A single source near a wall creates a virtual image source that interferes

Evidence

There are several acoustical situations which can cause interference effects. The obvious ones are when two loudspeakers radiate the same sound into a room, or when the same sound is coupled into a room via two openings which are separated. A less obvious situation is when there is a single sound source spaced away from a reflecting boundary, either bounded or unbounded. In this situation an image source is formed by the reflection and thus there are effectively two sources available to cause interference, as shown in Figure 1.28. This latter situation can often cause problems for recording or sound reinforcement due to a microphone picking up a direct and reflected sound component and so suffering interference. — David Howard, Jamie Angus, coustics and Psychoacoustics, 361

Categorization

• XREF: Explains why mics and speakers near walls misbehave — common recording-room lore given a mechanism.