Reverberation time (RT60, also written T60) is the time it takes for a sound in an enclosed space to decay by 60 dB after the source is stopped. It is one of the most fundamental parameters in room acoustics, building acoustics, and speech intelligibility assessment. Rooms with long reverberation times are often described as "reverberant" or "echoey" — they may sound impressive for orchestral music but are problematic for speech intelligibility, occupational noise, and learning environments. Standardised measurement procedures for reverberation time are defined in ISO 3382-1 (performance spaces) and ISO 3382-2 (ordinary rooms).
Reverberation time is determined by the room volume, the amount and distribution of sound-absorbing material in the room, and the frequency of the sound. Low frequencies typically have longer reverberation times than high frequencies. RT60 varies significantly between different room types: anechoic chambers have RT60 near zero; recording studios are designed for 0.3–0.5 seconds; classrooms should have 0.5–0.8 seconds for good speech intelligibility; concert halls typically range from 1.5 to 2.5 seconds for orchestral music; cathedrals and large reverberant spaces may have RT60 values of 5–10 seconds or longer.
Reverberation time can be measured using two main excitation methods: the interrupted noise method (a steady-state noise source is switched off and the decay is measured) and the impulse response method (a short impulsive sound is generated and the decay is extracted from the impulse response). The BRELUXE05 omnidirectional source is used for the interrupted noise method; the Bedrock BIMK balloon pistol kit is used for the impulse response method. Both methods are accepted by ISO 3382 and ISO 16283.
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