This is an acoustic boost which occurs in rooms sufficiently small and solidly built. If you have a room where the construction is fairly light, or one which is very large and open to other rooms, you may not have any significant room gain. If you have a small room, such as a bedroom, or if it is solidly built ie brick walls and concrete floor, then you may have quite a lot of gain.
Deon Bearden
Engineering Product Designer
Posted the following on diysubwoofers.org
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Room gain is an increase in spl below some threshold frequency, relative to an equivelant voltage / distance measurement in quarter space, halfspace, or free space. It comes from a change in mere boundary reinforcent to actual presurization and rarefractions caused as the woofer cone moves in and out. The frequencies it begins showing itself at are determined by the room's dimensions. The commonly accepted belief is that below the lowest eigentone ( standing wave mode, and can be calculated by dividing 570.5 by the room's longest dimension, this will give you the half wavelength at that freq ) of the room, the room can exhibit a rise in SPL of up to 12db per octave as you go down in frequency. IE if room gain started at 40hz, by 20hz the room could "add" 12db to the subwoofers output. In reallity, listening rooms are fairly lossy, and room gain begins well above most rooms lowest eigentone. I have never measured 12db per octave, about 9 is the best I've seen to date. An average could look like plus 1-2db at 70-80hz, plus 6db at 40-45hz, and plus 9db at 20hz.
