Why subwoofers blow: slam, bang, pop, and sizzle
Too much power or distortion damages subwoofers
An exploded view of the parts of a speaker. This particular example is the woofer from a set of Focal component speakers. (click the image to enlarge)
In a perfect world: the music sounds great
The sine wave below represents a signal that an amplifier sends to a subwoofer as a variation in voltage over time. The vertical axis represents voltage, the horizontal axis represents time. In AC (alternating current) signals, like music, the voltage swings between positive and negative values. Point A represents the point in time when the amplifier is telling the sub's voice coil and cone to be as far forward, toward the front, as that particular voltage (+V1) tells them to be. Point B is where –V1 voltage positions them toward the rear. Traveling back and forth rapidly, the cone pushes air and makes sound — and it's musical.
Overpowering your subwoofer – Slam and Bang
First we'll cover what happens when you give your subwoofer too much power. It's great to turn it up, but that extra volume starts to distort the music. Not only does it sound bad, but it can damage your speakers and subs, especially if you do this all the time.
Slam: The incoming signal tells the cone to move too far forward
Let's say that points A and B (and +V1 and –V1) are the maximum ratings of our subwoofer. If we increase the volume of the signal, the higher voltage (+V2) now tries to move the cone even further toward the front (C). The signal is still a clean signal, but now it is trying to move the cone and voice coil much further than they were designed to go, tearing the cone, spider, and surround apart, destroying the subwoofer.
Bang: The signal tells the coil to move too far backward
On the other side of things, when the signal (-V2) tells the voice coil to move too far toward the rear (D), the voice coil crashes into the back plate of the magnet assembly, cracking the coil and its former (the tube it coils around), and probably jamming it in the voice coil gap.
Underpowering your subwoofer — Pop and Sizzle
Underpowering a subwoofer isn’t inherently bad for the sub. Not giving it enough power just means that the music will sound weak and lack detail.
The danger is when that power is coming from an amplifier that's being overworked and sending out a clipped signal. The clipped signal tries to make the sub do things it's not designed to do, which leads to it tearing itself apart or overheating and burning out.
First, what’s a clipped signal?
Clipping a signal, or squaring its waveform, occurs when the volume of a source signal exceeds the electronic capability of a circuit. Let's say our amplifier can't play a signal more powerful than what voltage V1 can produce. If we tried to increase the volume at the source, the amplifier wouldn't produce more voltage, it would distort the signal, eventually into the form of a square wave.
Pop: A clipped signal tries to move the cone too quickly
You will notice that the sides of the clipped signal are vertical. That means that the signal will try to move the sub's cone from all the way forward (point E) to all the way to the rear (F) in zero amount of time, travelling at the speed of infinity. Nothing travels that fast, and the sub either tears itself apart trying, or the flapping cone wobbles just enough to jam the coil in the magnet's voice coil gap, killing the sub.
Sizzle: A clipped signal also tells the voice coil to hold still and heat up
The other parts of a square wave, the top and bottom, are horizontal lines that represent the times the signal is telling the cone to stay all the way forward or all the way back. Current flowing through a stationary coil only heats up the coil, which doesn't even benefit from a cooling breeze due to movement. The coil usually burns through one or more of its windings, or heats up enough to deform its shape so that it jams in the magnet's voice coil gap.
There's another, more complex reason voice coils burn when subjected to over-driven, clipped signals. A square wave carries twice the RMS power of a sine wave of the same amplitude (height). So not only is the signal telling the voice coil to pop into a position and sizzle, it's doing it with almost twice the power of the sub's maximum capacity. Usually, it's the glue holding the coil wire to the former that first melts under all the heat, and the coil crashes in its gap.
Distortion is the sub killer
Low power and low volume will not hurt a sub – but distortion will. A clipped signal is a sub's worst enemy. It isn't loudness that destroys an under-powered sub, it's trying to get bass volume by turning up a distorting signal that does it.
How to not blow your subwoofer — match the RMS power ratings
Ignore all "peak" and "maximum" wattage ratings, and use only RMS ratings. They may be harder to find, but RMS ratings are the only power ratings you should use. To safely drive a subwoofer, use an amplifier that can give it no more than its highest RMS power rating.
It's also crucial you set the amplifier's gain correctly. If you don't know how, you can check out Tuning Your Subs or Using Test Tones to Set Amplifier Gain for some helpful tips. And remember, if you want to run two or more subs, you've got to supply enough power for each and every one of them.