Decibel Shield
Reverb time calculator (RT60)
Reverberation time, RT60, is how long a sound takes to fade by 60 dB after it stops. Sabine's formula gives it from the room: 0.161 × volume ÷ absorption, in metres, where absorption is each surface's area times its absorption coefficient. Enter the room's size and surfaces for RT60 in each octave band. For mixing, enter a tempo for pre-delay and decay times that fit the beat.
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Room reverberation time (RT60)
RT60 at 500 Hz to 1 kHz, Sabine · 144 m³ room
1.37 seconds
| Band | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz |
|---|---|---|---|---|---|---|
| Sabine | 0.59 | 1.44 | 1.74 | 1.01 | 0.61 | 0.54 |
| Eyring | 0.52 | 1.38 | 1.68 | 0.94 | 0.54 | 0.47 |
Reverb times for a song's tempo
| Pre-delay | ms | Decay | ms |
|---|---|---|---|
| 1/128 note | 15.6 | Quarter note | 500 |
| 1/64 note | 31.3 | Half note | 1,000 |
| 1/32 note | 62.5 | 1 bar (4/4) | 2,000 |
| 1/16 note | 125 | 2 bars | 4,000 |
Room times assume sound spreads evenly through a box-shaped room with no furniture or people; both add absorption and shorten the time. The tempo table is arithmetic: a beat is a quarter note.
Target reverberation times
| Room | RT60 | Source |
|---|---|---|
| Classroom up to 283 m³ (10,000 ft³), unoccupied | 0.6 s or less | ANSI/ASA S12.60 (US) |
| Classroom 283 to 566 m³ (10,000 to 20,000 ft³) | 0.7 s or less | ANSI/ASA S12.60 (US) |
| Primary classroom, new build, unoccupied | 0.6 s or less | Building Bulletin 93 (England) |
| Secondary classroom, new build | 0.8 s or less | Building Bulletin 93 (England) |
| Lecture room for more than 50 people | 1.0 s or less | Building Bulletin 93 (England) |
| Reference listening room or studio control room | 0.25 × (V / 100 m³)^⅓ s: 0.25 s at 100 m³, 0.31 s at 200 m³ | EBU Tech 3276, ITU-R BS.1116 |
| Concert hall for symphonic music, occupied | about 1.7 to 2.0 s | Beranek |
Absorption coefficients
| Surface | 125 | 250 | 500 | 1k | 2k | 4k |
|---|---|---|---|---|---|---|
| Concrete or terrazzo floor | 0.01 | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 |
| Concrete block, painted | 0.10 | 0.05 | 0.06 | 0.07 | 0.09 | 0.08 |
| Concrete block, unpainted | 0.36 | 0.44 | 0.31 | 0.29 | 0.39 | 0.25 |
| Brick, unglazed | 0.03 | 0.03 | 0.03 | 0.04 | 0.05 | 0.07 |
| Plasterboard (gypsum), 13 mm on studs | 0.29 | 0.10 | 0.05 | 0.04 | 0.07 | 0.09 |
| Window glass | 0.35 | 0.25 | 0.18 | 0.12 | 0.07 | 0.04 |
| Heavy plate glass | 0.18 | 0.06 | 0.04 | 0.03 | 0.02 | 0.02 |
| Heavy carpet on concrete | 0.02 | 0.06 | 0.14 | 0.37 | 0.60 | 0.65 |
| Acoustic ceiling tile, suspended | 0.50 | 0.70 | 0.60 | 0.70 | 0.70 | 0.50 |
| Heavy curtains (velour, draped) | 0.14 | 0.35 | 0.55 | 0.72 | 0.70 | 0.65 |
| Open window or doorway | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
Coefficients from HyperPhysics (after Hall, Musical Acoustics), Acoustical Surfaces and Middle Tennessee State University; each row is one on which at least two of them agree. Suspended ceiling tiles vary by product: at 500 Hz Hall gives 0.6 and MTSU 0.83, and the calculator uses Hall's.
The formulas
Sabine: RT60 = 0.161 × V ÷ A, with V the volume in m³ and A = Σ S·α the total absorption in m² (metric sabins); in feet it is 0.049 × V ÷ A. Eyring, for rooms with a lot of absorption: RT60 = 0.161 × V ÷ (−S × ln(1 − ᾱ)), with S the total surface and ᾱ the average coefficient. Both assume the sound spreads evenly through the room, which is least true in small or long, narrow rooms and at low frequencies.
Sources: HyperPhysics, Modeling Reverberation Time; Middle Tennessee State University, Reverberation time; Nowoświat & Olechowska, Archives of Acoustics 2016; Beranek, Analysis of Sabine and Eyring equations, JASA 2006; Acoustical Society of America, Classroom Acoustics for Architects (ANSI/ASA S12.60); Department for Education, Building Bulletin 93 (2015); EBU Tech 3276, Listening conditions; ITU-R Recommendation BS.1116-3; Beranek, Aspects of Concert Hall Acoustics, AES 2007.
To time an effect rather than a room, Hz to ms gives every note length at a tempo, and the sound distance calculator covers how sound falls off outdoors, where there is no reverberation to speak of.
What is a good reverberation time for a room?
It depends on what the room is for and how big it is. The US classroom standard ANSI/ASA S12.60 asks for 0.6 s or less in a classroom up to 283 m³ and 0.7 s up to 566 m³, unoccupied; England's Building Bulletin 93 asks for 0.6 s in a new primary classroom and 0.8 s in a secondary one. Reference listening rooms and studio control rooms aim for about 0.25 s in a 100 m³ room (EBU, ITU-R), and the best-rated concert halls sit at about 1.7 to 2.0 s with the audience in. The table below lists them.
Sabine or Eyring: which formula should I use?
They agree in fairly live rooms and draw apart as absorption grows, Eyring's always giving the shorter time. Sabine's is the one most tables of coefficients were made for, and Beranek points out that coefficients should be used with the formula they were derived for, so treat both as estimates. The calculator shows both.
How do I set reverb pre-delay and decay to the tempo?
Work out a note length at the song's tempo (a quarter note is 60,000 ÷ BPM milliseconds) and use it as a starting point: a short note value such as a 1/64 or 1/32 note for the pre-delay, and a quarter note, half note or bar for the decay, so the tail ends with the beat. The table in the calculator does the sums; your ears decide.
How do I lower a room's reverberation time?
Add absorption, mostly on the surfaces that have little: soft furnishings, carpet or rugs, curtains, acoustic panels on the walls, an absorbent ceiling. Absorption is coefficient times area, so a large surface going from hard to soft changes the result most. As the carpet and curtain rows of the table show, most soft finishes absorb little at 125 Hz, so low frequencies need deeper treatment.
