As the title suggest, this node should provide a basic outline of the installation of subwoofers in a car. For the sake of the reader, I have included an outline of power ratings in car stereo components and a brief analysis of the process of choosing components, in addition to the actual installation guide itself. This should make things very clear.

Index

Introduction to Power Ratings
Choosing Equipment
Installing
Sources

An Introduction to Power Ratings

RMS and peak power ratings on car stereo components are often very confusing and true implications difficult to discern. Contrary to popular belief, an amplifier rated at 2,000 watts RMS is not always producing 2,000 watts. The sound produced by a speaker is a function of the frequency and amplitude of the current passing through it. What this means is that if the amplifier were to produce 2,000 watts continuously, you would have no control over volume. RMS power is generally considered to be the upper limit of the power the amplifier can produce with music over a sustained period of time.

I generally recommend that a customer purchase an amplifier capable of producing RMS power that exceeds the RMS power rating of the speakers in use. Why, you ask? There are two reasons: sound quality and damage potential. The sound quality of most amplifiers degrades as they near maximum output. If, on large musical transients, they exceed their capability to produce a clean signal, they will produce a clipped signal. Clipped signals often sound atrocious. Clipping is also very unhealthy for both the amplifier and the speakers. At the flat peak and trough of a clipped signal, the amplifier is producing its absolute maximum output. This creates a lot of heat inside the amplifier (more than it is made to handle) and usually an equally unhealthy amount of heat in the speaker (likely more than it is made to handle as well). Clipping is second only to installation errors in destroying amplifiers and speakers.

For a very long time I used a 2,000 watt RMS rated amplifier to run two 200 watt rated 8” midbass drivers in my Thunderbird. None of the components were damaged because I was careful with the volume knob. Too much power into a speaker can cause mechanical damage: the brute force of the input can separate the voice coil and cone from the suspension. You can often hear overpowering as a “popping” noise or reduced sound quality at high volumes. If you notice that you lose enough sound quality at high volumes for the difference to be audible, turn down the volume; you are almost certainly damaging something.

In general, more power than is necessary is safer than too little power and will always make it easier to upgrade in the future. Remember: A woofer will wear out over time no matter what you do, but a good amplifier that is not abused by constant maximum use can last essentially forever. Isn’t it nice that the amplifiers are the more expensive component, then?

Choosing Equipment

Before choosing woofers, or even amplifiers, take into account the limitations of your vehicle. Two Resonant Engineering XXX 18” subwoofers are probably not going to reside comfortably in the trunk of a Jetta. If you have a large vehicle, say a 1988 Chevrolet Suburban, you will certainly have room for said woofers, but you should remember that together they are going to demand between 10 and 20 cubic feet of enclosure for optimal conditions. This may sound neat, but most of us enjoy having a modicum of cargo space.

Amplifiers need space as well. Many medium-to-high power amplifiers are quite large. I use a Planet Audio 2250D in my car. It is approximately 22” inches long and 12” wide. At around 2,000 watts RMS, this is a relatively average-sized, average-output amplifier for two 15” to 18” woofers. Needless to say, you will probably not be hiding one of them under the front seats of your Civic. On the other hand, you probably won’t need that much power unless you’re a bass fiend like me. If you do, you’re almost certainly going to need to build an amp rack, which is really just a slab of wood to bolt and amp to.

When choosing woofers, pay no attention to power ratings. They tell you very little about how much sound the woofers can generate. Read reviews in respected audio magazines, consult online sources, and listen to equipment in vehicles. Unlike in home audio stores, sound rooms in car audio stores are very deceptive. They are usually optimal conditions for the equipment. Cars are generally not. Try to hear the equipment in a vehicle. The quality of an installation can greatly affect sound quality, however, so reviews are generally your best bet.

Size of woofers can make a significant difference in the performance of your sound system. Larger woofers are more efficient. What this means is that if you have a 12” and a 15” woofer, which are otherwise identical, the 15” will be louder with the same amount of power. This is an effect of increased displacement of air. Although these are by no means all-inclusive, the various common sizes of woofers typically share certain aspects in reproducing sound. Enclosure design can modify or eliminate many of these qualities, however, so we’ll assume the examples are in normal sealed boxes.

