Reverberation Time (RT60): The Sabine Equation and Its Limits
RT60 defined properly, the Sabine equation derived from a room's absorption, a worked example, and exactly why Sabine becomes unreliable in small, non-diffuse rooms.
Room Modes covers a room's response at specific, discrete frequencies. Reverberation is a different, related phenomenon: not a single resonant frequency, but the accumulation of thousands of reflections arriving from every surface, decaying over time after a sound stops. RT60 is how that decay gets reduced to one usable number.
What RT60 actually measures
RT60 is defined as the time it takes sound pressure level to decay by 60 dB after the source stops. Sixty decibels isn't an arbitrary round number — it corresponds to a millionfold drop in sound intensity (recall from What Is Sound that every 20 dB is a factor of 10 in pressure amplitude, so 60 dB is 10³ = 1000× in pressure, or 10&sup6; in intensity/power) — roughly the range from a loud transient down to something close to the room's own noise floor. A shorter RT60 means a more controlled, acoustically "dead" room; a longer RT60 means a more reverberant, "live" one.
The Sabine equation
The standard way to estimate RT60 from a room's construction, developed by Wallace Sabine in the early 1900s from empirical measurements of real rooms, is:
- RT60 — reverberation time, in seconds
- V — room volume, in cubic meters (m³)
- A — total absorption, in metric sabins (m²) — every surface's area multiplied by that surface's absorption coefficient, summed across the whole room
Each surface contributes S × α to the total — its area S (in m²) multiplied by its absorption coefficient α, a number between 0 and 1 describing what fraction of incident sound energy that material absorbs rather than reflects (0 = perfectly reflective, 1 = perfectly absorptive, both idealized extremes). A bare painted wall might have α around 0.02-0.05 at most frequencies; a thick acoustic panel can reach α above 0.8 in its effective range.
Worked example
The same 4.6 × 3.4 × 2.45 m bedroom studio from the Room Modes article has a volume V = 4.6 × 3.4 × 2.45 ≈ 38.3 m³. Say its combined surfaces (walls, floor, ceiling, furniture, and a modest amount of added treatment) work out to a total absorption A ≈ 12 metric sabins at 500 Hz — a plausible lightly-treated small room.
RT60 = 0.161 × 38.3 / 12 ≈ 0.51 seconds — inside the commonly-cited 0.3-0.6 second range often targeted for small mixing rooms, though see the limitation below before treating that range as a hard target.
Why Sabine has a real, well-documented limit
Sabine's equation was derived assuming a statistically diffuse sound field: reflections arriving from every direction in roughly equal proportion, with absorption spread fairly evenly across all surfaces. Concert halls and large, irregularly-shaped rooms with distributed treatment approximate this reasonably well. A small home studio, with six flat parallel-ish surfaces and absorption concentrated in a few specific spots, generally does not.
The practical marker for where this breaks down is the Schroeder frequency — below it, a room's response is dominated by individually countable, sparse room modes (see the previous article) rather than a smooth statistical reverberant decay; above it, modes are dense enough to behave more like the diffuse field Sabine assumes:
- fs — Schroeder frequency, in Hz
- RT60 — reverberation time, in seconds
- V — room volume, in m³
For the worked example above: fs ≈ 2000 × √(0.51/38.3) ≈ 2000 × 0.115 ≈ 231 Hz. That's a strikingly large fraction of a mix's fundamental energy sitting below the frequency where Sabine's own assumptions hold. This isn't a reason to distrust RT60 as a concept — it's a reason to treat a single calculated RT60 number as a genuinely useful planning estimate, and not as a lab-grade measurement of a small room's actual low-frequency behavior, which is dominated by the discrete modes Room Modes describes instead.
What actually changes RT60
More absorption anywhere in the room lowers RT60 — but not uniformly across frequency, because absorption coefficients themselves are frequency-dependent (see Acoustic Treatment for the physical mechanism). Thin panels absorb high frequencies efficiently but do very little in the bass, so a room can have a short RT60 at 2 kHz and a much longer one at 100 Hz simultaneously — which is exactly why this site's RT60 Calculator reports per-band results rather than one blended average.
Common questions
Is a shorter RT60 always better?
No. An RT60 near zero (an anechoic chamber) sounds unnaturally dead and is uncomfortable to work in for extended periods. Commonly-cited target ranges for small mixing/production rooms sit around 0.3-0.6 seconds, with the specific target depending on room use and size — a vocal booth generally wants shorter than a room used for general listening.
Why does my measured RT60 disagree with a calculated one?
Sabine's equation assumes a diffuse field and evenly distributed absorption, and, as this article explains, small rooms consistently violate both assumptions, especially below the Schroeder frequency. A real measurement (with a calibrated microphone and RT60 measurement software) reflects your room's actual, non-ideal behavior; treat disagreement between measured and calculated RT60 as expected in a small room, not as a sign either number is wrong.
What's the difference between RT60 and a room mode?
RT60 describes overall decay time from the accumulation of many reflections across the whole frequency range. A room mode is a single specific resonant frequency created by standing waves between parallel surfaces (see Room Modes). They're related — more absorption reduces both — but they describe different physical phenomena and neither one substitutes for the other.
Further reading
- Sabine, W.C., Collected Papers on Acoustics (1922) — the original empirical work behind the equation.
- Kinsler, Frey, Coppens & Sanders, Fundamentals of Acoustics — Schroeder frequency and diffuse-field assumptions covered in depth.
- Methodology — the exact absorption reference table and per-band Sabine calculation used by this site's RT60 Calculator.
Put this into practice
Run this on your own room -- the tool this article's physics actually explains.