The definition, in one sentence
RT60 (reverberation time) is the time in seconds for sound in a room to decay by 60 decibels after the source stops. A short RT60 means a dry, controlled room. A long RT60 means a live, reverberant space.
The technique to measure it: play a burst of sound, cut the source, measure how long it takes for the sound level to drop by 60 dB. Modern apps measure this with a phone in about 30 seconds per room.
Target RT60 by room type
Recording studios: 0.15–0.25 s (dead, controlled).
Home theatres: 0.25–0.35 s (dry, cinematic).
Podcast rooms: 0.20–0.30 s.
Boardrooms: 0.4–0.6 s (speech-friendly).
Open offices: 0.5–0.7 s (speech-clear but not dead).
Restaurants: 0.6–0.9 s (lively but not shouting).
Classrooms: ≤ 0.6 s (WHO guideline for learning).
Auditoriums: 1.0–1.2 s speech, 1.6–2.0 s music (dual-target rooms use variable acoustics).
Cinemas: 0.3–0.4 s.
How to hit your target
The Sabine equation says RT60 is proportional to room volume and inversely proportional to total absorption. Bigger rooms need more absorption; smaller rooms need less.
Practical rule of thumb: treating 20-30% of hard surfaces (walls and ceiling) brings a typical room into the 0.4–0.6 s range that most commercial applications need. Studios and theatres need 40-60% coverage.
Measure before you treat. Measure after. Adjust. Every room is different; the number tells you when you're done.