Everthing you need to know about car audio

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This is from the Polk web site, and is very true. There are some secrets, and some very helpful stuff in here. Please read this before asking what is better.

Listening To Buy, Or Little-Known Secrets Of The Sound Room

You could fall in love with a system in the sound room, only to want a divorce once that same system is installed in your car. This is because there are big differences between sound rooms and your car. Listening area, listening position, and something called "transfer function" can all make the sound in the sound room much different from the sound that ultimately ends up in your car.


Room vs Car

A sound room is different than your car. A sound room is just that (a room), while a car is… You get the idea. It's apples and oranges. A room has flat, hard walls set far apart. A car has any combination of soft curves and plush padding, hard vinyl and metal, all in an irregular space about the size of a refrigerator box. It's rare that you get to test a car audio system in an actual car, so you need to know the secret to turning these two completely dissimilar listening environments into more hospitable conditions.

Beware of Transfer Function

That's difficult because of a complicated physics equation called "transfer function." Transfer function is a measure of how the volume of an enclosure, such as a room or a car, effects the way a speaker sounds. A loudspeaker in your living room sounds different than a loudspeaker in your closet because the living room is a larger space, and thus puts less pressure, or "backward load," on the speaker. A loudspeaker in your closet sounds a lot like a speaker in your Miata, though, because they are both small, enclosed spaces.

You can get a good working demonstration of transfer function by listening to your current car audio system with the windows rolled up tight, and then with the windows rolled down. You'll hear that the bass is louder with the windows closed. That's transfer function at work for you. Remember that when you hear a speaker in a showroom, it will have less bass than it will have in your car.

Dead rooms and live cars

A "live" environment is one that is filled with noise. Your car audio system has to compete with your car's engine, other car's engines, sirens, road noise, and the sounds of the angry drivers you've cut off. None of that exists in the sound room, which is "dead" to extraneous sound (especially if it is an insulated, padded space). Professional car audio installers will often deaden a car's sound the same way room builders do when they build sound rooms. They apply padding, fill gaps in doors and behind dashes, tint windows, and even coat the inside of exposed metal with a dense, sound-dampening adhesive materials.

Listen different

So the best way to listen in a sound room is to try to duplicate the listening experience of your car. Keep in mind what kind of car you drive. Is it a big boxy metal car, or a snug soft two-seater? As a rule, different types of cars treat sound differently. Basic models, with lots of plastic and metal, tend to make highs louder, while more luxurious cars, outfitted with soft fabrics and more padding, will dampen the highs and make bass fuller. Keep the characteristics of your car in mind when you're standing in the sound room.

And maybe standing in the sound room is the wrong position in which to listen to car audio demos. Do you listen to your car speakers while standing in your car? Sit down in the sound room, with the speakers you're testing at dashboard level. Notice where the speakers will be placed in your car. Are you sitting off-axis, or directly in line with them? Pick an optimal pathlength by estimating how far from the speakers you will be sitting when you're in your car. Are you testing rear speakers? Don't stand in front of them, since that's not how you hear them in your car. Turn around and sit facing away from them. By keeping the differences between the room and your car-there will always be more bass response in the enclosed area of your car-in mind when you're testing out car audio, you'll be more prepared to make the right decision when it comes time to buy the best sounding system for your car.

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How To Avoid The 5 Biggest Mistakes of Buying Car Audio
When you walk into your local, friendly car audio sales emporium, the smiling pleased-ta-meetcha sales professional is going to lob a bunch of questions your way. You're first thought is: "Why should I share all this personal information with this stranger?" And that's a good reaction. But it's just a car stereo, man. The salesperson is there to help you pick the best system for your needs. They can take your end of the equation and add to it their own experience and knowledge of the products they sell, and help you come up with the perfect system for your budget and your car. So relax. Answer the questions. Now.


Question #1: How much do you want to spend? The smiling sales professional is not asking this question so he can decide how big a sucker you are, he's asking it so that he can help you budget your money in the most economical way. Your answer to this question helps determine where your money should be spent to build a system that's right for you. He'll probably suggest budgeting your money in the order of component importance: speakers, amplifier, and then head unit.

