PROBLEM: We just installed two speakers in a small auditorium. They are hung in a tight cluster at about a thirty-degree angle to each other. The problem is that we are experiencing drastic phase cancellation right down the center of the auditorium. It starts as an approximately three foot wide area and slowly expands to about nine feet and the rear of the auditorium. With an SPL meter we are reading about a 12-15db difference in and out of the area of cancellation.

SOLUTION: This could be a couple of things but my initial guess because of the severity is that you have a polarity reversal. If this was simply a case of phase cancellation caused by a minor misalignment affecting the arrival times of the two separate speakers the result would most likely be less severe and more frequency dependent. Start at the beginning of the signal chain (inputs) and check for a polarity reversal between pins two and three all the way through to the end (amplifier outputs). A more reliable way of checking for polarity reversals is of course with a “polarity tester”, which are available from several manufacturers. The advantage is this will also alert you to any wiring mishaps within the equipment itself. If you find no wiring reversals it could be at the speaker termination itself. Before changing anything if you have any DSP in the signal path be sure to check that as well. Some software programs actually have polarity reversal algorithms allowing you to make the change in the digital realm. If you still find no problems simply switch the (+) and (-) wires on the output side of your amp, but only for one channel. My guess is this will alleviate the problem.