The power rating of car stereo equipment, measured in Watts, is without doubt greatly misunderstood. The overall volume, and also to a large extent quality, depends on the amplifier/head unit power, but also equally on (a) the efficiency of the loudspeakers, and (b) the size of the car. The kind of music you are likely to listen to also plays an important role. What doesn't affect your volume directly is the power handling of your loudspeakers: more about that on the loudspeakers page. You could well say that it's not the power that counts, it's what you do with it. To put it simply, SPL equals POWER multiplied by EFFICIENCY divided by SIZE OF CAR

I recently took sound pressure level (SPL) readings in a small car with a stereo giving out 15 Watts RMS per channel. The average SPL at full blast was around 110dB, which is probably more than you'd experience at a rock concert without putting your head inside the bass-bins. Two reasons for this: (1) The car was fairly small, and (2) The loudspeakers were very efficient. (Those of you who don't think that 110dB is enough volume should take a trip to an airport and go and stand on the runway!) So, if you're one of those people (found in almost every neighborhood) that has a 1,000 Watt amplifier, and you have not deafened yourself before you've inched the volume control to half way, then you've a serious problem with wasting your power somewhere, and this is probably because you didn't look at the sensitivity rating of your speakers.

How much power do I need?

You need enough power to get the volume that you need, for the type of music you wish to listen, bearing in mind the size of your car and the efficiency of your loudspeakers.

Take another look at the formula given at the top of the page: SPL = POWER multiplied by EFFICIENCY divided by SIZE OF CAR

SPL stands for Sound Pressure Level, and is a way of measuring how loud a sound is. It is measured in dB, or decibels. For example: a noisy street might have an SPL of about 80dB. A quiet house may have 40dB SPL. Detonating 50 pounds of TNT ten feet away would have an SPL of 194dB. Generally, you want your stereo to provide as great an SPL as your budget permits, without going to stupid limits. It is always good to have extra volume in reserve, but not worth paying four times as much for your stereo.

EFFICIENCY is how difficult it is to make your speakers sound loud - most loudspeakers quote their efficiency in the technical specs. Sometimes it is quoted as sensitivity, sometimes efficiency, and the two terms are essentially interchangeable. The higher your sensitivity, the more SPL you can get for a given input power, and essentially that is what the sensitivity will indicate: usually how much SPL you will get with a 1 Watt input power. For a subwoofer more than 90dB sensitivity is good; for other speakers 92dB or more is acceptable, but for both these, the closer towards 100dB (and even higher) the better, as this gives you the extra SPL in reserve.

POWER is your amplifier output power (NOT your loudspeaker power handling), and the following applies. First, don't bother with a separate, additional amplifier unless you really need a LOT of deep bass. I have not found a good reason yet to use an amp to power loudspeakers other than subwoofers, provided you have got loudspeakers with reasonable efficiency. Your amplifier power rating must be equaled or exceeded by the power handling of your loudspeakers, otherwise they will blow. If your amplifier power rating is greater than that of your loudspeakers, expect them to go up in smoke - don't think that they must be cheap or not well made.

SIZE OF CAR is something I don't recommend you try and do anything about to affect your car stereo. But what you will notice is that you'll need less power in a two-seater sports car or pickup than you will in an MPV or SUV.

So how much power then? I'll give you a rough guideline. For an average-sized car, say a Ford Focus, with loudspeakers with 92dB efficiency, you will be looking at a head unit that gives out 20 Watts RMS per channel. For a subwoofer, with efficiency 90dB, you are looking at a 50 Watt RMS amplifier just for that. If you car is bigger, you will need more power all-round, or loudspeakers with better efficiency, but the figures I've just quoted will give you loud but smooth sound. If you want pounding bass then you're looking at getting a subwoofer with better efficiency or an amplifier with more power.

What's a Watt?

A Watt is a unit of power, and measures the amount of energy transferred per second (note the capital W because it's named after Mr Watt). What does "energy transferred" mean? In the case of your amplifier, it means the amount of electrical energy delivered to your loudspeaker. The more Watts on your amplifier, the more power it will deliver to the loudspeakers. With your loudspeakers, the "power handling" Wattage rating refers to how much electrical energy the loudspeaker can absorb before it blows (more correctly known as thermal power handling, which is explained on the loudspeakers page).

The problem is that there are different ways of using the term "Watts" to specify a piece of car stereo equipment: RMS, DIN, peak, max, PMPO. A lot of manufacturers like this flexibility because they can stretch the truth to make something seem a lot more powerful than it really is, because that will help them sell more products. BEWARE OF THIS. Before I give an example, let me explain the two most common terms used:

WATTS RMS - This stands for ROOT MEAN SQUARE. Very simply this is the AVERAGE power that is delivered to the loudspeakers. It is the average power, the RMS power, that your ear equates with the loudness of the sound. This is equivalent to the term DIN, which you might also see around.

WATTS PEAK - This is the peak power output, i.e. the maximum power that will occur for a fraction of a second during a loud piece of music. Most people think that the peak is twice the RMS, but that is not the case. The peak is not just twice the RMS, and you cannot guess the RMS power if you know the peak power. The two are related by something called the "crest ratio" which varies very widely according to what music you are listening to. Because the relationship between peak and RMS varies so greatly like this, manufacturers have the ability to interpret it how they like, based on an assumption about what music you will listen to; and this usually involves coming up with unreasonably large peak power figures. You CANNOT compare the peak power of amplifiers or loudspeakers between different manufacturers because of the license they have to interpret it different ways. In fact I'll go one further than that: peak power ratings in Watts are totally useless unless you are a manufacturer trying to rip people off.

