RT60 (Reverberation Time)
Definition
RT60 (Reverberation Time)
RT60 is the time required for sound energy in a room to decay by 60 decibels after the sound source stops. Defined by ISO 3382-1:2009, RT60 is the primary metric for quantifying room reverberance. SonaVyx measures RT60 in your browser using Schroeder backward integration of impulse responses.
RT60 (Sabine) = 0.161 × V / A, where V = room volume (m³), A = total absorption (m²)
How RT60 Is Measured
RT60 is measured by exciting a room with a sound source (log sine sweep or impulsive noise), recording the impulse response, then applying Schroeder backward integration to derive the energy decay curve (EDC). ISO 3382-1:2009 Clause 5.3 specifies that T20 or T30 extrapolations are used when the full 60 dB decay range is not achievable due to background noise.
Practical Example
A church sanctuary with hard stone walls and minimal furnishings might measure RT60 of 2.8 seconds at 500 Hz, making speech unintelligible for congregants. Adding 40 m² of broadband acoustic panels to the walls and ceiling reduces RT60 to 1.2 seconds, bringing speech transmission index above the 0.60 threshold required for clear communication.
Understanding the 60 dB Decay Criterion
Wallace Clement Sabine established the reverberation time concept in 1898 by measuring the time for sound to become inaudible in Harvard lecture halls. The 60 dB threshold represents roughly the dynamic range between typical speech levels (approximately 65 dB SPL at 1 meter) and the threshold of hearing in a quiet room. This standardized decay range enables consistent comparison between rooms regardless of excitation level.
T20 vs T30 vs EDT
ISO 3382-1 defines three variants of reverberation time. T20 evaluates the decay from -5 dB to -25 dB below peak and extrapolates to 60 dB, requiring at least 35 dB of impulse-to-noise ratio. T30 evaluates from -5 dB to -35 dB and requires 45 dB INR for reliable results. Early Decay Time (EDT) measures the initial 10 dB of decay, which correlates more closely with subjective perception of reverberance in occupied spaces.
Target RT60 Values by Room Type
Optimal reverberation time depends on room function. Speech-focused spaces such as classrooms and conference rooms target 0.4 to 0.6 seconds. Multipurpose halls aim for 1.0 to 1.5 seconds. Concert halls designed for orchestral music may target 1.8 to 2.2 seconds. Recording studios require 0.2 to 0.4 seconds for critical monitoring accuracy. These targets apply at 500 Hz and 1 kHz octave bands.
Frequency-Dependent Behavior
RT60 varies significantly with frequency. Low frequencies (125 Hz and below) typically exhibit longer reverberation times due to reduced absorption by standard building materials. High-frequency decay above 4 kHz is accelerated by air absorption, which increases with humidity. ISO 3382-1 Clause 4.2 recommends measuring across at least six octave bands from 125 Hz to 4 kHz for a complete characterization.
Measurement Requirements
ISO 3382-1 Clause 6 specifies minimum source and receiver positions based on measurement purpose. Engineering-grade measurements require at least two source positions and three receiver positions. The source must be omnidirectional, and the microphone should be at least 1 meter from any reflective surface and at least 2 meters from the source. SonaVyx automates Schroeder integration and validates INR before reporting results.
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