When sound is produced in an enclosed space multiple reflections build up and blend together creating reverberation or reverb. This is most noticeable when the sound stops but the reflections continue, decreasing in amplitude, until they can no longer be heard. The time it takes for the sound pressure level of the reverberation to decay 60 decibels is known as the reverberation time, or RT(60).
Large chambers, especially such as cathedrals, gymnasiums, indoor swimming pools, large caves, etc.