T30

T30 is a reverberation time measurement evaluated from the decay curve between −5 dB and −35 dB below the initial level, then extrapolated to a 60 dB range. Specified in ISO 3382-1:2009 §4.1 and ISO 3382-2:2008 §4.2, T30 requires a minimum signal-to-noise ratio of 45 dB. It offers better accuracy than T20 for rooms with good acoustic isolation because it uses a larger portion of the decay curve. T30 is the preferred parameter for performance spaces such as concert halls and theatres. If the room has high background noise, T20 may be more reliable. Both T20 and T30 should be compared to check for double-slope decay behavior.