[Wayne A. Pflughaupt]

Interesting post, Robert. As I understand it, though, a speaker’s (or sub’s) port is tuned below the driver’s front output. As such, there’s no way what you were experiencing could be caused by the port.

If you were getting that much energy in the 200-300 Hz range, the first thing I’d suspect would be the crossover. Most Dolby Digital receivers have crossovers with 24 dB/octave slopes. This means that anything generated at 300 Hz should be nearly 48 dB below what’s generated at the 80 Hz crossover frequency (since 300 Hz is almost two octaves above 80 Hz). You should be able to verify this with your SPL meter and some sine wave tones at those two frequencies.

Assuming the crossover is functioning as it should, it can be tricky to determine what’s causing the problem. My first guess would be that a room mode is boosting output in that region and essentially over-riding the crossover. If that’s the case, the sub’s output could be added on top of what the mains are doing there. But since you say you didn’t have this problem with your other sub (assuming nothing else changed, especially the room), we have to rule that out.

Either way, you did the right thing by eliminating the excessive output with the BFD. You could have also dialed in the sub’s built-in crossover on top of the receivers to effect a functioning slope of either 36 or 48 dB/octave, depending on the sub’s crossover’s slope. This would have reduced any output at 300 Hz to 72 or 96 dB below 80 Hz.

Regards,
Wayne