8” woofers typically do not extend low enough to satisfy car audio enthusiasts. Many 8’s make excellent midbass or reinforcement woofers for systems with large diameter woofers. 10” drivers tend to be punchy and fast to respond; they are accurate and good for people who listen primarily to fast rock music. They tend to be insufficient for extended low notes, however. 12” drivers are a good compromise; they can effectively reproduce most low notes in popular rap and are responsive enough to accurately play fast rock music. They also offer more output than a 10”. 15” woofers extend into very low frequencies and offer a lot of output, but suffer at higher frequencies. 18” woofers go deeper and louder. 15” drivers benefit from midbass woofers to fill in for their weak high frequency response, and I consider separate midbass drivers a necessity with 18” and larger subwoofers. I use two 15” woofers in my car. Recently I added supplemental 8” woofers, though the 15’s produced very enjoyable sound without the reinforcement.

I will explain the dynamics of enclosures in a later write-up, but I often recommend 12” drivers (one or two) in a medium ported enclosure for general use in cars. Such a setup will usually offer good low-end extension, high output, and flexibility. Pickups usually require smaller drivers and significant custom work. Large sport-utility vehicles typically demand more substantial equipment, but their large cargo areas are very flexible.

Amplifiers are a little easier to choose than drivers. Your primary concerns are finding quality units that produce enough power to run your speakers at the desired volume. You will usually be pleased by choosing an amplifier with RMS rated output near that of your woofers, or higher. A more powerful amplifier, used correctly, will damage nothing and often be more reliable. More power means upgrade potential later. If space is an issue, be careful to measure any amplifier you are considering. If it is not, then err on the side of more power.

With equipment choices out of the way, we can actually proceed to the installation of these components, which you undoubtedly spent the majority of a paycheck (or more) on. Since audio components are very expensive, safety comes first. My primary qualm with the original write-up is author’s the disregard for basic safety practices. In a 12 volt system, you are unlikely to harm yourself with shocks. On the other hand, it is very, very easy to destroy your equipment. Take this in mind as you continue.

Preparing to Install

You will need a few things to install your equipment. I am assuming you already have subwoofers in an enclosure, an amplifier, a head unit (deck / radio / cd player), and something resembling a car. I recommend you purchase an amp kit, which usually contains virtually every piece of wire, and every connector, you’d need to do the installation. These parts are as follows:

  • Power cable
  • Ground cable
  • Fuse Holder and Fuses
  • RCA Cables
  • Speaker wire (12 gauge for most woofer installations)
  • Remote wire (Any single wire of 18 gauge or so is sufficient)
  • Crimp Connectors (A few butt, two or three ring, possibly some spades)
  • Self-tapping Screws
  • If you have a stock stereo, you’ll probably need line-level converters. Talk to a stereo shop about these. I won’t explain their use.

If you don’t get these out of an amp kit, consult a car audio shop to obtain them. They can advise you better than any text tutorial and are generally the best place to obtain these parts. They have to use them every day, so they usually have plenty.
The following tools are also necessary, unless otherwise noted:

  • Screwdriver (Preferably cordless, with many bits)
  • Crimpers (Usually can cut wire too, but are inferior to good cutters)
  • Wire Cutters (for 0/1 gauge I recommend Craftsman Handicuts. Like a hot knife through butter.)
  • Digital Multimeter (not always needed, but handy)
  • Good Lighting
  • For 0/1 gauge, you may want a jack to run cable under the car. The stuff is huge.
  • Many amps use hex bolts, so you may need appropriate wrenches.

Wire gauge is very important and the best choice for any application is dependent on two factors: current and distance. Most car audio books have full charts to determine what size wire to use. Many of them are very generous and benefit the companies that make wire more so than the consumer. If you are going to do many installs, or even one major one, I recommend obtaining one of these books (some listed below). For the basics, I will eschew current and distance calculations and give simple guidelines (by RMS power, if they only list peak power, halve it for this purpose):

  • 300 watts and under – 8 gauge
  • 300 - 1200 – 4 gauge
  • 1200 - 1800 – 2 gauge
  • 1800 and over - 0/1 gauge

If the run of wire is much over 20 feet, bump it up to the next step. If in doubt, go larger. Don’t use 10 gauge, it’s glorified speaker wire. Extra-large wires almost never hurt anyone (Except your wallet. 0/1 usually costs around $4 to $5 a foot. Try to befriend a dealer to get it down to a reasonable price. That stuff hurts people in other ways too. I advise never to fight anyone holding a few feet of it).