So what does this mean in practice? In loudness terms all you have to know is the RMS value. RMS is a scientifically calculated term and is totally standard. It is the only term which relates directly to how loud the stereo sounds to the ear. Any of the peak or max terms can be confusing, and even if used correctly do not relate to how loud the music seems. The ear "hears" the RMS power, not the peak. Plus of course the business of the open-to-interpretation relationship between the RMS level and the peak level, depending on the music you may listen to.

An example; say you are going out to buy a 60 Watt light bulb. You go out and buy a 60 Watt light bulb - you know how bright 60 Watts is, no problem. If it said "60 Watts RMS, 85 Watts peak, 120 Watts max" you'd begin to think "well hang on, what's all this?". As it is, when you buy a light bulb, you buy it on a basis of RMS power and needn't be concerned with peaks or whatever. The same is true with car audio, except there's more money to be made from making your amplifier sound twice as powerful than there is from making your light bulb sound like it uses twice as much electricity.

Continuous RMS Power

You may also notice that the word "continuous" sometimes crops up in relation to RMS powers. Quite often it is possible for an amplifier to deliver an RMS power for a short period of time before it starts to overload. The same is true of loudspeakers: they can often handle very large RMS powers for short periods of time without damage. Manufacturers are aware of this behavior and often exaggerate their power ratings on a basis of this. However, this is a cheat. A stricter definition of RMS power is that it can be delivered continuously for thousands of hours without equipment being damaged or losing performance in any way, and this is what the word "continuous" refers to. If you're considering an RMS power, and it doesn't say "continuous RMS" then think carefully very carefully about whether that power could be exaggerated. Often an RMS power measured over a short period - e.g. a few seconds - can exceed the continuous RMS power by more than double. Federal legislation in the United States now requires that any RMS power used to describe a device be continuous, but this has not been completely adopted yet, and will doubtless be continually flouted by the manufacturers.

Amplifier Power Myths

So how much power can we get? How much do we need? Whether you want big volume or not, we need to take a brief trip back to high-school physics to see what amplifier power is all about.
The formula for calculating power from voltage and impedance is: P=V2/R.

We are therefore limited by the 12 Volt supply in our vehicles and 4 Ohm loudspeaker impedance to 25 Watts RMS of amplifier power - or, if you allow for the alternator running, a bit more than that (allowing for well-compressed music such as FM radio and a crest ratio of 1:2). This is typically as much as you'll expect to get from a good head unit. So how does a dedicated amplifier generate more power, given the limit of supply voltage and load resistance? You've seen amplifiers that deliver hundreds of Watts, right? These amplifiers all use a voltage inverter to transform the battery voltage to something higher in order to draw more power: a voltage inverter is simply the opposite of a mains transformer. Instead of plugging it into 220 Volts and getting say 12 Volts out, you plug it in to 12 Volts and get 220 Volts out. Any amplifier quoting much greater than 25 Watts RMS per channel into 4-Ohms must use some kind of voltage inverter. These devices are expensive, big, a hassle to fit, and the decision has to be made as to whether there are better ways of getting the extra power (like buying efficient loudspeakers).

How does SPL compare to how loud things sound?

I mentioned this briefly at the start of this page, but some further examples would be instructive. The easiest way to understand SPL and decibels is that adding 6dB to your SPL doubles your perceived loudness: reducing your SPL 6dB halves your perceived loudness. So if I decide my stereo needs to be twice as loud, I know that I need to increase my SPL by 6dB.

Note with the current in car SPL record you can't actually be in the car at the time - unless you want to be instantly, permanently deafened. Little point to this, methinks. For the average, good quality car stereo, you are really looking at a maximum SPL of between 100dB and 120dB, depending upon tastes. Higher SPLs will just be uncomfortable to listen to, and will also tend to cause vehicle body panels to vibrate enough that from outside, the car will be heard to "rasp", to use the polite word, in time to your music. Not very impressive, I can assure you.

How does SPL depend on power? If we increase our amplifier power, how does the SPL change?
The SPL that is given out by your car stereo depends logarithmically on the power you put in. What does that mean in practice? That if you replace the 5 Watt RMS stereo that came with your car with a 20 Watt RMS head unit, you will add 6dB to your SPL: equivalent to making your stereo twice as loud. To make it twice as loud again, you would need to get a 80 Watt amp, and new speakers that handle 80 Watts too. To make it twice as loud again, you'll need to get a 320 Watt amp, and bigger speakers still. Clearly we have a case of diminishing returns when it comes to increasing amplifier or head unit power: you're having to add hundreds of Watts to make things twice as loud, and will quickly need to add thousands of Watts to do so again.

This is where getting better loudspeaker efficiency comes to the rescue. The really simple way to double your output volume is to get loudspeakers with an extra 6dB of efficiency - a figure that is typically achievable by replacing the factory loudspeakers in your car. Even some quite expensive car audio loudspeakers have efficiencies as low as 84dB which could easily be replaced by a set with 90dB at the same price; equivalent to replacing your 100 Watt amplifier with a 400 Watt one, and giving you exactly the same sonic results.

So, do you want speakers with 93dB sensitivity driven off your head unit, or speakers with 84dB sensitivity driven from a 200 Watt amp, to get the same sound?