Question #2: What changes are you willing to make to your car? He's grinning when he asks this. Maybe he's salivating, too. You've got visions of a bunch of installers completely gutting your car just for the fun of it. You couldn't be more wrong. Answer this question, and establish boundaries for the extent of your installation. It will affect the type of system you buy, and the type of installation the expert suggests.

Question #3: How long are you going to keep your car? You agree to the most extensive and complicated installation of the most expensive system in the hemisphere. And when you get home you remember that you planned to sell your lemon at the end of the month. The answer to this question can help you avoid that. Question #4: What is the primary function of your car? Do you use your car to drive to and from work everyday? Is it a 10 minute jaunt, or an hour commute? Do you travel to distant destinations in your car? Does your car just sit in your driveway until you're forced to go to the grocery store? Answering this question helps an expert decide how much of a system you really need. If you simply drive from home to work, you probably don't need the same kind of extreme GPS navigation and DVD surround sound that you would want if you were traveling long distances on a regular basis. But then again…

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12V Glossary - How To Talk Like A 12 Volt Guy
'A' is for amperes, a measure of electric current. How much is enough? (We like to say that having too much amperage is like having too much money.) Your car's stock alternator probably provides enough amperage to power a basic car audio system. But if you want to add multiple high power amplifiers, you might require some additional current, and the installation of a new alternator or stiffening capacitors (see this link section with answers to your questions for more on alternators and stiffening capacitors). Ampere is commonly abbreviated as "amp" (not to be confused with amplifiers), but sometimes it's "I." Electricity is like that.


A crossover is an electronic circuit that directs which frequencies go to which speakers. For example, since subwoofers are designed to best reproduce only the lowest frequencies, a subwoofer crossover (a low-pass crossover) allows only the low frequencies to pass through to the subwoofer. Freed from the task of reproducing heavy bass, your other speakers will rejoice by performing better and playing louder. You need crossovers so that you don't send unnecessary signals to a speaker (which could damage it).

'dB' is for decibel, a measurement of power ratios and volume. All you really need to know is that to gain 3dB in volume (just enough to hear the difference), you must double your power. There. That's it.

'DC' is direct current, a type of circuit. In a DC circuit, the current always flows in one direction. In your car, you're dealing with a 12volt DC system (twelve volts of direct current). Hence the term, "12volt Guys." In a car, it's important to keep track of which wires are attached to the ground (or "negative") lead of the battery.

'Hz' is for hertz, a measure of frequency. One hertz is equal to "one cycle" per second. A cycle of sound is the duration between similar portions of a sound wave (between two peaks, for instance). Frequency can describe both electrical circuits and sound waves, and sometimes both. For example, if an electrical signal in a speaker circuit is going through one thousand cycles per second (1000Hz, or 1kHz), the speaker will resonate at 1kHz, producing a 1kHz sound wave. Got all that?

'Imaging' is a description of the illusion of being able to locate certain sounds as "coming from" certain places. If you have a system with good imaging, the sound should seem to come from different distinct instruments and voices, not from speakers. A singer would generally be in front of you (center stage), and the band would be arranged around them. See also "staging."

An Ohm is a measure of resistance and impedance that tells you how much a device (like a speaker) will resist the flow of current in a circuit. If the same exact signal is sent into two speakers, one rated at 4 ohms of impedance, the other at 8 ohms of impedance, twice as much current will flow through the 4 ohm speaker as the 8 ohm speaker. All things being equal, the 8 ohm speaker requires twice as much power to achieve the same volume level, since power is proportional to current. (See "dB.")

Can your impedance be too low? Yes it can. It all depends on how well your amplifier can handle the increase in current flow that comes with lower impedance. The more current, the hotter your amp will get. An overheating amp is trouble. A good amp will simply shut down when trying to generate too much current. A poor quality amp will burn. Makes sure your amp can handle the impedance of your speakers.