Installation

1) Disconnect the negative battery terminal on your car. This should always be the first step in any installation. Period. This eliminates any potential electrical flow in the vehicle and will prevent the sundry computers and devices in your car, in addition to your stereo, from being damaged by your tomfoolery. If you’re going to connect or disconnect anything from the car’s electrical system you should either do this or pull the fuse between the battery and that component. When in doubt, pull the terminal.

2)With the woofer box in the car, place the amplifier. For easy installations I like to affix amplifiers to the back of folding rear seats. That way they’re out of the way until you need to work on them, but when you do, you have access to the whole thing. Rear seats don’t usually vibrate much either, which can be a serious danger to an amplifier. Mounting an amp to a woofer box is often a bad idea; the walls flex with music and cause the amplifier to vibrate. These vibrations can damage solder connections and eventually cause the amplifier to self-destruct.

More dangerous than vibration, however, is heat. An amplifier in tight confines cannot effectively dissipate heat. Most amplifiers (Class A/B) are extremely inefficient and produce fantastic amounts of heat. This is why most amp chassis are generous heat sinks. If there isn’t enough air around the amp to absorb the excess heat, heat damage may occur. Placing amps that run hot underneath seats is a no-no and not even a particularly good way to jury-rig heated seats for the winter. Class D (so-called digital amplifiers) amps usually do not produce nearly as much heat and can be placed in tighter confines. If you put an amp in a tight spot, be sure to check it after playing it for a while to be sure it isn’t getting too hot.

You should also avoid mounting the amp directly to the chassis of the car. This can cause ground problems, since the car uses the body as a ground and your amp often uses its chassis for the same purpose. If you have trouble, mount a piece of wood to the car, and then affix the amp to that. Instant amp-rack. Once mounted, connect the speaker wires from the woofer box to the amplifier. You may need to use terminals for this, but most amps let you use normal stripped wire.

3) Run the power cable. This is the fun part. By “fun” I mean this is often the part where you lose small pieces of your fingers, discover vile pockets of debris, and learn that there is a surprisingly large amount of crap hidden underneath car carpeting. Affix a ring terminal to the end of the cable at the battery. Connect this to the positive terminal on the battery. Now, within a foot of that connection, find a place to put your fuse holder. Make sure the hood (bonnet) won’t hit it, but be sure you can get into it easily. You want to be able to replace it without trouble. Fuses are vital. If a wire breaks and comes in contact with metal, it will blow the fuse. If the fuse is not there, the wire will either catch on fire or melt due to the massive current flowing through it when it shorts, or the battery will boil and explode. These are bad things. Always use fuses.

Connect the wire from the battery to the fuse holder. If you have split loom, you should cover wire in the engine compartment with it. If not, don’t worry about it. Make sure the fuse is not in the holder. Connect the end of the rest of the wire to the other side of the holder. Now look for a good place to pass the wire through the firewall and into the cab of the vehicle. Look for grommets that have other wires passing through them or rubber plugs that occupy otherwise unused holes. Your goal here is to find a hole that leads on to either the passenger or driver side foot wells. If you can’t see anything from the engine compartment, pull up the carpet inside the car and check from that side. You’ll eventually find something.

Before poking the wire through, you’ll need to pull up the carpet and remove some interior trim pieces. Most of the plastic interior parts of a car are held on with screws. You’ll want to pull off the doorsills (the plastic at the bottom of the where the doors meet the body of the car) and probably the kick panels (the plastic plates that cover the sides of the interior of the car in the foot wells). With those out of the way, you can move the carpet at your leisure. Snake the wire under the carpet and to the doorsill. Most wire that is 4 gauge or smaller will fit under the sill and can be run alongside the edge of the passenger compartment all the way to the rear of the car.

