Boundary placement boosts sound intensity 3 dB per boundary by increasing directivity

Evidence

From these calculations, we can see that each boundary increases the sound intensity at a point by 3 dB, due to the increased directivity. Note that one cannot use the earlier equations on more than three boundaries because then the sound can no longer expand without bumping into something. … However, it is possible to have directivities of greater than 8 using other techniques. For example, horn loudspeakers with a directivity of 50 are readily available as a standard product from public-address loudspeaker manufacturers. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 321

Categorization

• DEFINE: Explains directivity factor Q and how boundaries convert spherical radiation into hemispherical, quarter-hemispherical patterns.
• XREF: connects to: Operating with three nearby boundaries gives a fixed 9 dB low-frequency rise — David Howard, Jamie Angus, coustics and Psychoacoustics, 2193