Early decay times and echoes

Early decay time: The initial rate of decay of reverberant sound appears to be more important than the total reverberation time. A rapid initial decay is interpreted by the human ear as meaning that the reverberation time is short.

Things to avoid in design include the following:

1. Echoes. An echo is a strong reflected sound that is sufficiently delayed from
   the direct sound that it can be heard as a separate entity rather than as a
   continuation of the original sound. When echoes are heard, the most likely
   culprit is the rear wall of the room.
2. Flutter echoes. Flutter echoes are a series of echoes that occur in rapid
   succession; they usually result from reflections between two parallel
   surfaces that are highly reflective.
3. Sound focusing. Focusing of sound can be caused by reflection from large
   concave surfaces. Certain sounds will be heard too loudly near the focus of
   a curved surface.

Sound shadows. Under balconies at the rear of an auditorium or church there
may be insufficient early sound, since most of the reflections from the side walls
and ceiling do not reach this area even though they are in a direct line of sight to
the performer and therefore receive the direct sound.