Musicians need sound reflections within twenty milliseconds to play together.

Evidence

There is evidence to show that, in order to maintain good ensemble comfortably, the musicians should receive the sound from other musicians within about 20 ms of the sound being produced. This means that ideally there should be a reflecting or diffusing surface within 10 ms (3.44 m or 11.5 ft) of the performer; the time is divided by 2 to allow for sound going to the reflecting surface and back. … Sometimes, however, the orchestra enclosure is so large that the reflections arrive later than this. If they arrive later than about 50 ms, the musicians will perceive them as echoes and ignore them. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1939

Categorization

• XREF: Directly instantiates the precedence effect / Haas fusion zone from the earlier psychoacoustics material: reflections under ~20 ms fuse with the direct sound, later ones split off as echoes.
• SHARE: The 10 ms / 3.44 m buffer-zone figure is a niche, gratifying fact for anyone who has played in an orchestra pit or designed a band room.