Reflections from adjacent walls create destructive interference nulls at one-half wavelength spacing.

Evidence

Figure 13.2 shows an acoustical interference dip at 188 Hz, caused by reflected sound from an adjacent boundary, the wall behind the loudspeaker in this case, destructively interfering with the direct sound radiated by the loudspeaker—the loudspeaker is omnidirectional at these frequencies. It shows up clearly in these outdoor measurements with only adjacent boundaries. In rooms there are multiple boundaries, some adjacent to and none very far away from the loudspeaker, each contributing something to variations in frequency response at listening locations. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1461

Categorization

• XREF: Connects to the standing-wave and room-mode material of Chapter 14; the 188 Hz null is the direct manifestation of the standing-wave pattern concept in a simpler, adjacent-boundary case.