Many guides advise that you not run signal wires alongside power wires. Good modern RCA cables reject most noise, and it is little known that since the entire body of a car constantly conducts electricity, it is actually emitting more interference than most any audio power cables. I usually run my power cable outside the car, since I use 0/1 cable (on other vehicles I go side-by-side). I won’t cover that process here, but it is somewhat more involved than running cable inside. For most applications, running power and RCA side-by-side is fine. If you find there is noise in the system, you may consider other options.

Snaking wire under carpet is very easy. Just push and pull the wire from wherever you can reach it until it is near the end of the car. You can usually just run it under the back seats and have it come out in the trunk. Stretch it to the amplifier and check the distance. Add a couple feet for safety and then cut it. Nothing is worse than having to move your amp or box later and not having enough cable to fit that length. Depending on the amplifier, you will either connect the wire to it bare or with a crimp-on connector, like a spade. Connect the amp now. Don’t worry, with the fuse out and battery disconnected, there is no power to the device.

Now we run the ground. This is very easy. Find a section of bare metal or a strong bolt that goes into the body of the car, within two or three feet of the amplifier. Shorter is better, but make sure the cable isn’t taut. It’s no fun to rip out your power terminals. If you go to bare metal, brush it so it is shiny and exposed, crimp a ring terminal to one end of the ground cable, and use a self-tapping screw to fix it to the body. If you use a bolt, polish the bolt and the hole, then use a ring terminal between the bolt and body. Connect the ground to the amp. Easy.

4) Run the Remote and Signal Wires. For this part, you’ll want to pull the radio out of your dash. This may be rather involved. In the back, you’ll find RCA outputs. Check the manual for your receiver to see which are the right ones to use. Remember this. Take the RCA cables and snake them either up into the dash from the foot well or down from the receiver, whichever looks easier. Connect them to the deck. At the same time, take your remote turn-on lead and connect it to the remote wire or antenna turn-on wire in the back of the receiver. Do not connect this to a constant power source, such as teh battery. Yes, the amp will work that way, but if the power is constant, the amplifier will always be turned on. This will quickly result in a dead car battery and a grumpy you. Use a butt connector to join the remote lead to the remote wire. If you don’t know what these are, look them up; they’re the most useful electrical connector known to man (I use hundreds every year).

Run the RCA cables and remote wire together, back to the amplifier. Connect them to the amp as described in the manual. Replace the receiver back in the dash of your car. Now double-check all your connections, move the wires exactly where you want them and where they will fit, then reassemble the interior of your car. The hard part is now done.

5)Test everything and choose settings. Reconnect the negative battery terminal. It may spark a little bit. This is normal. All the various components in your car are suddenly clamoring for juice. Don’t put the fuse in the fuse holder yet. Go turn accessory power on in your car. Make sure your radio and accessories work. If anything doesn’t, check for blown fuses in your vehicle’s fuse box. Replace any blown fuses. If it blows when you put a new one in, you’ve got a short circuit somewhere. This means you’ve probably nicked a wire you weren’t supposed to or connected something wrong. Recheck everything. If everything works fine, hooray, your car has possibly survived your tomfoolery.

Now, turn your car off and put the fuse in the fuse holder to connect the amp. Take a second to make sure that your subwoofer amplifier is set correctly (low pass filter, gain at minimum, and no bass boost to start with). Now turn your car to accessory mode and try the radio. If the previous steps went correctly, you should hear some bass with your tunes. Now you can take a few moments to adjust amplifier gain and radio settings to reach the desired results. Congratulations, you just saved yourself between fifty and two hundred dollars and gained a lot of practical knowledge.

Sources

These are some books, magazines, and websites that you may find interesting if you're into car audio, or looking to get into it:
Books
Auto Audio, Andrew Yoder (Fantastic book, one of the best to get started with)
MECP Study Guide (Anyone who wants to go pro needs this sooner or later.)
The Car Stereo Cookbook, Mark Rumreich (Excellent resource)
Loudspeak Design Cookbook, Vance Dickason (Very advanced, but extremely well written)
Magazines
Mobile Entertainment
Car Audio and Electronics Magazine
Websites
www.termpro.com (Look for DB Drag Racing and the forums)
www.sounddomain.com (Good forums)

This is my first node, BTW, so be kind.

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