RT60 Reverb Time Calculator (Sabine)

Clap once and listen: 0.2 s is a treated studio (the clap simply stops); 0.5 a comfortable furnished living room (brief warmth); 0.8 the slightly echoey empty apartment that makes voices ring; 1.5 a school gym where speech needs effort; 3+ a cathedral where each chord blooms into the next. The same physics that flatters Gregorian chant at 4 s destroys a lecture: optimal RT60 is purpose-dependent, which is why this tool grades rather than scores.

Add sabins: each m² of good absorber (a 5-cm rockwool panel, α≈0.9 mid-band) adds ~0.9 sabins, and RT60 falls as 0.161V/A — so HALVING reverb time means DOUBLING total absorption. A bare 73-m³ room with 4.7 sabins (1.0 s) needs about 5 more sabins — six standard 60×120 cm panels — to reach 0.5 s. The diminishing-returns curve is built in: the first six panels transform the room; the next six only nudge it. Soft furnishings count too: a fabric sofa is worth 2–4 sabins, a thick rug over underlay 1.5–3.

Absorption coefficients are frequency curves, not numbers: a 5-cm foam panel absorbs 90% at 2 kHz and nearly nothing at 100 Hz, carpet likewise — so untreated-but-furnished rooms typically decay fast in the highs and slowly in the bass (the 'boomy but dull' signature). A proper acoustic design quotes RT60 per octave band and aims for a gently rising-toward-bass or flat profile. The single mid-band figure this tool computes is the planning headline; the octave detail is where studio designers earn fees — and where bass traps (see our room-mode tool) do the heavy lifting.

At its assumptions' edges: it presumes a DIFFUSE field (sound energy uniform, arriving from everywhere), which fails in very absorbent rooms (α̅ > ~0.3 — use Eyring's logarithmic correction; Sabine overestimates RT there), in small rooms below the modal transition (~150–200 Hz where discrete room modes, not statistics, govern decay — our room-mode calculator's territory), and in oddly-shaped or coupled spaces. For a furnished rectangular room judged mid-band, it's honest to ~20% — Sabine's cushions built a science from less.