Sound Pressure Level and Reverberation Time

Spreading of Sound

Outdoors, a spherical sound wave can expand as much as it needs to. Indoors, however, the energy is trapped. Reverberation is the result of sound waves repeatedly reflecting off the surfaces of an enclosed space. Because the space is enclosed, the only way for the sound to dissipate is through atmospheric and material absorption, rather than spreading. Reverberation time (RT) is the time it takes for the sound pressure level (SPL) in a room, measured in decibels, to decrease by a stated amount, after a sound source stops emitting sound. RT60 measures the time it takes for the SPL to decrease by 60 decibels.

In general, rooms with mostly hard surfaces (Low NRC) tend to have high reverberation times, while rooms with many soft surfaces (High NRC) tend to have low reverberation times. The optimal RT60 for conference rooms is around 0.6s, according to the Well Building Institute.