'Speaker Sensitivity,' measured in dB, is how loud a speaker plays with a given amount of power going into it. Conveniently, the usual measuring stick is 1 Watt at 1 meter. A higher sensitivity rating means that the speaker will play louder using the same power as a speaker with a lower rating. So, should you always buy the speaker with the higher sensitivity rating? Not necessarily, because you'll usually end up trading off some other aspect of system performance like bass response or power handling. Sometimes a lower sensitivity rating gives a speaker a better (flatter) frequency response. How you announce your intention to spend your paycheck building a car audio system is your sensitivity rating.

Trust your ears, not the specs Specs can tell you how a speaker will sound, but they can't tell you what a speaker sounds like. Trust your ears. And if you don't know what to listen for, trust someone with listening experience, like the professional in the showroom.

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'Staging,' like "imaging," is a description of your system's ability to "fool you" into thinking that everything (including bass) is right in front of you. Like on a stage (hence the term "staging"), the singer should (in general) be in the center, and the band should be located to the left and right. Good staging (and good imaging) are not easy to achieve in a car audio situation. One of the hardest aspects of staging is getting the illusion that the bass is coming from the front of the car, even though the woofers are in the back. You may have to experiment with speaker locations, directions, and crossover roll-off points. Cheat the bass by overlapping the frequencies played by your mids and subs so that your semi-directional mids actually "pull" the bass to the front. To do this, use a high-pass crossover to roll off your midbass drivers as low as you can (without getting distortion). Then set your sub's low-pass crossovers at a slightly higher frequency. (See More About Crossovers) This will mix the bass coming from the front and rear, giving the illusion that the bass comes from the front. Adding a center channel improves staging as well, but that's next week's lesson.

'THD' is total harmonic distortion, or how much a device distorts a signal. These figures are usually given as percentages. THD figures below approximately 0.1% are inaudible, but like bad karma distortion adds up.

'V' is for volts, the measure of "electric potential." Voltages don't do anything, they simple measure stuff. 'W' stands for watts, another measurement of electrical power. One watt is equal to one volt times one amp of energy per second. Don't be mislead by wattage specifications. All things being equal, a good, expensive 50W amplifier will outperform a cheap, marginal 75W amp. Here's why: In order to play even 3 decibels louder, an amp must double its power output. The difference between 50 and 75W output is so small, maybe a dB or so, that you probably won't even be able to tell. The human ear just doesn't pick that up. To actually double the apparent volume, you'd have to have a 10dB increase in level. Basically, you're better off with a more expensive, more efficient, better-built lower wattage amp than with that "200 Watt" amp you picked up for $39.95 at the flea market.

Power Up!
How much power do you really need? A lot. But all car audio components come with their own power handling specifications, and you should start there when determining how much power you'll need.


On amps, for instance, you'll see two types of power specs: Continuous, and Peak (or Max) power output. The continuous power output rating is determined using a constant test tone. The peak power output rating describes how much power the amp produces in short bursts. This is more comparable to the nature of music, which tends to go up and down a lot.

For a speaker rated at 100 watts peak, you should get a 100 watts/per channel amplifier to safely get the greatest amount of volume from that speaker. If all you know is the continuous power of a speaker, use "The 3/4 rule": divide the continuous rating by .75 to calculate the maximum amplifier size. (For example, a speaker with a 50 watt continuous rating can be safely used with an amplifier of 70 watts/channel [50÷.75 = 66.7, round up to 70 watts]).

For most systems, 30 to 50 Watts (per channel) should be fine for primary speakers. Apply more (two to three times more, or 100-150 Watts) to your subwoofers. If you're powering your tweeters independently, they can get away with less power (20 - 40 Watts). A caveat: Speakers can be harmed when you push an amp beyond its power capabilities. It'll "clip" the signal, which produces both mechanical and thermal stresses on a speaker's voice coil. The speaker's voice coil gets banged around, overheats, and ultimately breaks. But you're actually less likely to blow a speaker by using too much power than you are by using too little power. If you like to play it loud, get a bigger amplifier

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More about crossovers
You can severely damage your speakers if you don't pay attention to the way your amps and crossovers work together. For example, if you're forcing too much high volume bass out of a smaller midrange or tweeter speaker, you're going to force the speaker to its "excursion limit," or its limit of movement. The voice coil in the stressed out speaker bangs around, gets bent out of shape, and destroys your speaker (see the page on how a speaker works for more about speaker movement).

