T30 and T20 measure RT by extrapolation and assume linear decay
Evidence
For practical reasons measurements are usually made from 5 dB after cessation of sound to 35 dB down, thus avoiding background noise and the need to produce very loud starting sounds. This is called T30, but a linear decay is assumed and it is doubled to get the time to decay 60 dB. A T20 version also exists, but they are all presented as 60 dB decay times. Therefore: T60 = T30 = T20, just measured in different ways. Obviously, if the decay is not linear, the estimate is wrong. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1244
Categorization
Category: Definitional Content
- DEFINE: Clarifies the T30/T20 measurement practice and its linear-decay assumption.