The Important Bits

While no part of the car stereo could be described as unimportant, the loudspeakers are without doubt the most important component in terms of quality of sound. They are generally the limiting factor in terms of both volume and frequency response, and so need careful selection and installation if we are to get the best sound quality.

Loudspeaker Efficiency

Sometimes known as sensitivity, loudspeaker efficiency is almost always overlooked by the average punter. It's not a difficult concept to grasp, it's just that people are misled from recognizing that the efficiency is just as important - if not more - than the power handling (in Watts). There is a staggering difference in efficiency between different loudspeakers that are available, and manufacturers realize that people usually buy speakers on a basis of "how many Watts?" instead of "does it use those Watts efficiently?". The result is that a lot of speakers with high power handling skimp on the efficiency because the manufacturers sell more units this way. Bear in mind that loudspeaker sensitivity is as important as your amplifier power, maybe more so in a vehicle where larger powers are difficult and expensive to obtain.

The efficiency of loudspeakers is measured in dB, or decibels, and loudspeakers on the market vary in efficiency between about 80dB and 100dB depending on the model. The problem is that for every 3dB you add to the efficiency of the loudspeaker is equivalent to doubling your amplifier power. Let me stress that again - adding 3dB to the loudspeaker sensitivity has the same effect as doubling your amplifier power. To give some examples:

Imagine you have a set of factory-fit loudspeakers in your car, and you replace them with a new set with a better sensitivity. Say the old speakers had 80dB sensitivity and the new ones had 100dB. With the new speakers in, the stereo will be more than eight times louder, equivalent to replacing your 20 Watt RMS head unit with a 2,000 Watt RMS monster amplifier - which would destroy your speakers anyway! Of course a really loud stereo doesn't necessarily sound good, but imagine being able to turn the bass up so the sound is full and punchy, above all of the engine and road noise.

Your friend Ricardo has a two channel 100 Watt RMS amplifier in his boot, feeding two 250 Watt RMS 6x9" speakers in the rear deck, each with 85dB efficiency [total SPL: 107dB] (he forgot that loudspeaker power handling doesn't affect the volume and bought high power speakers with low efficiency like the guy in the car stereo shop recommended to him). You, however, have a 25 Watt RMS head unit and have four 40 Watt RMS 6x9" loudspeakers, two front and two rear, with 93dB efficiency [total SPL: 113dB]. Your stereo will have an extra 6dB SPL, that's twice as loud, and since Ricardo has probably spent twice as much cash as you and gone to a lot of trouble fitting that extra amplifier, he will look one quarter as cool.

How do I calculate my SPL from my amplifier or head unit power and my loudspeaker sensitivity?

There is a simple way to calculate your SPL, given the power of your amplifier and the efficiency of your loudspeakers. The sensitivity or efficiency of your loudspeakers, quoted in dB, is how much sound they produce given 1 Watt RMS input power (sound volume being measured in dB). Starting with the sensitivity, and an input power of 1 Watt, add 3dB to your SPL each time you double your power, up to the most that your amplifier will deliver.

Find out the efficiency of your loudspeaker. Let's say this is 93dB. (Remember: this means 93dB SPL with 1 Watt input power.)

Find out the RMS power of your head unit or amp. Let's assume this is 25 Watts.

Work out how many times you have to double your 1 Watt power to reach the power of your head/amp; in this example, you have to double 1 Watt between four and five times to get 25 Watts - i.e. 1 Watt x 2 x 2 x 2 x 2 = 16; 1 Watt x 2 x 2 x 2 x 2 x 2 = 32. (In mathematics we would write 1 x 24 = 16 and 1 x 25 = 32.) The real answer is 4.6, although if you guessed 4.5 you wouldn't be too far off.

Multiply this number by 3 and add it to your efficiency - this is because you add 3dB for each time you double the power, up to the maximum power your amplifier/head can deliver. So if you double your power 4.6 times to get from 1 Watt to 25 Watts, in increasing the power delivered to the loudspeaker from 1 Watt to 25 Watts you will increase the SPL by 13.8 dB. Therefore, your maximum SPL will be 93dB + 13.8dB = 106.8dB

Add another 3dB each time you double the number of amplifier channels and loudspeakers to your setup. For example, if you have a four channel head unit driving identical loudspeakers, you would add 6dB, because you've doubled and then doubled again the number of channels and speakers. Total SPL therefore is 106.8dB + 6dB = 112.8dB

To summarize: a set of speakers have efficiency 93dB which means they each produce 93dB SPL with 1 Watt of input power. However, each one driven from a 25 Watt head unit will produce 106.8dB SPL, and four of them each driven at 25 Watts from the same head unit will give a total SPL of 112.8dB.

Another example:
Kicker Solobaric subwoofer - efficiency 88dB
Jensen XA2250 amplifier, RMS output power 125 Watts RMS into 4 Ohm load
Power is therefore doubled around 7 times from 1 Watt to reach 125 Watts
Difference in sound level will therefore be 7 x 3dB which is 21dB
Maximum SPL therefore 88dB + 21dB = 109dB
Compare this with the Ricardo example above: 109dB SPL from a Kicker Solobaric compared to 112.5dB from your setup with four 6x9's off your head unit!! So, having spent 500 quid on your subwoofer and amp you've still not got as much SPL from your subwoofer as the midrange and treble speakers give you running off your head unit! If only the people in the car stereo shop would have explained all this to you instead of running off with your money... (to be fair, you may have a better bass response owing to the superior frequency response characteristics of your sub, but nevertheless, it isn't going to match the rest of the stereo volume-wise). Now, going back to the power page - you can see why people run amplifiers of such huge sizes and still don't deafen themselves - it's because all that power is wasted with inefficient loudspeakers.

