1. Sound produced in an enclosure does not die out immediately after the source has ceased to produce it. A sound produced in a hall undergoes multiple reflections from the walls, floor and ceiling before it becomes inaudible. A person in the hall continues to receive successive reflections of progressively diminishing intensity. This prolongation of sound before it decays to a negligible intensity is called reverberation.
2. Reverberation time is defined as the time interval between the initial direct note and the reflected note up to a minimum audibility level.
3. More precisely, Reverberation time is defined as the time required, in seconds, for the average sound in a room to decrease by 60 dB after a source stops generating sound.

Reverberation role in acoustics

1. If a hall is to be acoustically satisfactory, it is essential that it should have the right reverberation time.
2. The reverberation time should not be either too long or too short. A very short reverberation time makes a room 'dead'.
3. On the other hand, a long reverberation time renders speech unintelligible. The optimum value for reverberation time depends on the purpose for which the hall is designed.
4. A reverberation time of 0.5 secs is acceptable for speeches and lectures, while a reverberation time of 1 to 2 secs is satisfactory for concerts.
5. In case of theatres, the optimum value varies with volume. 1.1 to 1.5 secs is suitable for small theatres, whereas for large theatres it may go up to 2.3 sees.