Formula and Method for Reverberation Time (RT60)
Reverberation time, commonly written RT60, is the time it takes for sound pressure in an enclosed space to decay by 60 decibels after a source stops. It was defined in 1898 by physicist Wallace Clement Sabine, whose empirical study of lecture halls at Harvard produced the first practical formula linking a room's geometry and finishes to how "live" or "dead" it sounds. This calculator estimates RT60 from room dimensions and an average sound absorption coefficient using Sabine's equation and, optionally, the more conservative Eyring equation.
How the calculation works
From the length, width, and height you enter, the calculator finds the room volume V = L × W × H and the total interior surface area S = 2(LW + LH + WH). Multiplying S by the average absorption coefficient α (a number from 0, fully reflective, to 1, fully absorptive) gives the total absorption A = S × α, measured in sabins (one metric sabin is one square meter of perfectly absorptive material; one imperial sabin is one square foot). Sabine's equation is then RT60 = 0.161 × V / A for metric units (V in m³, A in metric sabins), or RT60 = 0.049 × V / A for imperial units (V in ft³, A in imperial sabins). The Eyring equation instead uses −S ln(1 − α) in place of A, which better matches measured decay times in more absorptive rooms.
Sabine vs. Eyring: which to use
Sabine's equation is simplest and accurate enough for "live" rooms with an average absorption coefficient below about 0.2 — hard-surfaced spaces such as gymnasiums, lobbies, and untreated classrooms. It increasingly overstates RT60 as α rises, and it breaks down at α = 1, where a room with no reflective surfaces should have zero reverberation but Sabine still predicts a small positive time. Eyring's equation corrects for this and is preferred for well-treated rooms, recording studios, and any space with α above roughly 0.2-0.3. Neither formula accounts for air absorption at high frequencies, uneven placement of absorptive material, or unusual room shapes — for those, use frequency-dependent, per-surface calculations or dedicated room-acoustics software.
Real-world applications
- Concert hall and auditorium design targets RT60 around 1.5-2.2 seconds for orchestral music, where a longer decay blends notes and adds warmth.
- Classrooms and conference rooms aim for roughly 0.4-0.6 seconds so speech stays intelligible instead of blurring into echo.
- Recording studios and home theaters are typically treated to bring RT60 below 0.3-0.5 seconds, limiting the coloration picked up by microphones or listeners.
- Building codes and acoustic standards (such as ANSI S12.60 for classrooms) set maximum RT60 limits that designers verify with calculations like this one before construction.