The purpose of the subwoofer enclosure is to allow a subwoofer driver to produce the desired bass frequency response. Not everyone likes the same kind of bass. A 19-year-old hip-hop fan would want a different kind of sound from a 30-year-old jazz lover. The keys to getting what you want is understanding the advantages and disadvantages of each type of box and matching the right box to the right driver.
DON'T FORGET! To Ensure Good Bass from Your Box:
Sealed enclosures will take up less space in the vehicle. Use a subwoofer driver that is specifically designed for small sealed enclosures like the Polk dX and Polk/MOMO subwoofers. Sealed enclosures usually produce a flat, extended and "tight" bass response but are less efficient than ported enclosures, so you'll need a powerful amplifier to get loud bass. A sealed box is a good choice for customers looking for an accurate, more sophisticated sound. It is critical with this kind of design that the box be well constructed and absolutely airtight. Air leaks will cause noises that are easily mistaken as blown driver noises.
Ported or "vented" boxes have the advantage of delivering high bass output with less amplifier power than is needed for a sealed enclosure. Ported boxes can have very good deep bass response but will be less "tight" sounding than a sealed box. Vented boxes are larger than a sealed enclosure. At high volume levels the port can make a "chuffing" noise because of high speed air-flow in the port tube. Ported systems with high "Vent Air Velocity" (see Interpreting Subwoofer Technical Data) are not good choices for people who want to play their systems very loud.
This type of enclosure consists of two enclosure sections. The first section is a sealed and the second section is ported. The woofer is loaded into the sealed side and then the front of the cone is firing into the ported side. This design is called "Bandpass" because it will only reproduce certain bass frequencies |within a narrow section of the audio band. The advantages of this type of enclosure are very high efficiency and steeper filtering of unwanted midrange frequencies. Bandpass boxes can produce awesome amounts of bass with very little amplifier power. But there is a price to pay for this efficiency. Bandpass boxes tend to produce "one-note," somewhat "boomy" bass and are large. People who like to listen to Bass CDs tend to like bandpass boxes. They are also a good choice in systems where another woofer handles the mid-bass chores and the bandpass is needed only for reproducing the very lowest bass range. Even though they are ported, bandpass boxes have low Vent Air Velocity and can play very loud without excessive port noise.
Once you’ve picked out your electronics, you have to decide whether you are going to install them yourself, or let a professional installer do it. Maybe you’re handy with small repairs, or maybe the thought of installing speakers yourself seems daunting but you don’t know what qualities to look for in a professional installer. Here’s some help.The Pros of Pros
If you have any doubts, save yourself lots of grief and use a pro. Cars are more complicated today than ever before. Even basic models are packed with computer controls. Today’s state of the art upgrades, audio systems, navigation systems, even DVD systems, and their operating systems are interlaced in delicate and intricate ways.
Meddling with these critical systems can shorten the life of your vehicle and even endanger your life. Electrical problems, unseen shorts, wire deterioration, structural changes and loose connections could become safety issues.
Before you decide to let anyone work on your car, check his or her credentials. The Mobile Electronics Certification Program (MECP) administers a written test on the science and installation of car sound. Make sure your installer is MECP certified. Are they an ASE (Automotive Service Excellence) certified mechanic? Ask.
Check references as well. Ask to see examples of previous installs and speak with previous customers. Deciding on a professional installer is a lot like finding a reliable contractor or plumber.Power Tools don’t Frighten You
If you’re good with minor repairs, like changing your spark plugs, you probably won’t be intimidated by installing a sound system and you might consider doing it yourself.
There are lots of good reasons to self-install. If you’ve got the time, and enjoy the challenge, doing the install yourself might be pretty exciting. Customizing your own car is a great way to express your creativity. Poking around in the interior of your car makes you, well, “intimate” with your automobile.
Manufacturers and retailers are usually very helpful when it comes to DIY installs. Rely on them for parts, wiring harnesses, tips, templates and general good advice. Like this:
Avoid some of the pitfalls of doing-it-yourself simply by keeping some common-sense guidelines in mind:
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, in intensity, 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.
