Reverberation is the persistence of sound in an enclosed space after the original sound source has stopped, caused by the accumulation of thousands of reflections from the room's surfaces arriving at the listener in rapid succession. Unlike a distinct echo, which you hear as a separate repetition, reverberation blends into a continuous, decaying wash of sound that gives a room its characteristic acoustic signature.
Every room has reverberation. A tiled bathroom has a lot — clap your hands and the sound lingers for over a second. A carpeted bedroom has very little — the clap dies almost instantly. The amount and character of reverberation determines whether speech is clear or muddled, whether music sounds rich or harsh, and whether a room feels alive or dead.
Real-World Analogy
Imagine standing in a room full of mirrors and waving a sparkler. The single flame creates hundreds of reflections, each slightly dimmer than the last. From where you stand, you do not see individual reflections — you see a wash of light surrounding you from every direction, gradually fading as each successive reflection loses brightness.
Reverberation is the acoustic version. A single hand clap produces thousands of reflections that arrive at your ear within milliseconds of each other. Your brain cannot separate them individually — instead, you perceive a continuous "tail" of decaying sound. The louder the original sound and the more reflective the surfaces, the brighter and longer-lasting the tail.
Technical Definition
Reverberation is formally described by two characteristics: the reverberation time (RT60) and the energy decay curve.
RT60 is the time in seconds for the sound pressure level to decay by 60 dB after a source stops, as defined in ISO 3382-2:2008. It is the standard metric for quantifying reverberation.
The energy decay curve shows how sound energy decreases over time. In a well-diffused room, this curve is approximately a straight line on a dB-versus-time plot. Deviations from linearity indicate non-diffuse conditions, coupled volumes, or other acoustic complexities.
Reverberation vs Echo vs Delay
These three terms describe different perceptual experiences of reflected sound:
Reverberation is a dense, continuous decay composed of thousands of overlapping reflections arriving so closely together (typically separated by less than a few milliseconds) that the ear cannot distinguish individual arrivals. It sounds like a "tail" or "wash" after the original sound.
Echo is a distinct, identifiable repetition of the original sound, caused by a single strong reflection arriving more than approximately 50-80 milliseconds after the direct sound. At this delay, the auditory system perceives it as a separate event. Echoes occur in large spaces with distant, reflective surfaces — canyons, large halls, sports arenas.
Delay in acoustic terms refers to the time gap between the direct sound and a specific reflection. Early reflections (under 50 ms delay) fuse with the direct sound and enhance loudness and clarity. Late reflections (over 50-80 ms) may be perceived as echoes if they are strong enough. The term "delay" describes the physical timing; "echo" and "reverberation" describe the perceptual result.
The key distinction: reverberation is diffuse and continuous (many weak reflections blended together), while echo is discrete and directional (one or a few strong reflections clearly separated in time).
The Sabine and Eyring Models
Two models predict RT60 from room properties:
Sabine equation (ISO 3382-2 Annex A.1): RT60 = 0.161 V / A, where V is volume and A is total absorption. Valid for rooms with low to moderate absorption (average alpha below 0.3).
Eyring equation (ISO 3382-2 Annex A.2): RT60 = 0.161 V / (-S x ln(1 - alpha_avg)), where S is total surface area and alpha_avg is the mean absorption coefficient. More accurate for rooms with moderate to high absorption.
Why It Matters for Design
Reverberation is arguably the single most important acoustic parameter in building design:
Speech clarity. In rooms for spoken communication — classrooms, lecture halls, meeting rooms, courtrooms — excessive reverberation smears syllables together, reducing the Speech Transmission Index. Standards like ANSI S12.60 and BB93 set maximum RT60 values (typically 0.4 to 0.8 seconds depending on room type and volume) specifically to protect speech intelligibility.
Musical quality. Concert halls, rehearsal rooms, and worship spaces require carefully calibrated reverberation. Too little and music sounds dry and lifeless. Too much and fast passages become muddled. The ideal RT60 depends on the musical repertoire — symphonic orchestras typically need 1.8 to 2.2 seconds, while amplified contemporary music works best at 0.8 to 1.2 seconds.
Noise build-up. In restaurants, cafeterias, and open-plan offices, excessive reverberation amplifies the steady-state noise level. As conversations compete with reflected energy, people speak louder (the Lombard effect), which raises the overall level further in a self-reinforcing cycle. Reducing RT60 from 1.5 seconds to 0.6 seconds can lower the perceived noise level by 5 to 8 dB.
Occupant comfort. Spaces with RT60 well-matched to their function feel comfortable and effortless. Spaces with mismatched RT60 — a reverberant classroom, a dead concert hall — feel exhausting and wrong, even to people with no acoustic training.
How AcousPlan Uses This
Reverberation time is the primary output of AcousPlan's room acoustic calculator. When you define a room and assign materials to its surfaces, the calculator produces RT60 values at each of the six standard octave bands using both Sabine and Eyring methods.
The results are displayed as a frequency-dependent curve and compared against target values from your selected standard (ANSI S12.60, BB93, WELL v2, or custom). Compliance indicators show at a glance whether the reverberation is within the acceptable range at each frequency.
The auto-solve feature optimises material combinations to achieve your target RT60, and the AI co-pilot explains the relationship between the materials, the room geometry, and the resulting reverberation in plain language.
Related Concepts
- What is RT60? — The metric that quantifies reverberation
- What is Echo? — Discrete reflections versus diffuse reverberation
- What is Sound Reflection? — The mechanism that creates reverberation
- What is Sound Absorption? — The mechanism that controls reverberation
- What is Acoustic Damping? — The process that causes reverberation to decay
Calculate Now
Find out how much reverberation your room has — and whether it is too much or too little. Use the AcousPlan Room Calculator to calculate RT60 at every octave band and compare it against professional standards.