Originally Posted by stockmonkey2000
I'm interested in the PVG characteristics you are talking about - Never heard of that.
There exists room gain, and boundary gain.
First, Boundary Gain
comes from the driver operating in a constrained space. One begins with a theoretical full-space. This has no surfaces which interact with the acoustic output of the driver. Only a full tilt, entirely anechoic chamber can approximate a theoretical full space. That said, cutting a full space sphere in half with a ground plane, one creates half space. This is an outdoor environment, whereby zero acoustic interaction occurs between the driver, and the hemisphere above the ground. The ground merely halves the previous full space. This halving, theoretically mirrors another like powered driver, thereby resulting in a theoretical 6db gain. Each subsequent halving of space, with the addition of another boundary, also results in a theoretical 6db gain.
So boundary gain is essentially added acoustic support, by halving the existing acoustic quantity. After one accepts the benefits of the ground boundary, wall and corner loading is considered. The maximum additional SPL for each boundary is 6db each. So with a sub in the middle of the room, the floor adds 6db. Moving it to a mid-wall position would add an additional 6 db, totaling 12db. Full corner loading would yield 18db. However, one must subtract whatever movement is incurred by the boundary surfaces not being entirely inert. Any movement results in losses. A full reinforced concrete wall basement structure would typically yield the full 18db for corner loading. One must be mindful that boundary gain is limited to fully supporting 1/8 the wavelength. A sub-woofer crossed over at 80hz, with the 80hz wavelength being about 14 feet, the driver's acoustic center needs to be within 20 inches of the boundary to retain the entire benefit of the effect.
Now 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 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.