On this page
Clap once in an empty room and listen to what follows. The clap stops, but the room keeps sounding for a moment while the sound bounces between walls, floor and ceiling and dies away. RT60 puts a number on that moment, and that number shapes a design more than most people expect.
What RT60 is
RT60 is the time, in seconds, for sound to fall by 60 dB after the source stops. Sixty decibels is a millionth of the starting energy. In practice nobody waits for the whole 60 dB: a measurement times the first 20 or 30 dB of the decay and extrapolates, which is what T20 and T30 mean on a report.
RT60 changes with frequency, so it is given per octave band, usually from 125 Hz to 4 kHz, and often summed up as the average of the 500 Hz and 1 kHz bands. Bass usually lingers longest, because most finishes absorb it least.
For a feel of the numbers, the tool’s venue presets start a bedroom at 0.4 s, a living room at 0.5 s, a restaurant at 0.7 s, a sports bar at 0.8 s and a gym at 1.2 s. Its outdoor presets have no RT60 at all: with no ceiling there is no reverberant field to speak of.
Sabine: volume over absorption
Wallace Sabine’s formula is more than a century old and still the first estimate most designers make:
An absorption coefficient runs from 0, for a surface that reflects everything, to 1, for one that absorbs everything. Painted plaster, glass and tile sit near the bottom. Carpet, curtains, acoustic ceiling tile and upholstered furniture sit higher, and broadband acoustic panels come close to 1 in the middle of the range. People absorb sound too, so a full room is drier than an empty one.
A worked example: a room 10 m by 8 m with a 3 m ceiling has a volume of 240 m³ and 268 m² of surfaces. With hard finishes averaging an absorption coefficient of 0.10, A is 26.8 m² and RT60 is about 1.44 s, a lively room. Fit 20 m² of panels with a coefficient of about 0.9 to the ceiling and A rises to 42.8 m², because the panels add 18 m² and the ceiling they cover loses its 2 m². RT60 falls to about 0.90 s.
Eyring: when the room is absorptive
Sabine’s formula assumes sound is spread evenly and absorbed a little at a time. It works well in lively rooms and overestimates the reverberation of absorptive ones: by Sabine, even a room of perfect absorbers would keep some reverberation. Eyring’s version corrects for that:
The same room three ways, by its average absorption coefficient:
| The room | Absorption | Sabine | Eyring |
|---|---|---|---|
| Hard finishes | 0.10 | 1.44 s | 1.37 s |
| With 20 m² of panels | 0.16 | 0.90 s | 0.83 s |
| Furnished and absorptive | 0.30 | 0.48 s | 0.40 s |
The two agree in lively rooms and drift apart as absorption rises. The tool’s acoustic engine uses Eyring-corrected reverberation, with a term for air absorption that matters at high frequencies in large rooms. The temperature and humidity you can enter for each area set how much treble the air itself absorbs.
What the tool needs from you
In each area’s acoustics block you can type an RT60 you already know, or switch to From materials and pick the floor, ceiling and wall finishes from lists that carry their absorption per octave band. Two quick settings refine the estimate:
- Occupancy: empty, lightly occupied, seated or a full crowd.
- Furnishing: bare, light, furnished or soft-furnished. Furnished is the baseline the venue presets assume, because a design should describe the room as it will be used, not as an empty shell.
Why RT60 changes the design
Near a speaker you hear mostly direct sound. Further away, the reverberant field takes over. The distance at which the two are equal is the critical distance:
In the 240 m³ room above, with 90° ceiling speakers, the critical distance is about 1.8 m while the room is hard and about 3.1 m once it is furnished. Seated guests under a 3 m ceiling are already 1.8 m from the speaker overhead, and further from all the others. In the hard room they listen mostly to the room.
That has three consequences:
- Speech intelligibility falls. Reverberation smears the fast fluctuations that carry consonants, so STI drops even when the level is right. A lively room needs more speakers closer to its listeners, more directional speakers aimed at them, or treatment.
- The count goes up for speech. More speakers at a lower level each keep listeners inside their critical distance.
- The heatmap flatters. The reverberant field adds level everywhere, so coverage looks more even while clarity gets worse. Read the clarity figures too: C50 for speech, where 0 dB or more is good, and C80 for music, where about −2 to +4 dB is balanced.
Fixing a lively room
The tool’s treatment advisor compares an area’s RT60 with a recommended value for its volume, from a curve in the style of DIN 18041. When the room is too lively, it says how many square metres of broadband panel to add (50 mm thick, NRC 0.85 or better) and what STI and C50 to expect afterwards, worked out by running the acoustic engine again with the treated room. A failed clarity check becomes a line item you can price.
Where the room is not a simple box, such as an L-shape, a high ceiling, a big opening or two spaces joined, the statistical estimate assumes the same reverberation everywhere. The Ray-Traced Check (beta) fires 24,000 rays per speaker through the real geometry and reports RT60, C50 and STI at each focus pin, so you can see which seats the room punishes.
Measure it
RT60 is quick to measure and worth measuring. On a refit, measure before you settle the speaker count; on every job, measure at handover. The tool’s site tracking records the measured RT60, SPL and STI of each area through installation, beside the design values, and the prediction stays what it is: a planning baseline, not a guarantee.