Not surprisingly, this is a common problem with 4" and smaller full-range and coaxial speakers. And equally unsurprising, there's an easy way to prevent it. Simply "roll off" speakers with an in-line capacitor-or "bass blocker"-to keep the lowest bass frequencies from getting to that driver. (You won't be "missing" anything, since you'll be filtering out frequencies the speaker can't reproduce anyway, and your subwoofer will easily pick up the slack.) Any good car stereo dealer can help you choose a capacitor value that's right for your speakers. They probably have them in stock, too.

There are two types of crossovers: active and passive.

A passive crossover appears in the circuit after your amplifiers, and divides the signal that then goes to your speakers. A passive crossover has no power, ground, or turn-on leads and are rather inexpensive. But, they tend to be inefficient and can even add some distortion.

An active, or electronic, crossover does its job pre-amp (taking the signal directly from your head unit before it gets to the amplifier) and needs an external power source. Active crossovers give you control over which frequencies you want to use as the crossover points for bass and treble. Some active crossovers allow you to customize the crossover slope as well as the crossover point. Because they filter frequencies before the signal is amplified, active crossovers ensure that the amp gives its full attention to the filtered signal, which is very efficient. Chose your crossover points and crossover slopes by consulting the frequency response measurement on your speaker specs. The frequency response is the range of frequencies that the speaker can successfully reproduce. The frequency response of two separate speakers (woofer and midrange, for example) must overlap a little, or you will hear a "gap" in the music. The crossover point appears within this overlap. The crossover slope is a measurement of how abruptly the crossover cuts off the speaker's sound beyond that crossover point. If your speaker frequencies overlap just a little, use a steeper slope. The steeper the slope the narrower the range within which two speakers are producing the same signal, and the smoother the transition from one speaker to the next. The opposite is also generally true.

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Dealing with Noise
Hiss. Hum. Noise. Every system is going to have some noise. Unless you're going to enter a sound competition, you may not even have to worry about it. Usually, you won't hear that small hiss when you're driving. But if you're concerned, you can test your system. Make sure your amplifier is isolated from your car's chassis. Then, try this:


Check your RCA patch cords. Disconnect the cords from you amp and reinsert just one strand into the right and left input jacks of your amp. Now start your engine and turn on your system. If you get noise, try re-routing your cords, separating them from other connectors (like your power cable) by at least 18". Also, try a better quality of patch cord, consider trading in coaxial cable for "twisted pair" cables.
Check your antenna. Unplug it and see if the noise goes away. If it does, you need an antenna noise filter.
Check your head unit. If you stop hearing the noise when you jiggle or pull out your head unit while it's playing a tape, the noise may be radiating from an ungrounded component into your tape or CD system. Try shielding the back of your head unit with metal foil (available at retailers) or filtering the unit's power leads.
Check the electrical system. Is your battery filled? Have a mechanic check your alternator and give your car a tune up. A tune up will stop the noise at its source: the spark plugs, wires, distributor cap, and coil. Install resistor-type spark plugs with shielded carbon-core wires. Worse case, have a mechanic check the grounding of your ignition, charging, and injection systems. All of these things can radiate noise into your system.

Defeating Distortion

"Distortion" can destroy your speakers. Distortion is the audible expression of a speaker's limitations or an amplifier running out of clean power. Distortion happens when speakers are forced to play frequencies that they were not meant to play, which happens when speakers lack the proper crossovers (frequency blockers). If you don't have the crossovers, and you jam low frequencies through your little tweeters, you will hear distortion.

Since beauty, as it were, is in the ear of the beholder, there's lots of argument over what distortion actually sounds like. But it's safe to say that when the stuff you're listening to gets "muddy," when it starts to lose the definition of individual elements, when it all starts to blend together into a loud jumble… That, my friend, is distortion.

If you start hearing distortion, turn it down! Distortion is bad. Power, on the other hand, is good.