You will notice something about these calculations of how loud your speakers are going to be. The power handling (RMS Wattage) of the loudspeaker does not determine the output volume. At all. It merely has to be greater than the power delivered by the amp so that the speaker doesn't melt. It's like the fact you need a bigger fuse in your toaster than in your VCR. The bigger fuse does not make your toaster draw more power, it's just there to handle the larger amount of current required to brown your bread.

A further analogy: your head unit/amplifier is like a tractor that has to pull a very heavy trailer (your loudspeakers). You wouldn't expect the same tractor to pull a trailer full of bricks the same as it pulls an empty trailer, right? Well, the efficiency of your loudspeaker is like how many bricks your trailer contains. A very efficient loudspeaker is like a tractor pulling that empty trailer, and an inefficient loudspeaker is like the tractor is pulling the trailer full of bricks. So remember: 90dB efficiency or more is good; the closer towards 100dB (and even higher) the better, as this gives you the extra power in reserve.

Power Handling

The wattage rating of your loudspeaker refers to how much power they can take. When loudspeakers are working hard, the coil inside, that provides the electromagnetic thrust to move the cone, gets very hot. The power handling of the loudspeaker is more correctly referred to as thermal power handling, that is, the amount of power the loudspeaker can take before the coil gets so hot things begin to melt or burn and you cause permanent damage. The power rating is NOT the amount of power the loudspeaker can take before it distorts. What you will find is that with the bass control up, the loudspeaker will distort long before the specified power rating is reached. Celestion have a very neat disclaimer with respect to this on their website; they say that the amount of power your loudspeakers can take before they distort is dependent on the application. We all know that with the same power head unit you are likely to distort your loudspeakers with 2PAC long before you do so listening to Vivaldi. As a consequence, with rap/pop/drum'n'bass/jungle, you need to get loudspeakers that handle a lot more power than that dished out by your head unit. With rock music less so, and with classical least of all - you can probably get away with say 50 Watt speakers off a 25 Watt head unit.

Frequency Response

Frequency response is often the way by which most people judge a set of loudspeakers, and this is rightly so, for it is generally the frequency response of a loudspeaker system that governs the quality of the sound (as opposed to efficiency and power handling, which govern the quantity).
But what do these different frequencies relate to in reality? The frequency response will be given between two limits, say for example 100Hz to 17kHz (17,000Hz). The higher number represents the highest pitched note that the speakers can reproduce, and the lower number represents the lowest pitched frequency. But how high or low do we need to go?

Treble Response

There is a limit to how high-pitched a note the human being can hear. Generally this limit is quoted as 20kHz, but this varies from individual to individual. Young children are able to hear up to 20kHz, but a 40-year-old adult may have difficulty hearing 15kHz, and as the age increases the upper hearing limit will be reduced down to 10kHz or even lower. Taking 15kHz as a baseline figure for an average adult, we see that a loudspeaker system that responds to 22kHz is pointless (unless you want passing animals to hear the aliasing noise from your CD player). There are certainly no sounds in music that high anyway - bear in mind that FM radio is limited to 15kHz, and tape will struggle even lower than that. So as an upper figure in frequency response, 15kHz is certainly adequate, and 20kHz is plenty. Loudspeakers are often designed to very tight compromises, and so a system that responds well above 20kHz might mean you're paying for a lot of high-end response only your dog is ever going to hear. (Technical point in anticipation of somebody emailing me: there is of course the old argument about the non-linear behavior of certain equipment, and the ear itself, causing inaudible high frequencies to noticeably modulate sounds in the audible range. That is to say that stuff you can't hear has an effect on the stuff you can hear. This argument, if ever valid on old analogue equipment, is nonsensical with digital source material since any frequency component above half of your sampling rate is dominated by aliasing noise - we know this from the Nyquist Theorem.)

Bass Response

There is also a limit to how low a human can hear, but this is not called into question as much as the upper hearing limit. This is because it is extremely difficult to produce a system that responds that low at a reasonable size. The human ear can hear down to about 20Hz under the right conditions. The lowest note on a standard piano is 27.5Hz, and the lowest note on a bass guitar is 40Hz. An orchestral bass-drum has its lowest component at about 30Hz, whereas a dance bass drum is usually much higher at 50Hz (with most of its energy concentrated around 100Hz). There is a considerable difference here, and one that needs careful consideration given the expense of getting speakers with a deep bass-response.

Ideally you are looking for a system that responds down to 40Hz or lower, but a system that responds down to 60Hz is adequate. Any higher than this and things will start to sound tinny. There are some parcel-shelf speakers on the market that respond down to only 100Hz, and this is pitiful - avoid them.
These days there has been tremendous advances in the amount of bass delivered by loudspeakers, and in the in-car situation this is even more so with the advent of the subwoofer. A lot of subwoofers operate over a very restricted range of frequencies giving them a terrible lumpy sound, but this appeals to some people. If you want a nasty, lumpy, but nonetheless very very loud bass to annoy your neighbors with, be sure to get a subwoofer (I certainly have one but then I have to exact some revenge on the students that live next door, and leaving for work at 8am with the Fun Lovin Criminals is ideal for this). But unless you're that keen on huge amounts of bass, think about getting a pair of efficient 6x9's in the rear shelf instead - it may not kick passers by in the teeth but you'll achieve a much more professional sound inside the car. And isn't it inside the car that the sound should count?

