Room Acoustics and Reverberation Time

Overview

Reverberation time (RT60) is the time required for sound pressure level to decay by 60 dB after a source stops in an enclosed space. It is the single most important metric in room acoustics, influencing speech intelligibility, musical clarity, and noise comfort. Proper RT60 depends on room volume, surface materials, and intended use.

Key Formulas

Sabine Reverberation Time

$T_{60} = \frac{0.161 \, V}{A}$

The foundational equation relating reverberation time to room volume and total absorption.

The Sabine formula gives good results for ordinary rooms when the mean absorption coefficient is not greater than about 0.35. For rooms with a mean absorption coefficient greater than 0.35, the Eyring equation is preferred because the logarithmic form better represents heavily absorptive spaces.

Eyring Reverberation Time

$T_{60} = \frac{0.161 \, V}{-S \, \ln(1 - \bar{\alpha})}$

More accurate for rooms with highly absorptive surfaces where average absorption coefficient approaches unity.

Total Absorption

$A = \sum_{i} \alpha_i \, S_i$

Total Sabins of absorption equal the sum of each surface area multiplied by its absorption coefficient at the frequency of interest.

Room Axial Mode Frequency

$f = \frac{c}{2} \sqrt{\left(\frac{n_x}{L_x}\right)^2 + \left(\frac{n_y}{L_y}\right)^2 + \left(\frac{n_z}{L_z}\right)^2}$

Resonant standing-wave frequencies inside a rectangular room, where $n_x, n_y, n_z$ are non-negative integers.

Variables

Symbol	Description	Unit
$T_{60}$	Reverberation time	s
$V$	Room volume	m³
$A$	Total absorption	m² Sabins
$S$	Total surface area	m²
$\alpha$	Absorption coefficient	dimensionless
$\bar{\alpha}$	Average absorption coefficient	dimensionless
$f$	Frequency	Hz
$c$	Speed of sound	m/s
$L_x, L_y, L_z$	Room dimensions	m
$n_x, n_y, n_z$	Mode indices	integer

Absorption Coefficients by Material

Recommended Reverberation Times

Calculators

Sabine Reverberation Time

Absorption from Materials

Unit Converter

RT60 vs Total Absorption

Design Notes

• Frequency dependence. Absorption coefficients vary significantly with frequency. Always evaluate RT60 across the 125 Hz – 4 kHz octave bands, not just at a single frequency.
• Sabine vs Eyring. Use Sabine for typical rooms with $\bar{\alpha} < 0.3$. Switch to Eyring for heavily treated rooms, anechoic spaces, or when $\bar{\alpha}$ exceeds roughly 0.3, as Sabine overestimates absorption at high coefficients.
• Audience and furniture. Unoccupied RT60 can be 20–40 % longer than occupied. Design for the occupied condition and verify that the empty condition remains tolerable.
• Air absorption. At frequencies above 2 kHz and in very large volumes (auditoria, sports halls), molecular air absorption becomes a meaningful additional decay mechanism and is not captured by the Sabine formula.
• Flutter echoes and focusing. Large parallel reflective surfaces or concave walls can produce discrete echoes and focusing even when RT60 is nominally correct. Geometry matters alongside material treatment.
• Room modes. In small rooms (studios, control rooms), low-frequency modes create pronounced peaks and nulls. Mode spacing should be analyzed; widely spaced modes below 300 Hz cause audible coloration.

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Sound - Reverberation Time

References

• Original Source – Engineering ToolBox: Room Acoustics and Reverberation