Early reflections must arrive within 20 ms or they are perceived as echoes.

Evidence

The initial time-delay gap in the listening should be as large as possible, but it is clearly limited by the time it takes sound to get to the rear wall and back to the listener. Ideally this gap should be 20 ms, but it should not be much greater, or it will be perceived as an echo. In most practical rooms, this requirement is automatically satisfied, and initial time-delay gaps in the range of 8 ms to 20 ms are achieved. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2290

Categorization

• DEFINE: Clarifies the initial time-delay gap concept and its perceptual limit.
• XREF: Connects to the Haas effect/precedence effect found in other psychoacoustics literature on how the first-arriving sound dominates localization.