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Drive To Survive. How Not To Wax Your Car Speakers

Though many 12-volt fans might fantasize about it, you wouldn't like living in your car: It's a rough environment. Cars are full of vibration, countless road-surface impacts and temperature swings roughly equivalent to Mars. Unfortunately, even though autosound speakers are designed with this harsh climate in mind, many suffer shorter life spans than those disappearing Martian landers. Rarely, however, are the elements to blame for "blown" speakers.



Unlike a set of home speakers, in which the engineering is packaged in its own box, a car system contains many variables, including the enclosure, cross-overs, and power source. These elements are your responsibility, so avoiding major missteps in design and use can wildly extend the life of your car speakers.

The three most common killers of speakers are excessive power, way-ward frequencies and an overstressed power supply.

Much like your car itself, car speakers tend to perform best when properly driven. The likeliest culprit in speaker-cide is too much power. Overdriving a speaker means sending it more signal than it can physically produce. If force-fed via a ham-fisted volume control, any speaker will give its life trying to produce sound levels beyond its capabilities.

Simply because of the size of its components-which are less able to dissipate heat and offer less "suspension" to handle sudden power peaks-tweeters are usually the first to go. Also, unlike other drivers (which might produce a rapping or slapping sound if overdriven), tweeters may not give an audible sign of distress before checking out. "Edgy" and "harsh" are usually their last reported descriptions, and then they blip off the radar.

Overdriven is not to be confused with "loud." A properly installed audio system can render you near deaf without harming the drivers at all. "Properly installed" means amplifier levels are calibrated so speakers are not overdriven at normal volume settings, and each driver in a component setup (via the crossovers) only receives its appropriate frequencies.

Though it applies to all speakers, tweeters are also normally the first casualty in a system where frequencies are misrouted (1-inch dome tweeters were never intended to handle 50Hz drum kicks). Sub-woofers can easily suffer the same fate at the other end of the audio spectrum. Pumping up the bass (especially with an equalizer), in search of frequencies outside the realm of your sub's driver/ enclosure/amp will have the woofer's cone flopping back and forth worse than a cornered politician.

The search for subsonic bliss can also shorten your speakers' life span. Looking for bass beyond your woofer's setup design can put exponential loads on an amplifier, causing it to choke, or "clip," sending potentially harmful signals to your speakers. Amplifiers also tend to clip during periods of high demand due to an inadequate power supply (which includes everything from the car's alternator to your amp's internal power source).

If your car is not properly wired, when asked for some serious grunt, entire system voltage can drop, greatly lowering an amplifier's output capability. An amp starved for voltage can give up the ghost early, clip and take speakers down with it. To avoid all these pitfalls, proper setup is key. If you are popping speakers like aspirin, common solutions are a lower amplifier level (so that normal volume settings are not lethal), a re-tuned crossover frequency (giving the speaker a narrower range of sound to reproduce), or a steeper crossover slope (which rolls off more difficult frequencies more rapidly). Properly tuning your subs' crossovers and enclosures to provide sufficient bass for your musical tastes can also mean far less stress on your amps, extending the life of all your drivers. Find the balance between levels, crossovers and a stable power supply, and your speakers will live as long and loud as you like.

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Feed Your Tweeters Right (how to avoid blowing up your tweets)
In another "Advice" article 12 volter Paul Seredynski talked about some causes of automotive loudspeaker failure. Now we'll look specifically at tweeters-why they fail, and how keep them healthy longer.


Tweeters reproduce the very highest frequencies (treble). High frequencies require that the speaker move back and forth very fast, up to 20 thousand times per second! To move that fast, the moving parts (voice coil wire, former and cone) must be lightweight. Music is delivered to the tweeter in the form of electrical current. As current flows through the tiny voice coil wire in the tweeter, the coil gets hot and may burn out if fed too much current. A crossover network filters out the lower frequencies so the tweeter is only asked to reproduce frequencies and amounts of current that it can safely handle. Most tweeters should be crossed over at 3000Hz or higher. The lower the crossover point you choose, the greater the chance the tweeter will fail.

