Sound bounces off walls and decays. How long it takes to go quiet — the reverb time —
depends directly on room size and what the surfaces are made of. A clap in a bathroom
echoes differently than in a bedroom. Your phone can hear the difference and estimate
which one you're in.
forensic audio analysts can determine where a recording was made from its echo signature.
The reverb fingerprint of a room is unique — walls, furniture, carpet, and ceiling height
all leave a signature in the sound. Intelligence agencies use this to geolocate recordings.
The same technique, applied in reverse, works for navigation in GPS-denied spaces like
underground car parks and tunnels.
Press start listening, then clap once sharply near your phone.
The microphone will capture the reverb decay and estimate room size from it.
Works best in quiet rooms. Soft clap = bad result. Sharp, loud clap = good result.
—
RT60 = — ms
waiting
—RT60 ms
—est. volume m³
0claps taken
decay envelope
RT60
typical room
surface type
< 400 ms
bathroom / small tiled
hard, reflective
400–700 ms
bedroom / office
soft furnishings
700–1200 ms
living room / classroom
mixed surfaces
1200–2500 ms
hall / stairwell
hard floors, tall ceiling
> 2500 ms
large hall / church
stone / concrete, vast
▶ how does this work?
RT60 is the standard measure of room acoustics: the time it takes for a sound to decay
by 60 decibels after the source stops. It's calculated using Sabine's formula:
RT60 = 0.161 × V / A, where V is room volume (m³) and A is total sound absorption.
We invert this: measure RT60 from the clap decay, then estimate volume.
We measure the peak of the clap, find when the envelope has decayed to 1/1000th of that
(which is −60 dB), and call that time RT60. Then we back-calculate room volume using
typical absorption coefficients for furnished domestic spaces.
Accuracy: ±40% for volume, good enough to classify room type. Results improve with multiple claps.