Skip to content
RFrftools.io
Audio

RT60 Reverberation Time Calculator

Calculate room reverberation time (RT60) using Sabine and Eyring equations. Enter dimensions and average absorption coefficient to get decay time, critical distance, and Schroeder frequency for acoustic treatment design.

Loading calculator...

Formula

T60=0.161⋅VS⋅αˉT_{60} = \frac{0.161 \cdot V}{S \cdot \bar{\alpha}}
V— Room volume (m³)
S— Total surface area (m²)
ᾱ— Average absorption coefficient (0–1)
A— Total absorption (S × ᾱ) (sabins (m²))

How It Works

RT60 (Reverberation Time 60) measures how long sound takes to decay by 60 dB after the source stops. It is the single most important acoustic parameter for room design, affecting speech intelligibility, music clarity, and recording quality. The Sabine equation (Wallace Clement Sabine, 1898) gives RT60 = 0.161V/A, where V is room volume in cubic meters and A is total absorption in sabins (m2). This assumes diffuse sound field and low absorption. The Eyring equation (Carl F. Eyring, 1930) corrects for higher absorption: RT60 = 0.161V/(-S*ln(1-alpha_avg)), where S is total surface area and alpha_avg is average absorption coefficient. Eyring converges to Sabine for small alpha but is more accurate when alpha_avg > 0.2. The Schroeder frequency marks the transition between modal behavior (discrete room modes dominate) and diffuse field (statistical acoustics apply). Below this frequency, room modes create uneven response that absorption panels cannot fix; only bass traps or room geometry changes help. Critical distance is where direct and reverberant sound levels are equal; beyond this distance, reverberation dominates perception. Standards: ISO 3382-1 (performance spaces), ISO 3382-2 (ordinary rooms), ANSI/ASA S12.60 (classrooms require RT60 < 0.6s).

Worked Example

Problem

A home studio control room measures 5m x 4m x 2.7m. Current surfaces: concrete walls, carpet floor, plasterboard ceiling. Calculate RT60 and determine if acoustic treatment is needed for mixing.

Solution
  1. Room dimensions: L=5m, W=4m, H=2.7m
  2. Volume: V = 5 x 4 x 2.7 = 54 m3
  3. Surface areas: Floor/ceiling = 2 x 20 = 40 m2, Walls = 2 x (5x2.7 + 4x2.7) = 48.6 m2, Total S = 88.6 m2
  4. Absorption coefficients (1 kHz): Concrete walls alpha=0.04, Carpet floor alpha=0.3, Plasterboard ceiling alpha=0.05
  5. Total absorption: A = (48.6 x 0.04) + (20 x 0.3) + (20 x 0.05) = 1.94 + 6.0 + 1.0 = 8.94 sabins
  6. Average absorption: alpha_avg = 8.94 / 88.6 = 0.101
  7. Sabine RT60: T60 = 0.161 x 54 / 8.94 = 0.97 seconds
  8. Eyring RT60: T60 = 0.161 x 54 / (-88.6 x ln(1-0.101)) = 8.694 / 9.42 = 0.92 seconds
  9. Schroeder frequency: fs = 2000 x sqrt(0.97/54) = 268 Hz
  10. Critical distance: Dc = 0.057 x sqrt(54/0.97) = 0.43 m
Assessment: RT60 = 0.92s is too long for a mixing room (target 0.3-0.4s). Adding 12 m2 of acoustic panels (alpha=0.8) raises total absorption to 8.94 + 12x(0.8-0.04) = 18.06 sabins, giving RT60 = 0.161 x 54/18.06 = 0.48s. Adding bass traps in corners would bring it to the target range.

Practical Tips

  • ✓Target RT60 values by room use: recording studio control room 0.3-0.4s, podcast/voiceover booth 0.2-0.3s, home theater 0.4-0.6s, classroom 0.4-0.6s (ANSI S12.60), concert hall 1.5-2.2s, church/cathedral 2-5s. For speech intelligibility, RT60 must stay below 0.6s per ANSI S12.60; above 1.0s, word recognition drops below 85%.
  • ✓Quick absorption coefficient reference (at 1 kHz): bare concrete 0.02-0.04, glass window 0.03-0.05, plasterboard on studs 0.05-0.1, carpet on concrete 0.3-0.4, heavy curtains (draped) 0.5-0.7, 50mm rockwool panel with air gap 0.7-0.9, specialized acoustic foam 0.8-0.95. Furniture, people, and equipment also contribute absorption (a person = ~0.5 sabins at 1 kHz).
  • ✓The critical distance tells you microphone placement: record closer than Dc for dry/direct sound, farther than 3x Dc for ambient/room sound. In an untreated bedroom (Dc ~ 0.4m), you must record within 40cm for clean vocals. Treatment that doubles Dc to 0.8m gives much more freedom for microphone technique and movement.
  • ✓Budget acoustic treatment priority: (1) bass traps in corners first, they address the most problematic modes; (2) first reflection points on side walls and ceiling; (3) rear wall diffusion or absorption; (4) ceiling cloud above listening position. Per-panel cost effectiveness is highest for DIY rockwool panels (2-4x absorption per dollar vs commercial foam). 100mm thickness with 50mm air gap covers down to 200 Hz.

