Reverberation Time

A clap bounces around a hard room, losing energy each reflection; RT60 is the seconds it takes to fade 60 dB to near silence.

What it is

Reverberation time is how many seconds a sound takes to fade to near silence after the source stops.

Key facts

• RT60 = the seconds for sound to drop 60 dB (decibels) after the source stops; 60 dB = level falling to 1/1000th.
• Sabine formula: RT60 = 0.161 x V / A. V = room volume in cubic metres, A = total absorption in sabins, 0.161 = metric constant.
• A (total absorption) = each surface's area x its absorption coefficient, summed. Coefficient runs 0 (perfect mirror) to 1.0 (open window, total soak).
• Speed of sound = 343 m/s at 20C; a wave travels ~3.4 m every 10 ms, bouncing many times before silence.
• Speech needs short RT60: 0.4-0.6 s conference room, 0.6-1.0 s small theatre. Music wants more: concert hall 1.8-2.2 s, stone church 4-10 s.
• Empty gym/hall: RT60 often 2-5 s (mush). Carpeted lounge: ~0.3-0.5 s (dies instantly).
• Absorption coefficients ~500 Hz: bare concrete ~0.02, glass ~0.03, carpet ~0.3, heavy curtain ~0.5, 50 mm foam ~0.7, 100 mm rockwool ~0.9+.
• RT60 is frequency-dependent: low bass (125 Hz) rings longer than treble (4 kHz) because thin absorbers miss long bass waves.
• Inverse-square law: every doubling of distance outdoors drops level 6 dB. Indoors this stops at the critical distance, past which reverberant level dominates and clarity collapses.
• RT60 is a room property, near-independent of volume; turning up the PA raises direct AND reverb, so it does NOT fix smear and can worsen it.

How it works

1. Source stops: speaker, clap or starter pistol cuts dead.
2. Sound keeps bouncing off walls, floor and ceiling, losing a little energy each reflection.
3. Engineer measures the level decay on a meter or software (clap test or interrupted pink noise).
4. Time the drop over a 20 or 30 dB slope, then multiply up to estimate the full 60 dB fall = RT60.
5. Long RT60 means many reflections arrive late and overlap your direct sound = mush.
6. Cut it by adding absorption, aiming speakers at people not walls, and keeping levels sane.

Real examples

• Clap in an empty school gym: it rings 2-4 seconds, every word from the mic overlaps the last.
• Same clap in a carpeted, curtained lounge: gone in under half a second, speech crystal clear.
• Wedding in a stone barn: RT60 ~3 s, so the DJ's vocals turn to porridge until you hang drapes.
• Corporate AGM in a glass atrium: hard glass reflects, speech intelligibility tanks without delay fills and absorption.
• Outdoor festival stage: near-zero reverb (no walls), so the problem flips to wind and slap-back off distant buildings.

How it helps in live sound

• Do a clap test on load-in: count the ring. Over ~1.5 s of tail means you fight intelligibility all night.
• Hang heavy drapes, deploy stage skirts, bring 50-100 mm absorber panels; soft furnishings and a full audience drop RT60 hard.
• Aim mains DOWN at the crowd, never flat at the back wall; tilt and toe-in cuts wall reflections.
• Use a distributed/delay system in long reverberant rooms so listeners get close direct sound, not far reverberant smear.
• Roll off lows and tighten low-mids: bass rings longest, so high-pass vocals ~100-120 Hz and tame 200-400 Hz to clear mud.
• Keep SPL reasonable; cranking the PA feeds the reverb, it does not beat it. Lower, cleaner, closer wins.