About Crossovers
In coaxial speakers the "crossover" is usually a single capacitor. A capacitor reduces the level of low frequencies at a rate of 6dB per octave. That means that if the capacitor's filter value is 3000Hz, the current will be reduced by 6 dB one octave below 3000Hz or 1500Hz and by 12dB two octaves below at 750Hz (Fig. 1). More sophisticated crossovers use a capacitor with an inductor (coil) to make a "second order crossover" (Fig. 2) that rolls off the lower frequencies at a rate of 12dB per octave. To use our previous example, with a 12dB per octave cross-over, the signal is down 12dB at 1500Hz and 24dB at 750Hz. You can see by looking at the graph in Fig. 3 that there is far less low frequency energy getting to the tweeter with a second order crossover than a first order. A tweeter with a second order crossover will play louder and last longer than one with a first order filter. At the lower, more dangerous frequencies, much less energy gets to the tweeter, and thus much less heat.

Rule Number One
NEVER EVER RUN A TWEETER WITHOUT A CROSSOVER NETWORK!

Electronic Crossovers
In more sophisticated installations the passive crossover network supplied with the tweeter or component system is replaced with an external, electronic crossover. In this case the low frequencies are filtered out before a dedicated tweeter amplifier (Fig. 4). While this method has multiple performance advantages, there are potential dangers.

The crossover point is usually adjustable on an electronic crossover and the possibility for setting it incorrectly goes up exponentially. If you set the crossover "by ear" at low levels, it'll sound great at crossover frequencies as low as 1000Hz, but as soon as you turn up the volume, Poof! Bye, bye tweeter.

The calibration settings on an electronic crossover are rough at best, just because the knob says it's at 5000Hz doesn't mean it is exactly. To be absolutely certain that you have chosen a safe and appropriate hi-pass filter setting, use a signal generator and Volt meter to measure the filter characteristics of your crossover. Many professional car stereo installers have equipment capable of confirming cross-over filter characteristics.

Rule Number Two
NEVER SET A HI-PASS TWEETER CROSSOVER LOWER THAN 3000Hz.

Equalizers
Equalizers (or EQs) are electronic devices that can adjust the frequency range to achieve "flat" response. When misused, they are notorious tweeter killers. Many people adjust the EQ to yield flat response as measured by a RTA meter. But meters don't "hear" like people, and "flat" response is often too bright. Over adjusting an EQ for measured flat response is a sure-fire way to fry your tweeters. If your system isn't "bright" enough for you, don't compensate by turning the treble control or EQ all the way up. Either move the tweeters so they are more in line with your ears, or add another set of tweeters.

Rule Number Three
BE CAREFUL WHEN USING EQUALIZERS OR TONE CONTROLS. NEVER TURN THEM UP TO, OR EVEN NEAR, THE MAXIMUM SETTING.

Loud Vs. Distortion
We understand if you want to play your car system REAL LOUD. We also understand that if you want to play it that loud you'd better have plenty of speakers and amplifier power. Asking your one set of itty-bitty tweeters to keep up with the eight 12" woofers in your trunk is unrealistic. As soon as you hear any distortion, it's a sign that some part(s) of your system is operating at or beyond its limits. Rule Number 4
WHEN THE SYSTEM DISTORTS-TURN IT DOWN! THEN BUY MORE STUFF!

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The Basics (Or, The Stuff You Gotta Have)Head Unit
The head unit is the tuner, cassette deck, or CD player that sends the signal to the rest of your car audio system. Some head units have amplifiers built in (in which case you must make sure your speakers are efficient enough to play loudly with the relatively small amount of power in most head units - See the Power Up! section.). On a budget? Buy speakers first. Better speakers can make your stock head unit sound really good. You can upgrade it later. And you will want to. One thing to keep in mind: make sure the head unit has pre-amp outputs when you buy it. You'll need them when you're ready to add amplifiers later.



