Subwoofers. The bass. The loudness. The glory. The most obnoxious component in your system, and also, one of the most necessary.
If you've read my article about speakers, you're up to speed. If you haven't (and don't know anything about speakers or subwoofers), read it here.
Subwoofers reproduce the lowest frequencies of the audio spectrum, AKA bass. They are essential to both SQ and SPL systems, but for different reasons. Let's take a look now...
For SQ systems: Effectively reproduce the lowest frequencies making the music sound full and powerful. Instruments that play lower frequencies such as organs, drums and synthesizers will sound more realistic and powerful with a subwoofer installed.
For SPL systems: Because subwoofers are loud, they are the preferred speaker to make a whole lotta noise.
Also, because our ears are less sensitive to lower frequencies we need the extra bass for the system to sound balanced.
So, what do I have to look for to find a great woofer? I'll tell you!
Just like with every other part in a stereo system, power handling is important for subwoofers as well. Earlier, in my article about speakers, I talked about how you can send more power to them to increase their volume, but decrease the distortion. The same is true of subwoofers, but be careful! Since lower frequency distortion is harder to perceive, it's easy to overdo it, and blow your subwoofers. Anyway, with that said, it is smart to buy an amp that can put out exactly what your subwoofers are designed to take, but take it easy!
Subwoofers come in many sizes, but the most common are 8", 10", 12", and 15". This is probably also noted in metric for the european brands. As you can imagine, the bigger woofers are the ones that will play louder and play lower. Of course, the smaller ones are more musical, more precise, and sound "tighter". What size you decide on usually has to do with your own preferences, as well as installation considerations. Remember! 18" subwoofers don't just fit anywhere!
Voice coil info:
SVC: Single voice coil
This means that the speaker has only one voice coil. In other words, only one input.
DVC: Dual voice coil
This means that the speaker has two voice coils, or two inputs.
Ummmmmmm, ok. Why does it matter?
I could go into details here, but I know you don't really care. Basically, it boils down to this: With a DVC subwoofer, there are more installation options. Look at it this way, a DVC "looks" like two subwoofers to your amplifier. With some nifty wiring tricks (called series and parallel), you can hook up many subwoofers to a single amp--even if the amp is a "mono" or a two channel amp. In addition, you can manipulate how many ohms the amp has to push. If you have an amp that is stable to a half ohm (like some old school Orion HCCA amps), you could potentially hook up 4-8 woofers to a single channel! Imagine the noise you could make!
Some words about boxes:
Boxes are essential to most subwoofers. In other words, consider a subwoofer's box just as important as the subwoofer itself. Why? Because most subwoofers need a box to operate properly, sound good, and not damage themselves. Subwoofers that don't need a box are called "free air" or "infinite baffle" subwoofers.
Like I said, most subwoofers need a box. So, let's talk about boxes!
First, what types of boxes are there?
Sealed: The simplest kind. Essentially, a sealed box is a box where the inside and outside are not linked. There are no holes, and care is taken to ensure that the box is completely sealed.
Ported: A ported box is a subwoofer box with at least one hole, or "port" in it. The goal of the ported box is to emphasize a certain frequency. These boxes are used because they are often louder than sealed boxes.
Bandpass: A bandpass box is similar to a ported box because it has ports; however, the bandpass box tries to de-emphasize certain frequencies, while emphasizing others. In other words, it works like a bandpass crossover. The bandpass box emphasizes a frequency band while de-emphasizing frequencies lower than and higher than the band.
Isobarik: These boxes aim to fit more than one subwoofer into a small box. The subwoofers are set up to work in tandem. Sometimes both subs move in the same direction at the same time, sometimes one sub pulls while the other pushes. There are advantages to this type of setup, but box design is difficult.
OK, so now you know everything about SUBZ, but here are the specs that you'll need to know to choose your subwoofer:
Power Handling: Subs are speakers, and as such can only handle so much power (or distortion) before they blow! Power handling is measured in watts, and is often given in two specifications: RMS and Max (or peak). The RMS rating is the most important. If a sub says it can handle 200 watts RMS and 400 watts Max, be sure the amp will provide 200 watts RMS as well. Not 400 watts RMS. While it's true that the sub can handle 400 watts, it can only handle that kind of power for a short time. If you hook that sub up to an amp that puts out 400 watts RMS, you will ruin that speaker pretty quickly--because it is constantly being exposed to 400 watts, not short bursts of 400 watts. You dig? Good.
Sensitivity: This term means exactly what you think it means. In short, a subwoofer with higher sensitivty will be louder than a subwoofer with lower sensitivty when they are hooked up to the same amp. The spec is measured in db.
Frequency response: You obviously want a subwoofer than can handle a wide range of frequencies. However, the subwoofer does the most work under 100hz. If the subwoofer plays all the way down to 20hz, you know you've got a great sub. The good news is that even if it doesn't go that low, most music doesn't either. Sooooo, you'll still be fine. I would say the it's more important to have a low-playing sub in systems with DVD players and 5.1 sound. Also, frequency response varies depending on the box that the subwoofer is installed in. Ummmm, so with this in mind, just pick a subwoofer that will handle the power, and sound how you want it to.
Cone Material: When it comes to subwoofers, this is actually a pretty important thing to pay attention to. Because subwoofers are big and powerful, you need a cone material that can withstand lots and lots of abuse. I personally am a big fan of metals such as alluminum. This is because aluminum is light and stiff, and likewise, produces very tight and musical sounding bass. I know that many subwoofer manufacturers use exotic sounding materials for their subwoofers, which makes it difficult to determine which material is the best.
Surround material: Here's my advice: Always get a subwoofer with a rubber surround. Rubber holds up the best in the auto environment.
Impedence: Measured in Ohms, this spec tells you how much resistance the speaker presents to your amp. Too much, and you get no volume, too little, and your subwoofer will fry your amp! This spec will help you decide which amp to hook up to your subwoofer, whether you want to bridge your amp, and whether you want to wire multiple subs into parallel or series. Also, subwoofers sometimes have more than one voice coil (aka DVC). This means that the subwoofer will have two inputs, each with the same impedence. Subwoofers with dual voice coils are more versatile to install, especially in multiple woofer systems.
A basic subwoofer install is fairly easy. If you buy a pre-fab sub box, mount the subs and power them, you're going to get good bass. To install professionally, and achieve the best sound, much planning and work must go into the whole process. This is a situation where if you don't feel confident in your ability to build a box, tune it, and hook it up, you should visit a car audio shop, and have them do it for you. This can be relatively cheap, or quite expensive.
If you are planning on doing the install yourself, but lack the box building skills, you can buy a pre-fab box. I would also strongly recommend an EQ device to fine tune the bass. This will help you overcome road noise (which ruins bass), and also cut back on "boominess" and "muddiness". In the end, this route is the best compromise between a custom box and a pre-fab box. You'll get great sound, and save money!