If a sound is created in a room and then the sound is abruptly stopped, the reflections will linger in the room for a short period thereafter. This is particularly noticeable in a church, for example, where the sound may be heard for several seconds while it fades away.
A reverberation time measurement is used to calculate the time required for the sound to "fade away". That is, for the sound pressure to reduce by a predefined value. RT60 is the standard reverberation time measurement and is defined as the time it takes for the sound pressure level to reduce by 60 dB, measured from the moment the generated test signal is abruptly ended. The sketch below visualizes the basic principle of an RT60 measurement.
The RT60 reverberation time measurement is defined in the ISO 3382-1 standard for performance spaces, and the ISO 3382-2 standard for ordinary rooms. RT60 is measured in seconds and often stated as a single value. The actual measurements span the frequency band from 50 Hz to 8 kHz with 1/1 or 1/3rd octave resolution. Rooms have individual absorption capabilities for each frequency, so the RT60 values within each band will vary.
Typically the ambient noise in a room would create a noise floor of 40-50 dB. To measure a decay of 60 dB from a sound source, we have to inject the sound at 75 dB (with 15 dB headroom) above this noise floor. Creating such sound at 125 dB across the whole spectrum, and particularly at low frequencies, is technically often not feasible.
In practice, therefore, we measure the time taken for the reflections to decay by 20 dB or 30 dB only. If the decay is acceptably linear, these readings can then be linearly extrapolated to a decay time of 60 dB. Thus the reverberation time RT60(T20) is calculated as 3 * (time to decay by 20 dB) and RT60(T30) is calculated as 2 * (time to decay by 30 dB).