RT60 — Reverberation Time
Definition
The reverberation time (RT60) is the time, in seconds, that a sound takes to decay by 60 dB in a room after the source stops, which corresponds to one millionth of the initial energy density. Sabine's formula estimates it as T = 0.161 × V / A, where V is the room volume in m³ and A the total absorption in m². The formula assumes a diffuse sound field and becomes less accurate in very absorbent rooms. Recommended values depend on the use and the volume of the room. Spaces for speech are designed with short reverberation times, while large churches and cathedrals can reach several seconds.
In the field
In a room with a long reverberation time, the direct sound from the PA competes with the reverberant field, which reduces intelligibility. Common practice is to aim the loudspeakers at the audience, which absorbs sound, rather than at walls and ceilings, to add delay positions that keep listeners closer to a source, and to use less artificial reverberation in the mix.
Related lessons
Sources
- Ballou, p. 177 : décroissance de 60 dB, soit un millionième de la densité d'énergie initiale
- Reverberation (Wikipedia) : formule de Sabine T = 0,161 V/A en unités métriques ; limites en salle absorbante (Eyring) · en.wikipedia.org
- ISO 3382-1:2009 : mesure des paramètres acoustiques des salles de spectacle, dont le temps de réverbération · iso.org