How to Measure Reverberation Time (T20/T30): Methods, Equipment, and Standards

Reverberation time is the most widely used descriptor of room acoustic quality. It measures the rate of sound level decay after a source is extinguished, expressed as the time for the level to drop by 60 dB (T60). In practice, T60 is rarely measured directly — the background noise level in most rooms limits the usable decay range to 20 or 30 dB, from which T20 or T30 are extrapolated by linear regression. ISO 3382-1 covers concert halls and performance spaces; ISO 3382-2 covers ordinary rooms including offices, classrooms, and meeting rooms.

Measurement Methods

• Interrupted noise method — play broadband or 1/3 octave band noise through an omnidirectional loudspeaker, then suddenly switch it off and capture the decay. Requires a powerful source to achieve adequate signal-to-noise ratio.
• Integrated impulse response method — measure the room impulse response using a starter pistol, balloon burst, or swept-sine excitation, then integrate backwards from the noise floor (Schroeder integration). More practical for most spaces — no powerful loudspeaker required.

Measurement Parameters

• T20: time for sound level to decay from -5 to -25 dB below initial level — suitable for noisy environments with limited signal-to-noise
• T30: time for sound level to decay from -5 to -35 dB — provides higher accuracy but requires SNR ≥ 45 dB
• EDT (Early Decay Time): extrapolated from the first 10 dB of decay — correlates with perceived reverberance in occupied rooms
• C80 and C50 clarity indices — derived from impulse response; measures early-to-late energy ratio

Equipment Requirements

• Calibrated omnidirectional measurement microphone — Class 1 required for ISO 3382 compliant results
• Minimum 6 source-receiver combinations per ISO 3382-2; all receiver positions must be measured
• Multi-channel simultaneous recording — allows all receiver positions to be measured in one sweep
• Measurement software with backward integration (Schroeder) and linear regression for T20/T30

The PLACID PQ801 8-channel DAQ with 8 Class 1 microphones and PQ Analyst software provides ISO 3382-2 compliant reverberation time measurement in a single multi-position sweep. Contact PLACID for details of the building acoustics system configuration.

Uncertainty in Reverberation Time Measurement

Reverberation time has a well-characterised statistical uncertainty arising from the finite number of decay measurements. ISO 3382-2 Annex B gives the formula for the standard deviation of T20 and T30 estimates as a function of the number of averages and the bandwidth of the analysis. For building acoustic compliance measurements, a sufficient number of decay measurements must be taken at each source-receiver position to achieve adequate statistical confidence in the result. The measurement report should state the number of averages and the resulting expanded measurement uncertainty so that compliance with the design target can be assessed with appropriate confidence.