Reflections arriving later than about 20 ms are heard as a distinct echo

Evidence

However, this must be achieved while satisfying the need for even diffuse reverberation, and so the rear wall in such situations must have some explicit form of diffusion structure on it to assure this. 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: Defines the ‘initial time-delay gap’ as the interval between direct sound and first reflection, the key variable governing perceived spaciousness.
• SHARE: A recording-nerd friend who obsesses over room treatment would enjoy the concrete 8-20 ms window and the 20 ms echo threshold.