Speakers
Ultimately, the head unit source sends its signal to the speakers. Your speakers determine how your whole system will sound. No equalizer, amplifier, or processor can compensate for poor (or poorly installed) speakers. Even if you're on a budget, you should plan on spending the bulk of your allotted expenses on your speakers. (And if you're really on a budget, plan on a head unit and a set of speakers now, and worry about amps and processors later.)

Subwoofers
Subwoofers are the speakers that deliver the lower frequencies of the audio spectrum. They need to be specially installed, usually in a box designed specifically for them. They demand more power to play at acceptable levels without distortion, which brings us to...

Amplifiers
An amplifier boosts your signal power, resulting in a cleaner sound and more volume. And because more power is a good thing, an amplifier might be the next thing on your list. Be careful, though, because if you are planning on adding several high power amplifiers you may need to upgrade your car's electrical system with upgraded capacitors, battery, and-lastly-alternator.

More about amplifiers
Amplifiers can really turn your system on. With more power you'll achieve a cleaner, more dynamic sound at higher volumes. But installing an amp yourself can be tricky. Be sure to plan your install carefully.

Never mount amps or other components directly to the metal of your car. (That's just asking for noise problems.) Instead, use screws with rubber isolators when you have to mount to metal, or mount the component to a non-conductive board and then mount the board to your car's body. And before you drill holes to mount anything, hook the component up and give it a test run in your chosen location. How smug will you feel after finding that noise problem can be fixed simply by moving your amp to a new location… before you've drilled?

Amps are sensitive to electrical and motor noise, and they can interfere with your radio reception. They should be mounted at least 3 feet away from your head unit.

You can mount an amp under a front seat. This is close to your head unit, so you'll be able to use shorter cables to both the head unit and the speakers, but larger amps won't work here.

Mount an amp on the passenger side firewall; you won't have to remove the seat, but again only a very small amp will fit.

Better yet, mount your amp in your trunk, where it will have plenty of room to breathe, which is important because…

Amps produce a lot of heat. You'll see cooling fins on an amp. They radiate that heat into the surrounding air to help cool the amp. For these fins to operate properly, they need a few inches of air space around them at all times. Also, try to keep them vertical. Amps should not be mounted with the fins facing downward (because heat will radiate back up into the amp). A tip: Just because something is metal doesn't mean it's a good ground. Ground your amp directly to areas of heavy chassis metal only, not to a piece of metal that's merely attached to the chassis. Use ground wire run-lengths of 18"-24" maximum.

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More Toys (Or, Important Stuff You Also Need)
These are good times. There is no end to the cool stuff you can add to your car audio system these days. From the biggest, baddest subwoofers, to computerized navigation systems (some that are even voice-controlled). Soon, you'll even be able to get your hands on complete DVD "car theater" systems. The options are endless. What's more, there is even some cool stuff that you positively need (as opposed to the cool stuff you just want), like this:



You'll need RCA cables to carry pre-amp (low line level) signal, usually from your head unit to your amp and processors (crossovers, equalizers, etc.). Get 'em well-built, flexible and shielded, with sturdy connector ends that will withstand the stresses of car audio connections. It pays to buy quality cables.

Amplified signal (especially when it's going to your subwoofer) is much stronger, and requires a more capable cable. There's a lot of current zipping out of your amps. That's why they make speaker cables. Be sure to use cable that is 16 to 8 AWG (gauge) for subs and coaxes. (The lower the number, the thicker the wire the less resistance; thicker is better.) Tweeters and mids can use thinner cables (16-12 AWG).

For battery connections, a power distribution block can make wiring multiple electronics a breeze. Run a 4 to 6 AWG power cable from your battery to a power distribution block and use its multiple outputs for each component. Some power distribution blocks even come with their own fuses to protect your components. You can also find battery terminals that have secondary connectors to let you easily hook up the extra power cables. Told you it was a good time. To easily integrate a head unit to your car's factory wiring, use a wire harness. It allows you to do all sorts of alterations without harming your car's factory wiring. You want to replace your stock head unit, but you don't want to have to leave your custom unit in the car when you sell it in a year? So you use a wiring harness that's designed for your car model, and you can remove your stock head unit and attach a new one without cutting the original wiring. The custom unit just plugs right in to the harness!