Definition
RT60 (Reverberation Time) is the time, in seconds, that it takes for the sound pressure level in a room to decay by 60 dB after the sound source is switched off. The "60" refers to the 60 dB decay, and the result is usually written as T60 (or T60).
Reverberation time is a property of the room — not of a single material. It describes how "live" or "echoey" a space sounds, and it depends on the room's volume and how much sound-absorbing surface it contains.
What Does RT60 Measure?
RT60 measures the decay of sound energy inside a room after the source stops. A long RT60 means the room is reverberant (sound lingers, echoes build up); a short RT60 means the room is acoustically "dead" or well-damped.
RT60 does not measure how much sound passes through a wall or floor (that is sound insulation, described by STC or Rw). It is purely about the sound field inside the space.
Unit
The unit of RT60 is the second (s). Typical values range from about 0.2 s (a well-treated small room) to several seconds (a large, hard-surfaced hall or church).
Why RT60 Matters
Reverberation time is one of the most important room-acoustic parameters because it directly affects how well a space works:
- Speech intelligibility — long reverberation smears speech and makes it hard to understand.
- Music quality — some reverberation enriches music, but too much blurs detail.
- Noise build-up — reverberant rooms accumulate noise and feel louder.
- Comfort and productivity — in offices, classrooms and restaurants, excessive reverberation raises stress and reduces concentration.
How RT60 Is Measured
Reverberation time is measured by exciting the room with sound and recording how the sound level decays. The measurement is defined in ISO 3382. In practice, the procedure involves:
- Sound source: an omni-directional loudspeaker (often a dodecahedron speaker) or an impulsive source such as a balloon burst or starter pistol.
- Microphone: one or more measurement microphones placed at defined positions away from walls.
- Measurement positions: several source positions (typically two or more) and several microphone positions (typically three or more) are averaged to reduce spatial variation.
- Decay curve: after the source stops, the sound level is recorded against time, producing a decay curve.
- Frequency bands: the decay is measured in octave bands (commonly 125 Hz to 4 kHz) or one-third-octave bands.
- Calculation: because a full 60 dB decay is hard to measure cleanly above background noise, the standard uses a 20 dB or 30 dB portion of the decay, extrapolated to 60 dB. These are reported as T20 or T30.
This summary is for general understanding only — it does not replace the measurement standard.
Relationship Between Reverberation and Sound Absorption
Reverberation and absorption are two sides of the same process. Adding sound-absorbing material to a room reduces its reverberation time.
Absorption is a property of a material (rated by NRC or α); reverberation time is a property of the room (measured in seconds). More total absorption (more absorbing surface area, or higher-absorption material) shortens RT60.
Frequency Dependence
Reverberation time is frequency-dependent: a room can have different RT60 values at low, mid and high frequencies. Most ordinary rooms reverberate more at low frequencies (which are harder to absorb) than at mid frequencies. This is why RT60 is always reported per frequency band rather than as a single number.
Formula
The classic estimate of reverberation time is the Sabine equation:
T60 ≈ 0.161 × V / A
where:
- T60 — reverberation time (seconds)
- V — room volume (m³)
- A — total equivalent sound absorption area of the room (m² Sabins), calculated as the sum of (absorption coefficient × surface area) over all surfaces, A = Σ (α × S)
The Sabine equation is a model/estimate. It assumes a diffuse sound field and evenly distributed absorption. It is reasonably accurate for reverberant rooms but is not a substitute for measurement in every real room.
Practical Example
Consider a room of volume 100 m³ with a total equivalent absorption area of 20 m². The Sabine estimate gives T60 ≈ 0.161 × 100 / 20 = 0.81 s. If we add absorbing panels that raise the total absorption area to 40 m², the estimate drops to T60 ≈ 0.161 × 100 / 40 = 0.40 s — roughly halving the reverberation time. This example is illustrative and assumes the Sabine model's conditions; a real room should be verified by measurement.
Factors Affecting RT60
- Room volume (V) — larger volume generally increases reverberation time.
- Total absorption (A) — more absorbing surface shortens RT60.
