Reverberation Time Calculator

Reverberation time, commonly expressed as RT60, is one of the most fundamental parameters in architectural acoustics. It describes how long it takes for sound energy in a room to decay by 60 decibels after the sound source stops. A short RT60 produces a dry, intimate acoustic environment suited to speech clarity and recording studios. A long RT60 creates a lush, reverberant quality valued in concert halls and cathedrals. Understanding and controlling RT60 is central to the design of any space where sound quality matters.

What Is RT60?

RT60 stands for Reverberation Time — specifically, the time in seconds required for the sound pressure level in a room to fall by 60 dB after the source ceases. The 60 dB threshold was chosen because it corresponds to the practical limit of audibility: a sound that was initially loud enough to be clearly heard at a typical level becomes inaudible as it decays to the room noise floor.

Reverb is caused by the repeated reflection of sound waves off walls, floors, ceilings, and objects within a room. Each reflection carries slightly less energy than the previous one, because absorptive surfaces convert some sound energy into heat. The rate of energy decay determines RT60: rooms with many absorptive surfaces (carpets, acoustic panels, upholstered furniture) have short RT60 values, while rooms with reflective surfaces (concrete, glass, tile) have long RT60 values.

Sabine's Formula

In the early 1900s, Wallace Clement Sabine — considered the founder of architectural acoustics — derived an empirical formula relating RT60 to room volume and total acoustic absorption: RT60 = 0.161 × V / A. In this formula, V is the room volume in cubic meters, A is the total absorption in sabins (m²), and 0.161 is a constant derived from the speed of sound in air at room temperature and the natural logarithm relationships of energy decay.

The absorption A is calculated as the sum over all surfaces: A = Σ(αᵢ × Sᵢ), where αᵢ is the Sabine absorption coefficient of surface i (a dimensionless value from 0 to 1) and Sᵢ is the area of that surface in m². An absorption coefficient of 0 means a perfectly reflective surface, while α = 1.0 means a completely absorptive surface (an open window is the classic example). Typical materials range from α ≈ 0.02 for smooth concrete to α ≈ 0.90 for fiberglass insulation panels.

Absorption Coefficients by Material

The absorption coefficient α varies significantly with frequency — most materials absorb more at high frequencies than at low frequencies. For practical room acoustic calculations, mid-frequency values (typically measured at 500 Hz or averaged over the 250–2000 Hz octave bands) are commonly used as a representative figure. Concrete and brick are among the least absorptive common building materials, with α values of 0.02 and 0.03 at mid-frequencies. Glass has α ≈ 0.04, and plaster sits at α ≈ 0.05.

Soft furnishings introduce meaningful absorption. Thin carpet may reach α ≈ 0.20 at mid-frequencies, while thick carpet with a pad can achieve α ≈ 0.55. Heavy curtains provide α ≈ 0.50. Dedicated acoustic treatment products — acoustic ceiling tiles (α ≈ 0.70), acoustic foam (α ≈ 0.80), and fiberglass insulation panels (α ≈ 0.90) — provide the highest absorption per unit area. The placement and coverage of absorptive materials significantly affects both the total absorption and the spatial distribution of the reverberant field.

Ideal RT60 by Room Type

Different uses require different reverberation times. Recording studios benefit from very short RT60 values (0.2–0.4 seconds) for clean, controlled sound capture. Home theaters typically target 0.3–0.5 seconds to balance dialogue clarity with cinematic immersion. Classrooms are recommended to stay within 0.4–0.6 seconds for speech intelligibility, and offices function well at 0.5–0.8 seconds.

Concert halls optimized for orchestral music traditionally fall in the 1.5–2.5 second range, with some historic European halls reaching even longer. Cathedrals and large worship spaces may have RT60 values from 2.0 to 6.0 seconds or more, creating the distinctive reverberant quality associated with choral music and organ recitals.

Limitations of Sabine's Formula

Sabine's formula assumes a diffuse sound field — meaning sound energy is uniformly distributed throughout the room and reflections arrive from all directions with equal probability. This assumption holds reasonably well in large, reverberant rooms with relatively low absorption. In highly absorptive rooms (recording studios with extensive treatment), Sabine's formula tends to overestimate RT60.

For rooms with high average absorption, the Eyring formula provides a more accurate result: RT60 = −0.161 × V / (S_total × ln(1 − ᾱ)), where ᾱ is the mean absorption coefficient and S_total is the total surface area of the room. At low absorption levels, the Eyring formula converges to Sabine's formula. Additional corrections for air absorption become important in large spaces at high frequencies.

How to Control RT60

To reduce RT60, increase total absorption by adding carpet, acoustic ceiling tiles, fabric-wrapped absorption panels, or heavy drapes. Doubling the absorption halves the RT60. Rooms that start with very high RT60 (concrete gymnasium, empty auditorium) can be significantly improved with relatively modest treatment. Professional studios often require purpose-built acoustic panels, bass traps, and diffusers.

To increase RT60, reflective surfaces can be added or absorptive materials removed. Some concert halls include variable acoustics systems — motorized absorptive panels or adjustable curtains — that allow RT60 to be tuned for different performance types.

Measuring RT60 in Practice

RT60 is measured using an impulse or interrupted noise method. A loud impulsive sound (a starter pistol, balloon burst, or calibrated loudspeaker) is produced, and the subsequent decay is recorded with a calibrated microphone. Acoustic analysis software plots the energy decay curve and calculates the RT60 by linear regression. Because measuring a full 60 dB decay is sometimes impractical in noisy environments, related measures like T20 and T30 are often used as extrapolated substitutes.

Modern building acoustic standards often require RT60 measurements across multiple octave bands (125 Hz to 4 kHz) at multiple receiver positions. Room acoustic simulation software can predict RT60 before a building is constructed, allowing architects and acoustic consultants to optimize designs without physical prototyping.