W. C. Sabine, the acoustician, has defined reverberation time technically. It is the time that elapses from the instant a sound source is switched off until its energy decays to one millionth of its original strength (a drop of 60 decibels). As far as the ear is concerned, it is the time it takes for a sound to melt and be lost in the ambient noise. Echo differs from reverberation in that it is a repetition or partial repetition of a sound, due to reflection off a distant surface. Reverberation is also reflected sound but no separate repetitions of the original are distinguishable.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 446