- Frequency — low frequencies are usually harder to absorb, so RT60 is often longer at low frequencies.
- Furnishings and occupants — soft furnishings, curtains, and people add absorption and reduce RT60.
- Room shape and surface distribution — uneven absorption distribution makes the diffuse-field assumption less valid.
RT60 vs NRC
RT60 and NRC are different quantities and must not be confused:
- RT60 measures the decay of sound in a room (unit: seconds). It is the result of the whole room — its volume and all its absorbing surfaces.
- NRC is a single-number rating of a material's absorption (unitless), obtained from a standard test method (ASTM C423 / ISO 354).
NRC describes the material; RT60 describes the room. A high-NRC panel helps shorten RT60, but the final RT60 depends on how much of that material is used and where.
RT60 vs Sound Insulation
RT60 does not measure sound insulation. Sound insulation (STC or Rw) describes how well a wall, floor, door or window blocks sound passing from one room to another. A room can have a short, comfortable RT60 and still offer poor insulation between it and the next room — and vice versa. They are independent properties addressed by different treatments.
Common Misunderstandings
- "RT60 is a material property" — incorrect; it is a room property.
- "RT60 and NRC measure the same thing" — incorrect; one measures a room (seconds), the other rates a material (unitless).
- "Short RT60 means the room is soundproofed" — incorrect; RT60 says nothing about transmission through the building envelope.
- "The Sabine formula gives the exact RT60" — incorrect; it is an estimate under idealised assumptions.
- "RT60 is the same at all frequencies" — incorrect; it is frequency-dependent.
Limitations
- RT60 is a single decay measure that does not capture all aspects of room acoustics (for example, clarity, early/late energy ratio, or spatial impression are separate parameters).
- The Sabine and similar equations are models; real rooms deviate from their assumptions.
- RT60 is frequency-dependent, so a single average number can be misleading.
- In large or irregular spaces the concept of a single reverberation time is less meaningful.
Acoustic Treatment
The principal way to control reverberation time is to add sound-absorbing treatment — typically on the ceiling (baffles, clouds, acoustic ceiling panels) and on walls (PET, fabric or wood acoustic panels). The correct type, amount and placement depend on the room's volume, use and target performance, and should be confirmed by design and, where relevant, by measurement.
Relevant Standards
- ISO 3382-1 — Acoustics — Measurement of room acoustic parameters — Part 1: Performance spaces (auditoria, concert halls and similar rooms intended for music or speech).
- ISO 3382-2 — Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms.
ISO 3382 is published in parts because the measurement procedure and the parameters of interest differ between large performance spaces and ordinary rooms. Part 1 covers dedicated performance spaces; Part 2 applies to ordinary rooms such as offices, classrooms and dwellings. (A third part, ISO 3382-3, addresses open-plan offices.)
Related Yamamah Solutions
The following product families are used to control reverberation in rooms. They can contribute to reducing reverberation when appropriately designed and installed:
For room treatment design and reverberation control, contact us about acoustic treatment for your space.
See also the Sound Absorption vs Sound Insulation hub.
Read more in the four-part acoustic fundamentals series: What Is NRC? · What Is Rw? · What Is STC? — and see How Room Acoustics Are Measured.
Frequently Asked Questions
What is a typical RT60? It depends strongly on the room. Small, well-treated rooms can be around 0.2–0.4 s; larger or harder rooms can exceed 1 s. There is no single universal value.
What is the difference between T20 and T30? Both are practical ways to obtain reverberation time from a partial decay: T20 extrapolates a 20 dB decay to 60 dB, and T30 extrapolates a 30 dB decay. They are used instead of measuring a full 60 dB decay, which is often limited by background noise.
Does adding panels always reduce RT60? Adding absorbing surface reduces reverberation, but the amount depends on the material's absorption, its area, and its placement.
Can RT60 be calculated without measurement? It can be estimated with equations such as Sabine's, but the estimate is based on assumptions and should not replace measurement in critical spaces.
Verified Sources
- ISO 3382-1 — Acoustics — Measurement of room acoustic parameters — Part 1: Performance spaces.
- ISO 3382-2 — Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms.