37.5Hz is the second mode of the length of my listening space. Unfortunately, the seating works out to be about in the acoustic mid point between front and back walls, because the speakers and movie screen take up the front 4' of the room.
I've been making numerous measurements with Room EQ Wizard and my ECM8000 measurement mic.
One thing that shows up as a constant, no matter which bank of woofers is active, is the 37.5Hz notch. It's most prominent 15' from the front wall, which is smack in the middle of the length of the room. Moving the seating or speakers is.. well we're beyond that point and it was considered for other reasons related to projector requirements earlier. This response graph has no smoothing or filtering applied:
The only solution I can figure on now is diaphragmatic absorption--a set of plywood resonators at the back wall, tuned to exactly 37.5Hz.
The room is remarkably smooth from 17Hz to 100Hz, except for this amazingly-narrow notch at 37.5Hz where it's down 30dB with the doors shut and as much as 50dB down with the back door open! If I sit where the mic is and play a 37.5Hz tone from REW's generator, I can position my head where the tone completely disappears. I've done that with mid frequencies around 1-2KHz frequently, but this is the first time I can remember achieving a total null with a low frequency.
I'm almost certain that the standing wave off the back wall is responsible for this incredibly-deep null. It's amazingly narrow--5Hz to either side and the amplitude is normal. It looks a lot like the kind of notch I've seen in electronic filters that are very narrow, using resonant tank circuits. In fact, a good model of this is the coax "stub" notch filter we used to make for radio harmonic filters--a quarter wave length of coax with the end shorted. At frequency, it's an open circuit, but at 2X frequency it presents a short. The room is doing the same thing.
Measuring further forward (toward speakers) the null gets wider and less deep and moves down in frequency. Moving backwards to the rear of the space, the null frequency rises. At the center, it's narrow and very deep.
I believe the solution won't be cheap: resonant panels on the back wall, mounted in cavities with acoustic absorbers in the cavities. Each panel mass-tuned to resonate at exactly 37.5Hz.
I face some interesting construction projects ahead with this challenge. It's not a minor issue that can be corrected with stuffing fiberglass on the back walls. The front end already has a lot of treatment, hence the lack of deep comb filtering in the response curve.
My approach is probably going to be a pine 1x6 frame, 4'x8' and a plywood panel that is suspended in that space. Tuning would be achieved by adding mass to the center of the plywood diaphragm. It's probably going to take at least 1/2" plywood to satisfy MOST of the mass requirement while remaining compliant enough.
Any other ideas for high Q low frequency acoustic absorbers that are economical to construct?
