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What subwoofer is best for me?

So you want to buy a subwoofer. Here's what you should be thinking about:

1. Number of drivers - The first thing you should be considering is how many subwoofers you plan to install. A single driver is enough to strengthen the bottom end of your music, but bass isn't entirely omnidirectional, and you'll notice where the driver is placed. Two subs will always play louder then a single sub, and they tend to balance out the soundstage in a car interior. Three or more subs in a hatchback is only practical when you're competing in sound pressure level (SPL) competitions.

2. Size of drivers - The second decision you should make is what size drivers you want to run. Your options range from 8"-15" drivers. Since 8" drivers struggle to produce frequencies below 60Hz, most people tend to run the larger 10" or 12" drivers. 15" models are available, although they are rarely used because of their cost/size/power requirements.

3. Enclosure type - The next thing to consider is what type of enclosure the subs will be placed. Your choices are a sealed enclosure, a ported enclosure, and a bandpass enclosure. (There's others, but they're niche applications) Sealed enclosures provide the most accurate bass response and the least amount of airspace, but require more power to run. Ported enclosures are more efficient (louder) than sealed enclosures, but their response is somewhat muddied. Bandpass enclosures are a hybrid of both the ported and sealed design, and are capable of tremendous efficiency given enough airspace. Unfortunatley, they're difficult to "tune" properly and aren't practical in the back of an f-body. There's a limited number of subwoofer models that are designed to run in small airspaces. Since many of us have limited space to work with (stealth box, deck-top boxes), this is a critical parameter to calibrate. Don't forget to make sure your sub's mounting depth will fit in the box, too!

4. Sub Enclosure Rating - After you've settled on an enclosure, you need to find a subwoofer model that's designed to run with your given amount of airspace in your enclosure. Subs are rated with an optimum airspace volume -- they will be most efficient if they're placed in the correct volume enclosure. This parameter is one of the most important ones to be looking at when selecting a subwoofer.

5. Sensitivity - The next parameter you should compare among subwoofer models is sensitivity. Sensitivity is a measure of how the driver converts electrical energy into sound waves. This parameter is measure in dB. If you placed a microphone 1m away from the speaker and powered the speaker with a 1W signal of 1kHz, then the resulting SPL at the microphone would be X dBs, where X is the sensitivity of the speaker. Clearly, a higher number is better. Now here's where what's not commonly understood: increasing the sensitivity of a driver by only 3dBs is equivalent to doubling the driving amplifier's power! For example, an 90dB speaker under a 200W load will play just as loud as an 87dB speaker under a 400W load. I'll discuss this more in the amplifier power section. Suffice to say, that sensitivity is an important parameter to shop around for when you're choosing a subwoofer.

6. Frequency Response - Surprisingly, this isn't a very useful parameter to compare. Here's why: most subwoofers of the same size tend to do a reasonably similar job in producing low-end frequencies. Secondly, subwoofer spec sheets generally only give you the roll-off numbers. A roll-off is the high and low end of a band of frequencies where subs operate most efficiently in. It would be desirable to create a completely flat response in between the roll-off numbers, but you can't tell that just by looking at the two end-members. Without getting into a long discussion about Fourier transforms, it would be safe to just conclude that this parameter doesn't tell you much about a subwoofer when you're comparison shopping.

7. Power Rating - The last technical parameter you should compare is power handling. Power handling is defined in two ways: peak and RMS. Peak handling is a measure of the maximum amount of power a driver can handle for short periods of time. This number should usually be ignored. RMS handling is a weighted-average method of measuring a nominal power load that won't damage the speaker. Quantifying these values is as much of an art as it is a science. The vast majority of manufacturers just set their RMS wattage rating to half the peak rating -- these numbers aren't terribly fine-tuned. (i.e. a 400W RMS driver doesn't magically perform best when exactly 400W is applied) If a manufacturer sets the nominal rating too low, it's perceived as a low quality product. If he sets it too close to the peak rating, he'll be losing his a$$ to warranty claims. If you've selected an efficient subwoofer, then you don't need much power from your amp, and hence a small power handling capability would suffice.

8. Voice coil configuration - Finally, you'll need to decide between a single voice coil (SVC) and dual voice coil (DVC) model. Each voice coil can be considered an individual resistor. Since they can be run in either a series or paralell configuration DVC are often preferred since they offer more wiring configurations. Once you've settled on either SVC or DVC subs, you should determine what the final load at the amplifier will be. This would be a good time to check our FAQ to learn how to compute resistance across a series or paralell circuit.

9. Amplifer circuitry - Once you've selected your subwoofer/enclosure configuration, you should choose an amplifer to power your sub(s). The first of two technical details that you should compare is circuitry classification. Circuitry classification refers to the way transistors are addressed in the amplifiers internals. Class AB circuitry is found in the vast majority of amplifiers today, and is popular because it doesn't suffer from much distortion, even though it's an ineffiecient design. Inefficiency in amplifiers creates heat. Heat in car amplifiers tends to fry components. The other option is to buy a class D amplifier. Class D circuitry draws less current and produces less heat. Class D amplifiers have always been more efficient (cooler) than their AB counterparts, but they suffered from distortion problems which made them less than ideal for audio applications. Fortunately, in the mid-nineties, huge strides were made in the field of metal-oxides semiconductor field-effect transistors (MOSFETS). These are ideal for class D amps because they're inexpensive, efficient, and fast-switching. Today, class D amps are capable of producing outstanding power with minimal distortion. There's really no reason not to buy a class D amp now, especially for car audio applications.

10. Amplifier power rating - Again, amplifers are rated with Peak and RMS power outputs. Power is measured in Watts. Select an amplifier whose RMS power specification is as close as possible to your subwoofer's RMS power handling capabilities. If the amp is rated slightly better than your sub, there's no cause for concern. You are however, probably voiding your subwoofer's warranty. But how much power do you need? Again, it's a common misconception that you need a lot of power to drive a subwoofer. That's only true if you've selected an inefficient sub. Here's some background info: A human can detect a 3dB change in sound at nominal volumes. Any smaller change is unnoticable to the average ear. A 10dB change in SPL is perceived as twice as loud. It takes twice the power from an amp to increase the SPL by 3dB. It takes twice the power from an amp to increase the SPL by 3dB. You read that twice because it's important. A 600W amp will increase your SPL by only 3dB when compared to a 300W amp. It's much more economic to select an efficient sub and place it in the correct enclosure, rather than buying a ridiculously powerful/expensive amplifer. Bear in mind that some of the highest-end home audio tube amplifers run on less than 10W.

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