T20

T20 is a reverberation time measurement derived from the portion of the decay curve between −5 dB and −25 dB below the initial level, then extrapolated to a 60 dB decay. Defined in ISO 3382-2:2008 §4.2, T20 requires a signal-to-noise ratio of at least 35 dB. Because it uses a smaller section of the decay curve than T30, T20 is more tolerant of background noise in field measurements. It is the most commonly reported reverberation time in ordinary room assessments. In practice, T20 and T30 should agree within 10%; larger discrepancies indicate non-exponential decay or measurement problems such as inadequate dynamic range.