Absorption coefficients are typical mid-frequency (500 Hz) values; real materials vary with thickness, mounting and frequency.
The Sabine formula
RT60 is the time it takes for sound to decay by 60 dB after the source stops. Wallace Sabine found in 1898 that RT60 ≈ 0.161·V / A, where V is the room volume in m³ and A the total absorption (each surface area × its absorption coefficient).
The Eyring formula is more accurate for absorbent rooms, where Sabine overestimates the reverberation. Both assume sound is evenly spread through the room.
Good reverberation times
Recording booths: under 0.3 s. Home studios, offices and classrooms: 0.4–0.8 s. Living rooms: about 0.5 s. Concert halls: 1.8–2.2 s. Cathedrals: 4–10 s.
Frequently asked questions
How do I reduce reverberation?
Add absorption: rugs, curtains, upholstered furniture, bookshelves and acoustic panels – especially on parallel hard walls and the ceiling.
Why does my empty room echo?
Hard, parallel surfaces reflect almost all sound energy, so it takes a long time to decay. Furniture and people add absorption.