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.Crossover
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 levels. All you really need to know is that to gain 3dB in volume (a moderate increase in playing volume), you must double your available 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’
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
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.‘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.” Think of volts as the pressure and current as the flow of all those electrons. If you have high pressure and plenty of flowing electrons that means lots of work can be done. Another word for work is Power!‘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.
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 (just kidding!). 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-damping 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.
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.
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. Many top car audio installers recommend running a ground wire all the way back to the car's negative battery terminal.
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!
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. 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.Active
An active, or electronic, crossover does its job before the 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.
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.
Simply, speakers are air pistons that move back (on the negative cycle of an electrical signal) and forth (on the positive cycle) creating different degrees of air pressure. These movements translate into different frequencies that translate into Mozart or Metallica at your eardrum.
To do this, an amplifier produces electrical impulses that alternate from positive to negative voltages and create an electromagnet when they reach the "voice coil" (a spool of wire) inside the speaker. This electromagnet will then either be repelled or attracted by the fixed magnet at the bottom of every speaker. The voice coil is attached to the speaker cone, and it moves the speaker cone back and forth as it's attracted or repelled. The surround (a rubbery circle that joins top of the cone and speaker's metal basket) and the spider (a circle of corrugated material that joins bottom of cone to the speaker's magnet) make the cone return to its original position. All this moving around makes "sound." Speaking of Speakers
Coaxial speakers (or "three-ways") are two (or more) speakers built into the same frame.
They can be inexpensive and are usually easy to install, often fitting into factory speaker locations without cutting or major modification to the car. Because coaxial woofers and tweeters are housed in one frame, you're assured good imaging.
They lack the positioning flexibility of separate components, and with many models you can't aim the tweeter toward your listening position. Better speakers have two-way crossovers that filter bass out of the tweeter and treble out of the woofer so that each part of the system is playing the frequency range it can do best. Most coaxials come with simple crossovers that only filter the bass out of the tweeter but allow the woofer to run unfiltered (full-range) and that's a bad thing. Polk's db Series coaxials are unusual in that they have two-way crossovers for better sound.
Separates, or components, are sets of separate woofers and tweeters with an external crossover.
More placement/system flexibility, and a better crossover. Best sound quality.
Placement flexibility means you'll be drilling holes in your car to put separates in custom locations. You may be able to mount the woofer in the place vacated by your factory speaker, but you'll still need to make a separate hole for the tweeter. (The most common place to install tweeters is in the top front corner of the door panel.)
Subwoofers add the lower frequencies to a whole system. We could have a whole section on subwoofers alone. In fact, here comes one now:Subwoofer Knowledge (or, The Sub is Greater than the Parts)
Some things are just true. The sun rises and sets. Christmas comes every year. Subwoofers need power. They need more power than anything else in your system. This is simply because they are bigger, and need more power to move their cones farther than other types of speakers. If you have an amp supplying 50W to each of your four front and rear speakers, you can bet on needing at least 100W for your sub(s). And you need to use a low-pass crossover to block the high frequencies from getting to your subwoofer and messing up the mix.
Low frequency signals reproduced by subwoofers are "non-directional." That is, it's most difficult for humans to tell where lower frequencies are coming from. So you can (theoretically) put a subwoofer anywhere without worrying about a loss of sound quality due to poor aiming and direction.
To make bass response more effective, subwoofers must be housed in a subwoofer enclosure. That would be a box. With a subwoofer in it. Because of this, subwoofers are most often placed in a car's trunk. A subwoofer enclosure can be simple ("sealed"), or complex (bandpass, that's pretty complex). Each kind of enclosure has its own characteristics, and the "one size fits all" rule of stretch denim and baseball caps does not apply here.
Ask us (go ahead, ask us), and we'll say that bigger is better. But-honestly-a good quality, well-enclosed 8" will outperform a cheap 12" sub any day. Big, cheap subs have slower responses and can sometimes be "boomy," while smaller subs tend toward a tighter sound. A bigger sub also needs a bigger box in which to enclose it. And if you fill up your trunk with a giant sub box, how are you going to haul the bodies (not to mention golf clubs!)? If you're going to go "big," don't skimp. Do it right. Check out our "Enclos-R-ama" for the basics of sub enclosures.