The direct-to-reflection gap must stay under 30ms or become audible echoes.

Evidence

A gap between the direct sound and first reflections. This happens naturally in most spaces and gives a cue as to the size of the space. The gap should not be too long—less than 30 ms—or the early reflections will be perceived as echoes. Some delay, however, is desirable as it gives some space for the direct sound and so improves the clarity of the sound, but a shorter gap does add “intimacy” to the space. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2260

Categorization

• XREF: Connects to the precedence/HAAS effect literature on how the ear fuses direct and reflected sound within a window.