Reverberation Time

The subjective feeling of reverberation is called reverberance, which is best known of all the subjective room acoustic aspects. When a room has too much of this property, speech loses intelligibility because important details, i.e., consonants, are masked by louder, lingering vowels. In music listening condition, however, reverberance adds attractive fullness to the sound by binding adjacent notes and blending sounds from different instruments in ensemble.

The reverberation time, $T$, is a traditional objective measure of this subjective quality. It is defined as the time it takes for sound to decrease by 60 dB after a sound source has been switched off. In practice, the evaluation of the reverberation time can typically be done by three ways: using 10 dB decay range (0 to -10 dB), 20 dB(-5 to -25 dB), and 30 dB(-5 to -35 dB). They are called the early decay time ($EDT$), $T_{20}$, $T_{30}$, respectively. Note that all three measures are corresponding to a 60 dB decay range, e.g., EDT is 6 times the time duration for the sound to lose the first 10 dB, etc. When the sound decay rate is unchanged over time, the three reverberation times must be identical.

Reverberance is known to be best represented by $EDT$. All these reverberation time measures are calculated using the energy decay curve by Schroeder's backward integration and via a least-square linear fitting.