Common Mistakes

  • ✗Applying Sabine equation in highly absorptive rooms (alpha_avg > 0.3) where it significantly overestimates RT60. The Sabine equation assumes energy lost per reflection is small, breaking down when surfaces absorb most of the incident energy. Use Eyring for treated rooms, studios, and anechoic environments. The difference can exceed 30% at alpha_avg = 0.5.
  • ✗Ignoring the Schroeder frequency when planning acoustic treatment. Absorptive panels and diffusers only work in the diffuse field (above Schroeder frequency). Below it, discrete room modes dominate and require bass traps, membrane absorbers, or Helmholtz resonators. A typical small room has Schroeder frequency around 200-400 Hz, meaning standard foam panels do nothing for bass problems.
  • ✗Using a single RT60 value without specifying frequency. RT60 varies significantly with frequency: untreated rooms typically have RT60 2-3x longer at 125 Hz than at 4 kHz due to the frequency-dependent absorption of common materials. Always specify RT60 at octave bands (125, 250, 500, 1k, 2k, 4k Hz). ISO 3382 requires measurement at minimum 6 octave bands.
  • ✗Placing acoustic treatment uniformly on all walls. Absorption should be distributed to avoid flutter echoes (parallel reflective surfaces) while maintaining some reflections for natural ambience. The reflection-free zone (RFZ) design places absorption at first reflection points only, keeping rear wall partially reflective for diffusion. IEC 60268-13 studio standard recommends non-uniform treatment distribution.

Frequently Asked Questions

For a mixing/control room: 0.3-0.4 seconds provides accurate monitoring with minimal coloration. For a recording/tracking room: 0.4-0.6 seconds retains some natural ambience without muddying recordings. For a voiceover/podcast booth: 0.2-0.3 seconds gives the driest possible sound. These targets apply at mid-frequencies (500 Hz - 2 kHz); bass RT60 is typically 1.5-2x higher and requires dedicated bass trapping to control.
Sabine (1898) assumes low absorption and a perfectly diffuse field: RT60 = 0.161V/A. It overestimates RT60 when average absorption exceeds 0.3. Eyring (1930) accounts for the logarithmic nature of absorption: RT60 = 0.161V/(-S*ln(1-alpha)). Use Sabine for untreated rooms (alpha < 0.2) and initial estimates. Use Eyring for treated studios, acoustically designed spaces, or any room with significant absorption. For a room with alpha=0.5, Sabine gives RT60 = 0.322V/S while Eyring gives 0.232V/S (28% lower and more accurate).
Method 1 (balloon pop): Pop a balloon, record the decay with a measurement microphone, analyze in REW (Room EQ Wizard, free software). Method 2 (sine sweep): Use REW with a calibrated mic (UMIK-1, ~$80) to play a logarithmic sine sweep and measure impulse response. REW calculates RT60 (T20 or T30 extrapolated to 60 dB) per ISO 3382. Measure at 3+ positions; average results. Octave-band analysis (125 Hz to 8 kHz) reveals frequency-dependent problems. T30 (extrapolated from 30 dB decay) is preferred over T60 (requires 60 dB dynamic range, hard to achieve in small rooms).
The Schroeder frequency (fs = 2000*sqrt(RT60/V)) marks where room acoustics transition from discrete modes (below) to diffuse statistical behavior (above). Below fs, sound is dominated by room resonances (modes) that create peaks and nulls at specific positions. Standard acoustic panels only work above fs. For a typical bedroom studio (30 m3, RT60=0.8s): fs = 2000*sqrt(0.8/30) = 327 Hz. This means standard foam panels do nothing below 327 Hz; you need bass traps, membrane absorbers, or room geometry changes for low-frequency problems.

Related Articles

Audio Engineering

Passive Speaker Crossover Design Guide

Master passive speaker crossover design with precise calculations, component selection, and real-world engineering insights for audio electronics projects.

Related Calculators

Audio

Room Modes

Calculate room axial modes using f = n·c/(2L). Find all standing wave frequencies, Schroeder frequency, and mode spacing for acoustic treatment and speaker placement.

Audio

Speaker SPL

Calculate speaker SPL at any power and distance from the rated sensitivity (dB/W/m) specification.

Audio

EQ Q Factor

Free EQ Q factor calculator — enter center frequency and bandwidth to get Q, octaves, and 3dB points. Convert between Q factor, fractional bandwidth, and octave bandwidth for parametric equalizer design.

Audio

Delay Time

Calculate audio delay time in milliseconds from BPM and note value — quarter, eighth, dotted, triplet. Also computes acoustic propagation delay from speaker distance for live sound alignment.