Mounting Considerations

It is important to mount car speakers that suit the volume of air behind them. Some loudspeakers designed to sit in the parcel shelf and fire into the boot/trunk space require many cubic feet of cabinet space. Speakers designed to go in the dash only require cubic inches. If you try to use either in the wrong place, they will not sound good. If you understand Thiele-Small loudspeaker design you can work out exactly what speakers suit the location in your car the best (on a basis of this I selected a pair of loudspeakers to fit in my parcel shelf resulting in a system Q of 0.6, i.e. approaching the all-desirable Bessel-Thompson response). If you don't know the difference between a Butterworth maximally flat passband characteristic and a Chebychev equi-ripple response, it is not such a great problem - in fact the solution is easy. Obtain the fitting recommendations from the manufacturer. Most manufacturers recommend enclosure design for their loudspeakers. In the old days, one bought a pair of loudspeakers and then had the challenge of producing a good cabinet design by hand. Nowadays, with computer simulated design, loudspeakers are specifically designed to fit a certain application; that is, the manufacturers arrive at a loudspeaker design by first thinking "We need to sell loudspeakers that fit the doors of Lincoln Towncars or Chevrolet Malibu's" - and then coming up with the hardware. So find out just where the manufacturers intended their loudspeakers to go; and you are onto a winner.

Wiring And The Car Stereo

I've had a few questions about wiring, and it seems that this is misleading a subject as any to the car stereo installer; so this page is devoted to explaining wiring myths and how to work out what wiring is required. There are a number of different types of wiring you might need to install with your in car entertainment, including but not limited to:

• Speaker wiring
• Power wiring
• Signal/RCA wiring
• RF wiring
• Interconnects

Speaker Wiring: Impedance Matching And Connection Options

The important task with wiring speakers - especially when it comes to wiring more than one of them to one or more amplifiers - is to wire them in such a way that you achieve impedance matching. Before I cover how to do this, I need to explain a few simple ideas.

What is impedance?

First off - what exactly is impedance? Quite simple - resistance. It is measuring how much of a load is presented to the amplifier - it is not called resistance because impedance varies with frequency and resistance doesn't - that's the difference. The impedance quoted is almost always the lowest resistance that occurs at any frequency. But why quote the lowest resistance the loudspeaker is going to present? Something that is important to note here is that the LOWER the impedance of the loudspeaker, the GREATER the load presented to the amplifier. This is not quite how you'd expect things to work, but that's how it is. A four ohm loudspeaker presents more of a load to an amplifier than an eight ohm loudspeaker, for example. Therefore, the impedance indicates the maximum amount of work that the amplifier will have to do. This is not to say that you're going to be better off with a higher impedance because your amplifier does less work; this is where impedance matching comes in.

Impedance Matching

Amplifiers will quote amongst their specifications the minimum impedance they will drive, i.e. what is the lowest impedance loudspeaker you can connect before the amp starts to work a little too hard (and maybe starts smoking). If this equals the impedance of the loudspeaker which you have connected, then you have said to have achieved "impedance matching" - it's simply a case of matching the impedance of your loudspeakers to that which your amplifier states it will drive. If your loudspeaker has a greater impedance than the minimum that your amplifier is specified to drive, then you will not get as much power and volume as you can, but the amplifier will work away quite happily (without smoking).

There is more than one way to achieve impedance matching, however, and this comes down to the options you have when you wire up your loudspeakers.

How do I wire my loudspeakers?

Ok so this is where you get to test your math skills. The reason loudspeaker wiring is sometimes elusive is because it requires basic math that some people can't be bothered to understand. (All the people I went to school with who were too cool to pay attention in their math classes now have crappy car stereos because they can't do the math to work out the wiring.)

There are two ways of wiring your loudspeakers, and these can be mixed in any combination in order to achieve impedance matching. These are:

Parallel Connection and Series Connection

You might have noticed that some mathematics has appeared next to the wiring diagrams, which deserves some explanation. The formulas given enable you to work out your total impedance, ZT, which is presented to the amplifier. Given that these are the only two methods of connecting speakers up, you only have to learn the way that each one works then the math is simple. I'll start with the series connection (the lower of the two diagrams). If Z1, Z2, Z3 are the impedances of your loudspeakers numbers 1, 2, 3 etc. (there can be any number of speakers) then the total impedance is simply the sum of all the individual impedances.

Example: What is the impedance of four Kicker Solobaric subwoofers connected in series?
Answer: Each loudspeaker has impedance 4 Ohms; therefore the total impedance is 4 x 4 Ohms, i.e. 16 Ohms.

Now look at the upper diagram, i.e. parallel connection. This is where things get a little bit more complicated. Again, your total impedance is ZT, and the impedances of each loudspeaker you are going to connect are denoted by Z1, Z2, Z3 etc. then the total impedance is given by the equation in question. Again, you can connect any number of speakers like this. If the loudspeakers all have the same impedance then the math becomes a bit easier though. For two speakers in parallel, the total impedance will be half that of each loudspeaker. For four speakers in parallel, the total impedance will be one quarter of that of each loudspeaker.

Example: What is the impedance of four Kicker Solobaric subwoofers connected in parallel?
Answer: Each loudspeaker has impedance 4 Ohms; therefore with the four of them in parallel the total impedance will be a quarter of the impedance of one of them alone; i.e. 1 Ohm.

