Hi Ethan, I am starting a bass trap project, and it looks like you have a lot of experience in this field. I have a dip in the frequency response around 140 Hz. Not really bothersome, but knowing that it is there, and enjoying small projects like this make me explore my options. The frequency corresponds to the wavelength of a reflection from the wall behind the speaker. The front of the speaker is about 2' away from the wall, so the reflection would be 1/2 wavelength, 4' - 140 Hz. Since my room is over the garage, I can replace the sheetrock walls behind it, if I want to, up to a height of 40", then the ceiling slope starts anyway. I could also build a membrane in front of the wall studs. Behind it is insulation, and the attic, no problems there. What would you recommend, as far as the membrane is concerned?
That null could be related to SBIR - speaker-boundary interference - caused by your speaker being about 2' from the wall. That distance is 1/4 wavelength of 140Hz. The best way to mitigate this is by adjusting placement - 'all the way in or all the way out' is the motto. Barring that, you can reduce the effect with absorption placed at first reflection points which can be found with a high-tech gadget called a mirror.
Yes, that’s what I was trying to convey, the cancellation is because of the reflection from the wall. So, if I move the speakers, I am afraid I might merely move the frequency of the cancellation. I don’t like the idea of putting a foot of absorptive material behind the speakers, diffusion is out due to wavelength. That’s why I am exploring the possibility of a bass trap.