Reverberation time is the time it takes for sound in a room to fade by 60 decibels after the source stops. That 60-decibel drop, roughly the difference between normal conversation and near-silence, is the standard benchmark acousticians use to describe how “live” or “dead” a space sounds. The measurement is often abbreviated as RT60, and it typically ranges from a fraction of a second in a small carpeted room to several seconds in a cathedral.
How RT60 Works
When you clap your hands in an empty gymnasium, the sound doesn’t vanish instantly. It bounces off the walls, floor, and ceiling, losing a little energy with each reflection until it drops below the threshold of hearing. RT60 captures this decay process as a single number in seconds. A room with an RT60 of 0.5 seconds feels tight and controlled. A room at 3 seconds feels expansive and echoey.
The 60-decibel threshold was chosen because it represents a millionfold decrease in sound energy, a point where even a loud initial sound becomes effectively inaudible. In practice, measurement systems sometimes can’t capture a full 60 dB drop because of background noise, so they measure a smaller decay (often 20 or 30 dB) and extrapolate.
What Determines a Room’s Reverberation Time
Three factors control how long sound lingers: the room’s volume, the total surface area of its walls and objects, and how absorptive those surfaces are. A larger room gives sound more space to travel between reflections, so bigger rooms generally have longer reverberation times. But surface materials matter just as much.
Every material has an absorption coefficient between 0 and 1. A coefficient of 0 means the surface reflects all sound energy. A coefficient of 1 means it absorbs everything. Normal concrete sits very low, around 0.05 to 0.10, meaning it reflects most of the sound that hits it. That’s why parking garages and stairwells echo so much. Specialized acoustic panels, thick curtains, and upholstered seating absorb far more. Some porous acoustic materials reach absorption coefficients of 0.65 to 0.73 in the mid-frequency range, soaking up the majority of sound energy on contact.
The classic formula for predicting reverberation time, developed by physicist Wallace Sabine in the early 1900s, ties these variables together: RT60 equals 0.161 multiplied by the room’s volume (in cubic meters), divided by the total absorption in the room. Total absorption is calculated by multiplying each surface’s area by its absorption coefficient and adding them all up. So a large concrete room with bare walls produces a long RT60, and adding carpet, curtains, or acoustic panels shortens it.
When the Simple Formula Falls Short
The Sabine formula works well for rooms with moderate, evenly distributed absorption, like a typical auditorium with mixed surfaces. But in rooms where absorption is very high (a fully treated recording studio, for example), it can overestimate the reverberation time. An alternative formula, developed by Carl Eyring, accounts for rooms where surfaces absorb a large fraction of sound energy. In practice, either formula can produce accurate results as long as the absorption coefficients plugged into them were measured in spaces with similar size, shape, and layout.
Ideal Reverberation Times by Space
Different activities need very different acoustic environments, and RT60 is the number that guides those design decisions.
- Classrooms and lecture halls: Below 1 second. Short reverberation keeps speech crisp and prevents words from smearing into each other. When RT60 climbs too high in a classroom, students at the back hear a muddy overlap of direct speech and reflected sound.
- General-purpose auditoriums: Around 2 seconds. This balances the clarity needed for speech with enough warmth to support live music.
- Concert halls for orchestral music: Roughly 1.7 to 2.1 seconds. Vienna’s Musikvereinsaal, widely considered one of the best-sounding concert halls in the world, has an RT60 of 2.05 seconds. Boston Symphony Hall comes in at 1.8 seconds, and Carnegie Hall in New York at 1.7 seconds. These numbers are not accidents; they were achieved through careful choices about room dimensions, seating materials, and wall geometry.
- Recording studios: Often well below 0.5 seconds in the control room, giving engineers a neutral listening environment with minimal coloring from the room itself.
How Humidity and Temperature Affect Sound Decay
Air itself absorbs sound, and this effect becomes significant at higher frequencies. At 4,000 Hz (the range of consonant sounds in speech and the shimmer of cymbals), air at 40% relative humidity absorbs about 105 dB per kilometer. Raise the humidity to 70%, and that drops to 63 dB per kilometer. The difference is large enough to change the character of a concert hall between a dry winter evening and a humid summer night with a packed audience.
This air absorption depends on frequency, temperature, humidity, and atmospheric pressure. Below about 1,000 Hz, the effect is small, only around 5.6 dB per kilometer regardless of humidity. But above 2,000 Hz it grows rapidly. This is why large rooms often sound warmer or darker than small ones: high-frequency sound decays faster as it travels longer distances through the air, leaving the lower frequencies to dominate the reverberation tail.
Why Reverberation Time Matters for Hearing and Fatigue
Excessive reverberation doesn’t just sound bad. It makes your brain work harder. When reflected sound overlaps with new speech, your working memory has to sort through distorted and incomplete information to reconstruct what was said. Research on cognitive load during speech communication shows that long reverberation times cause missed and distorted information, reducing the natural redundancy that makes speech easy to follow. Over time, this extra processing effort contributes to cognitive fatigue, gradually depleting working memory resources.
The effect hits people with hearing loss especially hard. Studies comparing normal-hearing and hearing-impaired listeners found that in reverberant or noisy conditions, people with hearing loss performed measurably worse on both speed and accuracy in language processing tasks. Their main difficulty was identifying unfamiliar words, exactly the situation where clear acoustics matter most. This is why classroom acoustics standards are so strict: for children still developing language skills, or for anyone with even mild hearing loss, a room with poor reverberation control can turn a lecture into an exhausting guessing game.
How Reverberation Time Is Measured
The simplest way to measure RT60 is to make a loud, sharp sound in the room and record how long it takes to die away. Common sound sources include a starting pistol, a balloon pop, or an electrical spark generator. Even a loud handclap can work for a rough estimate. The key is that the sound needs to start at a high level and stop abruptly, giving the room’s natural decay a clean starting point.
A microphone captures the decay, and software calculates how long it takes for the sound pressure level to drop by 60 dB. Professional setups use calibrated measurement microphones and specialized acoustic analysis software that can break down the reverberation time by frequency band. This frequency-specific measurement matters because a room might have an RT60 of 0.8 seconds at 500 Hz but 1.4 seconds at 125 Hz, meaning bass frequencies linger much longer than midrange ones.
Smartphone apps can give you a ballpark measurement for casual purposes, but they lack the calibrated microphones and controlled sound sources needed for professional accuracy. For designing or treating a space, professional measurement with proper equipment remains the standard.