficult to pinpoint the singer's location. You have just put your speakers out of phase acoustically by moving! Move back to the middle and have 10 minutes listening carefully to that one track.

Now, without listening to anything else, and without talking to anyone go back to the car as quickly as possible... Have a listen to the same vocal in the car and you'll probably be gutted at how grating and wooden and "out of phase" and hard to pinpoint the singer sounds. This is a normal reaction having just listened to a home hifi in phase. (Unless your car is very well set up and tweaked - which it probably won't be otherwise you wouldn't be reading this!)

Turn off any rear speakers, turn off your sub and reset the balance control to the middle.

If you have coaxial front speakers then it is easy to phase your system because there is only really one condition - either out of phase electrically or in phase electrically! Simply swap round the terminals on one of your speakers (it can be drivers or passenger side) and listen again. I find it helpful to extend the cables into the cabin so I can do it in my hand as I listen. The bass response may get a bit worse (remember the front speakers don't share the same rear airspace so it won't totally kill the bass) or it may get better (!) but what you will notice is that the vocal will either be more focused and easier to locate or even worse. Choose the best option and reconnect everything. You can play with the phasing of both rear speakers relative to the fronts and of course the subwoofer - the idea is to get the most realistic and integrated sound where the woofer doesn't "lag behind" the music. You will have to listen carefully and you can't really phase properly for more than a few minutes at a time otherwise you'll get brain overload and it will sound awful! I recommend a tea break and another listen to the home hifi as a good way of "re-referencing" your ears.

Once you are happy with your fronts you can perhaps click in a little bit of balance to get the sound "centered" - beware because often this can kick out the phasing again so best to recheck it.

You can see that this job takes a while to get right - and on some cars you'll never get it anywhere near right. This all comes back down to having better system design in the first place with respect to speaker positioning and installation. It almost never comes down to actually having better speakers themselves!

If you have a component kit then it is even more time consuming. You guessed it - you have to unwire the tweeters and first phase up the mids separately and then bring in the tweets. You've also got to be aware that as well as the left/right phasing you have to consider relative phasing of the pair of tweeters compared to the mids; i.e. you have a fair few possible phasing combinations - and that is without even trying different tweeter locations and directions, which is certainly something you will want to consider.

If you have a 3 way front kit (i.e. 6"mids, 4"mids and tweeter) then it is a nightmare as you have dozens of phasing combinations!

On a happier note, you generally find the big audio gains from phasing pretty quickly and the finer improvements are much harder to quantify - so don't worry if you can't hear the difference, say, when you swap the tweeters.

Sub Failure - Mechanical or Electrical

Too much power (or sometimes too much power for the box design) causes the speaker to physically move too far in the suspension of the speaker, and creates a horrible banging noise. Meanwhile, as you are playing the coil with perfect AC voltage (no distortion) from the amp when the sub fails it fails MECHANICALLY. This means that something physical rips or breaks in the woofer, and when you strip the woofer you see a shiny perfect coil but lots of deformation of the bobbin where it has been hitting the xmax. This produces NO smoke.

When you get distortion coming from the amp caused by too much gain for the load impedence or badly set up subsonic then what happens is that the amp sends DC voltage in the music rather than pure AC. DC can kill a speaker in literally a few seconds, especially if you are sending a lot of it (ie it is loud and highly distorted!) The problem is that it is nowhere near as easy to hear a ton of distortion in a woofer that is being tortured by DC as it is to hear the banging you get with over-powering. This means that you fit your woofer, crank it up and close the boot - you get bass and everyone is happy except the sub, which, due to the crossover is masking the horrible noise that it is receiving. You CAN hear it if you listen carefully with all other speakers turned off but if you set up your system with all speakers playing it is very, very tricky - what happens is that the DC voltage begins to heat the coil, and the coil dissipates that heat. Soon though, the coil can no longer get rid of enough heat and the resin coil glue starts to melt (smoke) - at this point the sub is wrecked. This is known as ELECTRICAL failure of the woofer and it is never covered by warranty.

When you set up ANY woofer you are only a fraction of a twist of a knob away from wrecking it - the setup is THAT important. If you go from one box to another or one sub to another even you can alter the electo-mechanical relationship that affects the damping factor of the amp and actually cause the amp to edge into distortion (the sub presents a load to the amp, so the amp actually reacts to that) - that is why setup is so incredibly important and needs to be done in a controlled, careful and skillful way.

Even once you have the perfect setup it is still easy to have the gains at such a position that actually inserting a disc with a higher recording level can create distortion and if you're not listening carefully destroy the equipment.

Most professional installers therefore under-drive the equipment they supply by a mile, therefore, so that the 'headroom' in the system is enormous, allowing you to run full volume (where the head unit doesn't clip!) without damage.

Thompsons Ltd


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