Originally Posted by Dennis Erskine
FOH...let's avoid this terminology. Perhaps better as where the x.x wave length is longer than the room dimension. There is too much misunderstanding about "bass not fitting into a room".
"as any freq with half a wavelength bigger than will fit into the room"
I'm guessing many understood, but you're right, I'm sure there's those that misunderstand. I certainly didn't want to muddy up the waters. Many savvy enthusiast don't get it, muct too much confusion and the whole point is clarity.
Typically, I'm more clear (read verbose) with regard to PVG, like I did in this excerpted post;
"Pressure Vessel Gain (PVG), or room gain, is the scenario whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization. A typical myth is a small cabin cannot support the lowest frequencies.... nothing could be further from the truth. The manner in which the sound is reproduced into the space changes from a normal cyclic propagation, to pressurization because the wavelengths are too big for the space. The frequency at which this occurs is approximately the point whereby half the wavelength of a given frequency is equal to the rooms longest dimension. So, a 20 hz frequency has a wavelength 56.5 feet. So half of that, 28.25 feet, is the point of transition. Any frequency below that point pressurizes the room, and any frequency above that point propagates freely. So in this room that's approximately 28 feet in the longest dimension, from 20 hz downward, the room gives back acoustically. This is room gain, cabin gain, or more specifically PVG...Pressure Vessel Gain.
At this frequency, the results are a gain in acoustic pressures in the room that grows as the frequency decreases. This acoustic support reciprocity, is theoretically 12db per octave. The percentage of the 12 db/octave gain one achieves, entirely depends on the integrity of the boundary walls and surfaces. If it was the theoretical concrete bunker, a full 12db/octave boost would occur. Typically, somewhere between 6-10 db octave could result. Also, in addition to the walls and surfaces flexing, other aspects may affect the point at which room gain begins. Furniture, cabinets etc, anything that consumes a certain measure of cubic feet, may slightly alter the transition frequency merely because the items take up space.
This acoustic pressurization, room gain, is the proverbial free lunch. It is essentially headroom that's thrown back into the system. And unlike horn subs, the distortions and non-linearities are not magnified. An IB sub system is a sealed alignment. Sealed alignments roll off second order. Room gain also is second order. So one can see how integrating a sealed alignment may offer substantial benefit when attempting to integrate the system to the room via time and frequency equalization. The -3db point of the IB, may typically be deeper than the transition point where room gain begins. Properly adjusted, this would result in substantial headroom added back in for significant capability for the big LFE effects."