Reflections from a nearby hard wall can double the sound amplitude toward the listener

Evidence

This is true, but if the loudspeaker is close to the wall, something much more remarkable happens. When sound reflects off a hard surface, compressions are reflected as compressions, and rarefactions as rarefactions. If the distance to the wall is short enough, such that the round-trip from the loudspeaker to the wall and back to the loudspeaker is under about a quarter of a wavelength, the reflected wave has not changed phase very much and adds in-phase (constructively) with the direct wave. Using fλ = c, the “quarter-wave or less” round-trip requirement translates to frequencies f < c/8d, where d is the distance to the wall. The result is that the wave amplitude heading toward the listener is about twice what it would have been without the wall. — Eric J. Heller, Why You Hear What You Hear- An…, 509

Categorization

• XREF: Connects to why singers/players hear themselves differently near walls, and to the coupling/resonance theme in this book; also connects to how one’s own voice sounds different singing near a wall.