How this instrument works
Reverberation time, RT60, is how long a sound takes to decay by 60 decibels after its source cuts off — roughly the gap between a shout and silence, about a millionth of the original sound energy. Sabine's formula puts a number on it: RT60 = 0.161 · V ⁄ A, room volume in cubic metres divided by total absorption in square metres of Sabins, scaled by a constant tied to the speed of sound.
The constant 0.161 is not arbitrary. It falls out of a statistical model of a diffuse sound field: energy bounces between surfaces roughly 4V ⁄ S metres apart on average, loses a fraction of its intensity at each hit, and the whole assembly decays exponentially at a rate set by the speed of sound, about 343 m/s. Work through 24 ln(10) divided by that speed and you land on 0.161 — the volume term grows the distance between reflections, the absorption term speeds the energy loss per bounce, and that is exactly why one sits on top of the fraction and the other on the bottom.
Sabine derived this in 1898 by timing organ pipes decaying in Harvard's Fogg lecture hall, and it remains the working formula for architects sizing lecture halls and acousticians treating home theatres and recording booths. Its blind spot is highly absorptive rooms: as the average absorption coefficient climbs toward 1 — thick foam, deep carpet, a near-anechoic space — the formula keeps predicting a finite decay time when the room should go almost silent instantly. Eyring's 1930 correction fixes that limit; Sabine's version stays accurate and far simpler whenever a room is only lightly to moderately damped, which covers most living rooms, offices, and classrooms.
- Enter the Room volume in cubic metres, or switch the unit menu to cubic feet — measure or estimate it as length times width times height.
- Enter Total absorption (Sabins), the sum of every surface's area times its absorption coefficient — carpet, drapes, upholstery, and an audience all count.
- Read Reverberation time (RT60) in seconds — the time for the room's sound to decay 60 dB after the source stops.
- Change Total absorption (Sabins) alone to see how added treatment — a rug, acoustic panels, more furniture — shortens RT60 without touching the room's volume.
Worked example — a 150 m³ living room
A 150 m³ room — a modest living room or small open-plan office — carries about 30 m² of Sabin absorption from carpet, upholstered furniture, and curtains. Sabine's formula gives RT60 = 0.161 × 150 ⁄ 30 = 0.805 s: just over three-quarters of a second for a clap to die away 60 dB, comfortably conversational and not noticeably echoey.
Double the absorption to 60 m² — heavier carpet, acoustic panels, more soft furnishing — while keeping the same 150 m³ volume, and RT60 halves to 0.4025 s, because absorption sits in the denominator: twice the A, half the decay time. That is the audible difference between a room that sounds 'live' and one that sounds 'dead,' achieved without moving a wall.
Questions
What does RT60 measure exactly?
RT60 is the time for a sound to decay 60 decibels below its starting level after the source stops — roughly a millionth of the original intensity. It is measured by exciting a room with a sharp sound, a clap or a burst of noise, and timing the exponential decay on a sound level meter, or computed directly from a room's volume and absorption with Sabine's formula.
Why does the formula use 0.161 as the constant?
Because it packages the speed of sound into a single number for metric units: 0.161 is 24 ln(10) divided by 343 m/s. In imperial units, feet and square feet, the equivalent constant is 0.049, so a US textbook version of the same physics looks different only because its units differ, not because the underlying acoustics changed.
What counts toward Total absorption (Sabins)?
Every surface's area multiplied by its absorption coefficient, summed across the room. A fully open window has a coefficient of 1 and contributes its full area — that is the original definition of one Sabin. Painted concrete sits near 0.02, thick carpet near 0.3, and a seated audience is one of the most absorptive surfaces a room has.
Does Sabine's formula work in every room?
It works well for lightly to moderately absorptive rooms with a roughly diffuse sound field — most homes, offices, and classrooms. It breaks down as average absorption climbs toward 1, heavy acoustic treatment or a near-anechoic space, where it keeps predicting a nonzero decay time instead of near-instant silence. Eyring's 1930 correction handles that regime; Sabine's version is the right tool whenever a room is not unusually dead.
What is a comfortable reverberation time for a room?
It depends on use. Living rooms and offices sound clear around 0.4 to 0.6 s, classrooms aim for roughly 0.6 to 1.0 s so speech stays intelligible, and concert halls target about 1.8 to 2.2 s so orchestral music has room to bloom. Recording booths and studios often push well under 0.3 s to stay dead for close-miked work.
How much absorption do I need to hit a target RT60?
Rearrange the formula: A = 0.161 × V ⁄ RT60. For the 150 m³ room in the example, targeting a brisker 0.5 s calls for A = 0.161 × 150 ⁄ 0.5 = 48.3 m² of Sabin absorption — about 18 m² more than the room's baseline 30 m², roughly what a few acoustic panels or a heavier rug would add.