As I mentioned above, you can mix and match parallel and series connections to result in different impedances. For example, consider the connection below:

Mixed Connection

This scheme is a combination of series and parallel that is sometimes used with four loudspeakers. The reason I have printed this particular scheme is that it has a very useful consequence - as long as all your loudspeakers have the same impedance, your total impedance will be the same as with a single loudspeaker connected. In other words, you can connect four speakers up like this and they will have the same impedance as just one of them.

Tri-Mode Connection

Something that is often used to connect three loudspeakers off an amplifier with only two outputs is tri-mode connection. This is not something that works for all amplifiers, however, so you must check in your amplifier's instructions whether tri-mode is suitable for the model that you have. Tri-mode connection looks like this:

Tri-mode Connection

Dual Voice Coils (DVC's)

A lot of subwoofers come with dual voice coils, or DVC's, to increase the options that you have in terms of impedance matching. A loudspeaker with DVC's will appear to have four connections on it; two for each coil. The impedance quoted will be for each coil; so a speaker that is quoted as 4 ohms dual voice coil means that it has two coils, each at four ohms. It is therefore easiest to think of these as two four-ohm loudspeakers sharing a magnet and cone structure - two loudspeakers in one. You can connect one coil to each channel of your amplifier: i.e. one left and one right. Or, you can connect both coils to a single channel in either series or parallel. So for a parallel connection with a 4 ohm DVC speaker your overall impedance would be 2 ohms, and for a series connection, 8 ohms. This gives you quite a bit of flexibility with connection options, but the main advantage is that it allows you to get a single speaker with an overall 2 ohm impedance, something that is not easy to achieve with a single voice coil due to other constraints of loudspeaker design, number of windings required to get the correct magnetic flux, etc.

Table Of Wiring Combinations And Resulting Impedances

Now that I've got you to look at the math - and hopefully understand it - for those that still find all this a little difficult I've included below a table of typical loudspeaker impedances and the wiring combinations required to match different numbers of them to different amplifier impedances. Hopefully, you can use this table to plan exactly what speakers and wiring you require to match to the impedance of your amplifier:

Speaker Impedance

I've tried to stick to combinations of loudspeakers that result in useful impedances, to keep this table to a manageable size. The table can be used in a number of ways, depending on whether you want a certain number of loudspeakers and can choose the model to suit; whether you don't mind how many loudspeakers but want to get the impedance matched as best as possible; or whether you know what kind of speakers you want and how many and are deciding on an amp that best suits them. Once again, some examples would be instructive here:

Example One: You have a Jensen XA2150 amplifier that can drive a minimum of 4 Ohms in bridged mode. You decide that you want to use Kicker Solobaric subwoofers, each with impedance 4 Ohms. You want to know how many subs you would need to get the best impedance match, and hence maximum power out of your amplifier.

Answer: You have a number of options. The easiest is one sub, since the impedance already matches that of the amplifier. Or, you could use four subs in mixed configuration; this would also match your impedance correctly (and your power handling would be better). There is not a good way to connect two or three subs of this type onto this amplifier.

Example Two: You have a Jensen XA2150 amplifier that can drive a minimum of 2 Ohms on each of two channels. You would like to drive four speakers from this. What impedance speakers should you get and how should you connect them?

Answer: You could get four speakers with impedance four ohms, and drive two of them in parallel off each channel. You could also get four speakers with impedance eight ohms, and drive two of them in parallel off each channel but your impedance match would not be as good - 4 Ohms off each 2 Ohm channel - meaning you would only get 50% power. However, if you were hell-bent on getting four speakers and wanted some with 8 Ohm impedance this would be a way of doing it. REMEMBER, you can always exceed the stated amplifier impedance, you will just lose power. You must not present a loudspeaker impedance that is lower.

Example Three: You would like three subwoofers in your trunk. What amplifier impedance, subwoofer impedance and connection method would give best results?

Answer: You have a number of options. You could get an amplifier that drives 4 Ohms, and connect three 2 Ohm subs in series. This would mean a load of 6 Ohms on a 4 Ohm amp, which is ok but you'd lose a little power. You could use three 4 Ohm subs in series off the same amp but you'd lose a lot of power - too much really. If you got an amp that could drive down to 2 Ohms, you could drive three 8 Ohm subs in parallel and this is the ideal solution with the best impedance match. There are other solutions, however, and I recommend you explore them with the help of the table above. Just remember that the combined impedance of your loudspeakers must not be less than that your amplifier will say it can drive.

Speaker Wiring: Choice Of Cable And Current

It is commonplace these days to find a whole range of cables designed specifically for speakers and sold at extortionate prices, but how much do they really contribute to sound quality? There is an argument that the clarity of sound is helped by good quality cable, which is true to a certain extent. However this argument has become a little over-used in the attempt to sell cable at ridiculous prices. OFC, or oxygen-free copper loudspeaker cables are quite common; all this means is that the cables are manufactured in an environment without oxygen so that they will be less susceptible to corrosion. Surface corrosion is said to contribute to loss of sound quality; but only to a marginal extent - so it may be better to spend your hard-earned money elsewhere.

