The persistence of sound even after the source is stopped is called reverberation.
Repeated reflections of a sound in a hall will persist until it is reduced to value where it is no longer heard by us. This repeated reflections that persists are called reverberation. In big theatres and auditoriums reverberations are not welcome. So, sound absorbing acoustic materials like fibreboard, rough plaster or draperies are used to line the walls and roofs of these halls.
The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer. In order to provide a reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. The reverberation time can be modeled to permit an approximate calculation. The picture given below is the reverberation room in which one can experience reverberations.
The below graph shows that the persistence of sound exists for a short time period depending upon its amplitude.