RT60 Acoustic Calculator — Frequently asked

It depends on the room's VOLUME, not on what the room is called — which is the single most common way this number is quoted wrongly. The governed bands are: Boardroom / small meeting room — 0.4–0.6 s (under 50 m³); Office / classroom — 0.5–0.7 s (50–150 m³); Conference room / lecture hall — 0.7–1.0 s (150–400 m³); Auditorium / multi-purpose hall (speech) — 1.0–1.4 s (400–1,000 m³); Concert hall / large auditorium — 1.6–2.2 s (over 1,000 m³). A "boardroom" is the clearest illustration, because the same word covers the first two tiers: a small meeting room under 50 m³ targets 0.4–0.6 s, while a larger one in the 50–150 m³ tier targets 0.5–0.7 s. Both figures are correct; quoting either one against the bare word "boardroom" makes them look like a contradiction. Always cite the volume tier alongside the band. These bands are mid-band (500 Hz / 1 kHz) design targets for the OCCUPIED room — an empty room measures materially longer, because an audience is itself absorption.

The switch is on mean absorption, not on room type. The model computes ᾱ = A ÷ S — total absorption in sabins divided by total surface area — and uses the Sabine form below ᾱ ≈ 0.2 and the Eyring form above it. The reason is that Sabine over-reads a well-treated room: it assumes absorption is small, so as a room gets genuinely absorptive the plain Sabine equation predicts a longer decay than the room actually has. Eyring ≤ Sabine at the same ᾱ, and both fall as absorption rises, so the property that matters to a designer — add treatment, get a shorter RT60 — holds continuously across the switch. The calculator prints which form produced the number, and says so in a warning when it is running Eyring, because a figure quoted from one equation and compared with a figure from the other is not a like-for-like comparison.

Four things, each of which matters at a different stage. (1) OCTAVE BANDS — every finish carries one Sabine coefficient at 1 kHz, so this is a single mid-band estimate. Real rooms behave differently at 125 Hz and at 4 kHz, and a treatment specification is written from the band-wise picture, not from one number. (2) AIR ABSORPTION — the 4mV term is not applied. It is negligible in a meeting room and it is not negligible in a tall hall, which is why the calculator raises a warning above 8 m ceiling height rather than quietly absorbing the error. (3) MODAL BEHAVIOUR — below roughly 200 Hz a room's decay is governed by its modes, not by a diffuse-field average; Sabine and Eyring both assume a diffuse field and neither predicts that region. (4) GEOMETRY — a strongly non-square room supports flutter echo that a diffuse-field model cannot see, so the calculator warns when the plan aspect ratio passes 2.5:1. None of these are gaps to be filled in later by the same tool; they are the boundary between a brief-stage estimate and an acoustic design.

No, and the calculator says so itself. The 1.00–1.40 s band shown is a speech-use auditorium target. Dedicated cinema / mixing rooms at this volume target a far lower RT60 (≈ 0.4–0.7 s per SMPTE / Dolby guidance) — do not use this band for a cinema. That warning fires on room volume alone, because the tool cannot know that the 500-seat shell in front of it is going to be a cinema rather than a lecture hall. A dedicated cinema, mixing room or critical listening room is a measured specification, not an estimated one: the target is lower, the tolerance is tighter, and the sign-off is an impulse measurement rather than a calculation.

Because the room is already inside its target band, or below it. Recommended area is only computed when the predicted RT60 sits ABOVE the top of the band; a room that is already within band needs no addition, and a room below band needs less absorption or more diffusion rather than more panelling. The area itself is derived by inverting whichever equation produced the RT60 — Sabine inverted for a Sabine result, Eyring inverted for an Eyring result — and then dividing the shortfall in sabins by the 0.85 coefficient of a 50 mm fabric-wrapped panel. Inverting the wrong one is not a rounding difference: it overshoots, and the recommendation then lands the room below the band the same screen just printed.

Take a 7 × 5 × 3 m room with a polished stone / marble floor, painted drywall walls and a concrete / glass / hard plaster ceiling. Volume is 105 m³, total absorption 7.9 sabins, mean absorption ᾱ = 0.06 — below the 0.2 switch, so the Sabine form applies. The predicted RT60 is 2.15 s against a target of 0.5–0.7 s for that volume tier, and the tool recommends 23.9 m² of treatment to bring it into band. That is the whole point of running the number before the finishes are locked: the same room, specified with a soft floor and a treated ceiling, never needs the retrofit.

At design stage, an octave-band model with per-frequency coefficients, air absorption applied, and occupancy accounted for. At handover, a measured reverberation time to ISO 3382-1 with calibrated equipment, reported per octave band from 125 Hz to 8 kHz and filed against the design target. The measured noise floor belongs beside it: a room designed to 0.5 s with a noisy air-conditioning system is not a quiet room, and speech intelligibility is bounded by both numbers at once. Re-measure after any change to the room — added seating, a lighting refit, new upholstery — because each of those moves the absorption budget.