Each added boundary surface boosts sound intensity by 3 dB up to three boundaries

Evidence

2. By placing the speaker carefully, with respect to the boundaries, one can eliminate some of the variation in the response. However, one still has a 9 dB rise in response at low frequencies. This is well known and the 9 dB rise at low frequencies is inherent in operating in an environment with three boundaries. In general, the lowfrequency responses of loudspeakers designed to be placed close to boundaries is tailored to compensate for it. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2193 (#corroborates, via linked claim)

Categorization

• XREF: connects to: Adding equal sound intensities yields a 3 dB total increment; A reflecting wall doubles total sound power output from any source; Horns and boundaries both concentrate sound into restricted solid angles (+8 more)

Broader Connections

• Operating a loudspeaker near three room boundaries imposes a 9 dB low-frequency rise #corroborates (0.95) — David Howard, Jamie Angus, coustics and Psychoacoustics, 2193
• Sound intensity falls off as distance squares only in free field, boundary-free conditions #qualifies (0.78)
• Surface reflection reduces total sound power and favors high frequencies #complicates (0.75)