Originally Posted by J_P_A
I'm guessing your mic was near the floor?
Yes, the mic was lying directly on the riser, as in the photo in post 1124. In fact, it's still lying on the riser right now. I haven't touched it at all since I set it down.
Originally Posted by J_P_A
Could you measure about halfway up the wall?
I can, and I will. But it may not be soon or easy - I need to buy a mic stand.
Originally Posted by J_P_A
I wonder if the mic being close to floor is seeing an acoustic shortening of the room due to the riser and stage height? I really have no idea how you would go about estimating the acoustic length, or if being close to the floor will make any difference or not. That is, does the acoustic length change as the height changes?
I think you're thinking of it the wrong way. The mic either sees the high pressure of the modal response, or it doesn't. The modal response exists because of the real dimensions, and to some degree the storage of energy in the boundary. If we position the mic at a place (near a boundary) where the pressure of a standing wave must be high (pressure is high, velocity is low), then the mic should record the highest pressure possible - the highest of any place in the room. If we position the mic at a node of the standing wave, the pressure should be the lowest of any place in the room - theoretically zero at that frequency.
There is a separate question of what frequency of wave constructively interferes in a resonant way between any two surfaces (or combination of several surfaces). This is the reason for this test.
In theory, we can just plug the numbers into the calculator and know what frequency the resonances will be centered on, but it's always more complex than that. I'm not fully educated on the intricacies here, but this is a common area of misconception I think, so let me lay out what I know and think I know. (This may seem condescending, but I hope you don't take it that way. I know you are well-educated in a lot of this, but others are not. Maybe others will find this appropriately informative.)
If we start with the idea of a simple impulse wave, we see the wave reflects off a rigid boundary.
If instead of a single pulse wave, an endless series of full waveforms (both positive and negative halves) propagate through the medium toward a boundary, there will be some frequency where the reflected waveform exactly superimposes with the incident waveform, and perfect constructive interference is achieved. If this phonomenon is present at both ends of a confined medium, we have a standing wave. In this case, no energy is lost to the perfect boundary, and the wave reflects or resonates forever, even without further input from a source driving it.
The length of the space between the boundaries and the speed of the wave reflecting between them are the only two factors which determine the frequency of maximum constructive interference - the resonant frequency. But again, this is the theoretical perfect scenario. Real boundaries do not perform this way at most frequencies - especially low frequencies.
Instead, with real boundaries we see some portion of the wave energy will be transferred from the particles of the incident medium (air) to the particles of the boundary medium. The energy now contained within the motion of the boundary medium can do one of two things: either remain in the medium and be gradually dissipated as heat (frictional losses, in effect) or it can be transmitted back out of the boundary medium into whatever is adjacent (air). With most home construction partitions, the energy of low frequency sound will pass into the boundary and out the other side. Some of the energy will be immediately reflected at incidence. Some of the energy will be transmitted into boundary, but not immediately through to the air space on the other side. It's this energy that is interesting.
Imagine a boundary confined at the edged, but free to flex in the middle. It's fundamentally like a drum head in that way. Each small section of drywall is like this, and as a whole, the wall exhibits this behavior as well. When the high pressure of sound impedes it, it flexes. That flexing is the sound energy being transmitted out of the air, and into the wall structure. Once that happens the energy deflects the wall stretching it like a rubber band storing energy within it elastically. The pressure lessens, and the wall begins to return to its original shape. The wall, as drum heads and basically any other deformed object, will rebound past its original shape and oscillate until the energy dissipates. When the wall flexes back into the room with the sound source, it has become an acoustic source of its own, emitting sound back into the room. However, the sound that comes back into the room is delayed by whatever time it took the wall structure to flex back and forth. This will vary with mass and construction technique - it's difficult to predict, even in lab conditions as I understand it. So now the wall must be added into the model of how the standing wave is formed. Instead of the speed of sound and the distance between the walls being the only variables, we've added in an extra bit of time (not just the time that the wave needs to move at its own natural speed in air). This extra bit of time has the effect of lowering the resonant frequency in the standing waves. Two boundaries that would establish a standing wave at 35Hz if perfect might establish a standing wave of 32Hz if one or both is flexible.
There are other more complex phenomena present in the complete analysis of the modal interactions and decay. At some point, we'll need to consider the bandwidth of the resonances, and the way that through overlapping bands one resonance can stimulate another. The acoustic coupling of adjacent spaces (as I suggested for the 30Hz region in my test) is another one, but I don't think I have the knowledge to deal with that.