Acoustic Terms and Definitions: Reverberation time
Reverberation time or decay time is a measure of the time required for reflecting sound to "fade away" in an enclosed area after the source of the sound has stopped. Reverberation time is important in defining how a room will respond to acoustic sound.
The optimum reverberation time for a space in which music is played depends on the type of music that is to be played in the space. Rooms used for speech typically need a shorter reverberation time so that speech can be understood more clearly.
See also: what is "reverb"?