Cable thickness is important, but only so that it can handle the maximum amount of current delivered to the loudspeakers. You can work this out from the power of your amplifier output, using the following equation: I= sqroot (P÷R)

Where I is the maximum current your cable will need to carry: P is your RMS power, and R is the impedance of your loudspeakers. So therefore, if you have a 400 Watt RMS amp driving a loudspeaker of 4 Ohms impedance, the current delivered to the loudspeakers will be 10 amps. You will need to specify the amount of current your loudspeaker cable will handle to be larger than this figure; a safety factor of two would be a good thing here, so with the amp and speakers just described you would require 20 amp cable. Much more than this is pointless as it will be expensive and NOT contribute to your sound quality. Bear in mind a smaller amplifier power would not demand such a heavy cable, for example a 100 Watt RMS amplifier would only demand 5 amp cable, so again doubling this for safety, we would still only need 10 amp cable.

The number of strands of copper in your cables is a subject much talked about recently, with the consensus these days seeming to be the more strands the better. This may or may not be true: the argument for having more strands is that high frequencies in music tend to travel along the surface of the cable, rather than right through the middle of the copper. Therefore, the more strands for a given overall cable thickness, the more the surface area hence the less resistance at high frequencies. I'm not sure where this idea comes from, (probably somewhere in the marketing department of some cable company) but it might help to put it in perspective to consider your television antenna lead. Typically this carries frequencies thousands of times higher than the highest frequencies used in your speakers; yet antenna leads are almost all solid-core, i.e. single stranded. If it were true that the higher the frequency the more strands of cable required then TV and radio antenna leads would never work unless they had literally billions of strands! Clearly somebody is having a laugh with this one.
The rule for choosing loudspeaker cable is work out what current your speakers will use with the above equation and get cables that handle double that or a little more. Don't bother with OFC cables or mega-multi-stranded ones unless they are cheap - save your money and spend it on something that will improve your sound - such as better loudspeakers.

So I've got this car stereo. What do I do to make it sound better? What's the best thing to do to upgrade it?

There are no straightforward answers to these questions: making something sound better depends on what was there beforehand. However, from experience in recent years, a pattern has emerged of how to upgrade car stereo that can work for almost any car - almost. By writing a procedure on how to do this, the last thing I would want is to discourage you from coming up with your own creative way of doing things. If you are going to take a risk and try something different you have to live with what you end up with!

#1: Get good loudspeakers and run them off your existing head unit.
Most loudspeakers installed by car manufacturers are not very good (to put it mildly), and probably the single greatest thing you can do to improve your sound to start with is to buy a better set. Bear the following things in mind, though:

Get quality loudspeakers. Make sure they have a good efficiency. If you want me to recommend a single automobile loudspeaker manufacturer you cannot go wrong with I will: JBL.
If you have some speakers in the front already - the first step is to get your bass response sorted out. Get a pair of 6x9's in the parcel shelf. Most parcel shelves I have seen can be easily cut to support a pair of these. Remember what to look for: the highest efficiency you can get. Check manufacturer's data to make sure they are designed to work in free air and not in a closed box.
Once you have a big enough pair of loudspeakers in the back for reasonable bass, upgrade the front speakers for better efficiency and quality. If you have dash mounted speakers your hands may be tied with this (i.e. there may be size/fitting restrictions). If your speakers are mounted in the doors you are likely to have more options. Again look for decent efficiency.

#2: Upgrade your head unit
This really needs to be done as a priority if you do not already have a CD player. Even if you do, it is worth upgrading to a more up-to-date system if it can offer more power or control. Remember to look for the RMS power-rating and base your decisions on this. It is also essential that any head you buy offers an RCA/phono output in case you decide to drive any further equipment in your car; an RCA/phono input is also useful but not found so commonly. There doesn't seem to be a tremendous amount of difference in terms of sound quality between different head units, but I bought a cheap one once and found the radio reception was appalling. There are lots of choices, so have a look at them all, and take your time. I personally do not advocate the use of CD multichangers - I have so many CD's I find it easier to keep a stack of them in the car with me and pop them in and out of the head as required (rather than stop the car, open the trunk, take out ten CDs and put another ten in every few miles). Some head units have very impressive displays on them, but again, I am a minimalist when it comes to this kind of stuff - (a) because you soon get bored with the same flashing lights and (b) because unless your stereo sounds really good as well you're going to look a bit stupid with a Christmas tree on your dash.

#3: Get a subwoofer and amplifier
This is the one step that's really going to make your setup sound impressive. Take it easy though, and don't go for stupid sizes and powers - you want to maintain a balanced sound in the car. There are two options with the subwoofers themselves: in a box in the boot, or cut into the rear shelf. It is important to use the correct type of subwoofer for your application. My personal preference is to use free-air subs in the parcel shelf, because this means you can still stick your golf clubs or guitar case in the trunk - although you will need to go to the trouble of making a new parcel-shelf out of wood because your old one is almost certainly not strong enough for the job. Subwoofers in boxes will generally give you a tighter sound, although unless the boxes are very large they're most likely not as efficient as good free-air ones. You pays your money, you takes your choice.

I also have a curious preference to having two modestly-sized subwoofers instead of one big mofo. The reason for this is stereo. Most audiophiles are of the opinion that bass does not affect your perception of stereo, and I would argue with this. I have two subs in the rear shelf, and with a 40Hz tone, I can tell whether it is coming from the left or right speaker from a normal driving position. Yes, you can get away with one, but two for stereo is better.

Amplifier power. This depends on the efficiency of your subwoofers. Mine are 93dB efficient and I find that a 75W RMS per channel amplifier is plenty - you could probably get away with 50 Watts RMS. I saw a sub recently in a magazine that was 87dB efficient, so to get the same output level with this you'd need to be running a 400 Watts RMS amplifier. Remember the discussion about efficiency? Do you want to spend $400 on a 400 Watt RMS amplifier or $100 on a 50 Watt RMS amplifier to get exactly the same result? Right.

