Setting Subwoofer Phase the Easy Way
The "Bass Management Boot Camp" article about subwoofer setup that I wrote for the November S+V inspired an in-turn inspiring e-mail from reader Bruce Erwin. Bruce recalled his days setting up sound systems when he'd use a 1.5-volt battery to test subwoofer phase. If the sub and main speakers were in phase, he'd hear a single thump when he connected the battery to the wires leading to the sub's terminals. If they were out of phase, he'd hear a double thump.
That's with passive subwoofers, though. Almost all of today's subs have built-in amps, so this method wouldn't quite work with them. Still, it got me wondering if I could come up with a simpler phase check than the ones I suggested in the article, which required either measurement or careful listening. My e-mail convo with Bruce gave me some ideas and, ultimately, a method that's simpler than my original. And all you need to do it is a piece of free software and a meter you've either already got or can get for nothing.
First, let me digress a little with an explanation about phase. If your subwoofer and main speakers are in phase, the woofer cones on the subwoofer and the speakers will move forward and backward in sync at the subwoofer crossover frequency. They'll reinforce each other's output. That's good. If the sub and mains are out of phase, the woofer cones on the main speakers will move backward while the subwoofer cone moves forward, and vice-versa. They'll cancel each other's output. That's bad.
But it's not accurate to think of your sub and main speakers as being in or out of phase, except maybe at one particular frequency. That's because the phase of both devices changes with frequency. Even if you can get them in phase at 80 Hz, they might not be at 100 Hz.