What should I know about my speakers' impedance?

Impedance is the load value that your speakers present to your power source — the amount of resistance they provide to the current flowing from your amplifier. While it's convenient to refer to a speaker's impedance as being a fixed value (say, 4 ohms) over a speaker's entire frequency range, the impedance typically varies with the frequency of the input signal. So when we say a speaker is a 4-ohm speaker, think of this rating as a useful approximation.

The key things to know about impedance are your speakers' ratings, your amplifier's ratings, and whether they match. That's enough to keep you out of trouble, and even let you get a little creative when you're designing your system.

In the car audio world, the typical stereo amplifier is stable down to 2 ohms in stereo, or 4 ohms in mono (or "bridged") operation. Most car audio speakers are rated at 4 ohms, but 2-ohm, 8-ohm, and dual voice coil subwoofers are increasingly common. Car audiophiles know that they can actually make their systems hit harder by combining higher-impedance woofers.

Bridging a stereo amplifier down to a single channel is a great way to maximize power to a single speaker. You could use this single channel to drive a single 4-ohm woofer. But some fans of aggressive bass prefer to hook up two parallel-wired 8-ohm woofers — the subwoofers' total impedance remains the same but the overall surface area is increased, raising the system's sound pressure level. If you've invested in an amplifier that's stable down to 2-ohm mono (1-ohm stereo), you can further raise the stakes in this game by introducing up to four parallel-wired woofers to your bridged amp. You'll get more bass from the increased surface area, and more power output as well.

But before you rush out to run your car audio amplifier at the minimum impedance it can handle, bear in mind that there are trade-offs involved. As you decrease the impedance your amplifier sees, the amplifier's distortion spec will rise. On the other hand, some people claim that, in the hard-thumping arena of car audio subwoofers, such changes are inaudible and insignificant.

So what's the bottom line? Make sure you know your speakers' impedance ratings, as well as the minimum impedance of your amplifier in bridged and stereo modes. Then experiment carefully and go with what sounds right