#4: Get separate tweeters and a crossover for the front
Assuming you didn't get a component set when you replaced your speakers to start with. This will take care of your stereo image. Place the tweeters where they can be seen by both the driver and passenger in the front of the car.

#5: Get a graphic equalizer and two-channel amplifier for the front speakers
This can easily deafen you if you make the mistake of going for stupid amplifier powers - keep that amplifier power to 50 Watts RMS or less. You will need an audio test CD and a sound level-meter to set the graphic equalizer up - this is not a job I would leave to trial and error.

Following these steps you'll end up with - providing you have bought the right gear and done a good job of installing it - a good quality car stereo setup. But don't expect this little lot to win you any competitions - if you're competing for either SQ or SPL then you have to work to a totally different set of ideas.

One last thing: it is worth trying to reduce the rest of the noise made by your car. This will make a great difference to how loud you need things to be to sound good. The first thing is always stick to a manufacturer's exhaust; don't be tempted to buy a cheap one, and don't be tempted to get a big fat one to try and get more power out of your engine. The power increase you will get is negligible compared to the huge amount of noise you end up with. If you get big fat wheels put on your car you will increase the road-noise and again, this will have a detrimental effect on the quality of sound you get inside the car. So keep the quiet tires on there. Investigate sound-proofing the inside - sound proofings are quite expensive (often about 10 quid a sheet) but if you do the whole of the inside of your car you can reduce the road and engine noise by up to 10 dB - that's a LOT; probably more than your fat exhaust would add.

The Wonders of the In-car Environment

On the Power page of this site I referred to the size of car making a difference on the amount of power required to achieve a certain loudness inside the car. Really this is a simplification of things. The fact is, the acoustic behavior inside a car is very unusual compared to what we would experience elsewhere in our lives when listening to music. In some respects it is similar to listening to music with headphones on - it just has a knack of sounding good with the right equipment. Let us not forget how our own automobile is such a precious and personal environment and how great it is to be able to install a quality car stereo. Hallelujah!

So what is so special about the acoustics inside an automobile? And how can we take advantage of this?

There are three fundamental acoustic differences of the innards of a car compared to the other environments in which we would normally listen to music - i.e. in a house or pub or at a concert - even in a supermarket. First and most important of these is the physical volume (i.e. how many liters of air) of the environment. In a car this is small compared to even the smallest room in your house - especially so in two-seater sports cars or pick up trucks. And since the amount of power you need to fill out the bass sound depends on the size of your listening environment, it follows that with a reasonable amount of power we can achieve a bass sound inside the car that we would struggle to ever get in our houses. To quantify this, the amount of power needed depends on the size of listening room as follows, assuming average efficiency of loudspeakers:

Size of Room
Power needed for 100dB
Basketball Arena
10,000 Watts
Living room (30'x20'x10')
100 Watts
Small Van
10 Watts
Automobile
2 Watts

The significance of this? With minimal amplification we can get enormously loud deep bass just because we are in such a small space. Just like how we seem to get enormous amounts of bass listening to music through headphones: they are working with a space of only a few cubic inches, if that. Therefore our personal stereo can run from tiny batteries but produce enormous bass. And that is why other people's personal stereos always sound so trebly and hissy when you're on the bus next to them - the headphones do not have the power to excite the bass frequencies in the larger air-space outside of the listener's ears.

The second main acoustic feature of the inside of a car is the fact that you are in a fixed position when you are listening. This sounds obvious, right? But think of the situation compared to listening to a hifi at home or a jukebox in a pub - there you will get a small region of good sound while you're directly in front of the speakers, but otherwise you are rarely facing both of them head-on and in the same position. In your home you have to position your loudspeakers as a compromise to getting good sound throughout the room; in the car that's a compromise you don't have to make. You are precisely in the right place to get good stereo each time, providing you design your loudspeaker setup correctly.
The third noticeable acoustic feature is the lack of reverberation in the car, and how this can make the sound much clearer. The walls of the car are generally partial absorbers of sound, and while this may serve to accentuate certain frequencies and color the sound, we can minimize this with good speaker placement and maybe even a graphic equalizer. The lack of reverb contributes to more of a clean, dry sound.

Of course there are disadvantages, acoustically, of getting good quality sound in a car. Because of the small size, any standing waves that are set up will be at higher frequencies and hence more noticeable than in very large spaces. And of course there is the problem of tire and engine noise, which even in today's quiet cars can mean that the volume has to be increased considerably to be able to hear the music with any clarity. There are ways around this of course - it is quite popular to buy soundproofing materials for cars that are used to line the inside of door panels, for example. These can be quite effective but not as good as getting a nice quiet car in the first place.

Incidentally, a word of advice about such sound absorbers - there is a very simple way to judge how good they are going to be at doing their job. It arises from a simple acoustic phenomena: that the amount of sound absorbed by a non-porous material is proportional to its weight. Simple as that. A 50lb slab of plastic absorbs the same amount of sound as a 50lb slab of concrete. So if you really want to cut the road noise out, get the heaviest sound absorbent you can lay your hands on. Something that's very good, cheap, and very heavy for its thickness is roofing felt. The problem with this is finding a good way to stick it in place, because badly affixed sound deadening rattles and vibrates like crazy and sounds terrible.