Reverberation Time (RT60): The Number That Matters

Ask ten people what "good acoustics" means and you'll get ten vague answers. Ask an acoustician, and the conversation almost always starts with one number: RT60, reverberation time. It's the closest thing our industry has to a single, comparable figure for how a room behaves. It's worth understanding properly, rather than just quoting the number a supplier gives you.

What RT60 actually measures

RT60 is the time it takes for a sound to decay by 60 decibels after the source stops. That is roughly the drop from a sound at its loudest to the point where it disappears into typical background noise. In a room with a long RT60, such as a stone-floored hall or an empty warehouse, speech and music keep bouncing around for a second or more after someone stops talking. In a room with a short RT60, the decay dies quickly. Sound stops when the source stops.

You hear the result long before you measure it. Words blur into each other. People raise their voices to be understood, which makes the room louder still. Teachers, receptionists and call-takers finish the day tired without quite knowing why.

How it is measured on site

In practice, few rooms let you watch a full 60 dB decay. Background noise from traffic, ventilation or the building itself gets in the way first. So measurements follow BS EN ISO 3382-2, the standard for reverberation time in ordinary rooms. The decay is measured over a shorter range, 20 or 30 dB, and extrapolated to 60 dB. You will see these results written as T20 or T30.

The test signal is either a burst of noise that is switched off, or an impulse such as a balloon burst. Readings are taken at several source and microphone positions and averaged, because no single spot tells the whole story. The room's condition matters too. An empty room and the same room furnished and occupied give different results, so a good report states which one was measured.

One number, many frequencies

Reverberation time is not the same at every pitch. Most rooms hold on to low frequencies longer than high ones, because hard surfaces and thin panels absorb bass poorly. That is why the full result is a set of values, one per octave band.

Standards simplify this into a single figure. BB93, the acoustic standard for schools in England, uses the mid-frequency reverberation time, Tmf. It is the average of the 500 Hz, 1 kHz and 2 kHz octave bands, the range that carries most of the information in speech. A room can pass on Tmf and still sound boomy, so ask for the octave-band values as well as the headline number.

The Sabine equation: the formula behind the number

The standard way to calculate reverberation time is the Sabine equation. Physicist Wallace Sabine developed it in the 1890s, and acoustic consultants still use it as their working formula today:

RT60 = 0.161 × V / A

V is the room's volume in cubic metres. A is the total absorption in the room, in square metres of fully absorbent surface (metric sabins). Each surface contributes its area multiplied by its absorption coefficient, a figure between 0 (reflects everything) and 1 (absorbs everything).

The equation tells you two practical things. Bigger rooms need proportionally more absorption to reach the same reverberation time. And every hard surface, whether glass, concrete or plasterboard, adds to the room without adding much to the absorption side.

A worked example

Take a meeting room or classroom of 10 m × 8 m with a 3 m ceiling. The volume is 240 m³ and the total surface area is 268 m². Assume plasterboard walls, a hard floor, a plain ceiling and some glazing, with an average absorption coefficient of about 0.05. The figures below are illustrative. A real calculation uses the actual finishes, octave by octave.

• Untreated: A = 268 × 0.05 = 13.4 m². RT60 = 0.161 × 240 / 13.4 ≈ 2.9 seconds. Conversation in this room is hard work.
• Target 0.6 seconds: the room needs A ≈ 64 m², almost five times what it has.
• 50 m² of Class A ceiling panels (absorption around 0.9): A ≈ 56 m², RT60 ≈ 0.7 seconds. Better, but not there.
• The full 80 m² ceiling in Class A panels: A ≈ 81 m², RT60 ≈ 0.5 seconds. On target, with some margin for a room that is later emptied out.

Two lessons come out of this. First, the jump from "noisy" to "comfortable" needs a lot of absorption, not a few decorative panels. Second, the area you treat matters as much as the product you choose.

Where Sabine runs out

Sabine assumes sound is spread evenly through the room and that the room is fairly reflective. Once a room is heavily treated, with an average absorption above roughly 0.2 to 0.3, Sabine starts to overestimate the reverberation time. Consultants then switch to the Eyring formula, or to computer modelling. In the fully treated example above, Eyring gives about 0.4 seconds against Sabine's 0.5.

Sabine also cannot see where the absorption is. A room with an absorbent ceiling and two bare, parallel walls can hit its target on paper and still produce a ringing flutter echo between those walls. See acoustic wall panels. Placement is a design decision, not something the formula makes for you.

What counts as a good number, and why it changes by room type

There's no single "correct" RT60. The right figure depends on what the room is for.

• Classrooms (BB93): 0.6 seconds or less in a new-build primary classroom, 0.8 seconds in a secondary one. Refurbished classrooms are allowed 0.8 and 1.0 seconds.
• Teaching spaces for pupils with special hearing or communication needs (BB93): 0.4 seconds or less, averaged over a wider band from 125 Hz to 4 kHz, and no more than 0.6 seconds in any octave band.
• Assembly and multi-purpose school halls (BB93): 0.8 to 1.2 seconds in new build, 0.8 to 1.5 in a refurbishment.
• Sports halls (BB93): 1.5 to 2.0 seconds depending on floor area, from 1.5 for halls under 280 m² to 2.0 for halls over 530 m².
• Common corridors and halls in blocks of flats (Approved Document E): no seconds target, but a coverage rule. Under the simple method, corridors and entrance halls get an area of Class C absorber, or better, at least equal to the floor area, usually on the ceiling.
• Homes: a furnished living room typically sits around 0.4 to 0.6 seconds without any special treatment.
• Concert halls: designed for orchestral music, they deliberately target a longer reverberation, often 1.8 to 2.2 seconds. That is what makes the music sound rich rather than dry.

An untreated open-plan office or an empty industrial unit can easily run to 1.5 to 2 seconds or more. Getting the target backwards, treating a speech room like a concert hall or the other way round, is one of the most common specification mistakes we see. See our advice by space for the wider picture.

Why the same treatment plan performs differently room to room

RT60 is driven by the ratio of absorption to volume. So identical treatment, the same number and type of panels, gives a different result in a small meeting room than on a large open-plan floor. This is why a generic "how many panels do I need" answer is close to useless without the room's volume and its existing finishes. It's also why a consultant calculates the required absorption area first, and only then specifies panels.

What RT60 does not tell you

Reverberation time describes how long sound lingers inside a room. It says nothing about noise coming through the walls from next door. That is sound insulation, a different problem with different fixes. See Approved Document E explained.

It also isn't the only measure of whether speech works. Open-plan teaching areas under BB93 and open-plan offices are judged on speech intelligibility and how far conversation carries, not on reverberation alone. Background noise from ventilation or traffic matters too. A room with a perfect RT60 and a loud air-handling unit still fails the people in it.

Common questions

Can RT60 be too short? Yes. An over-absorbed room can feel dead and unnatural, particularly for music or performance spaces where some reflection is part of the intended sound.

Does furniture count as absorption? Yes. Soft furnishings, carpets, upholstered seating and the people in the room all add to the total absorption. That is why an empty room measures very differently to the same room furnished and occupied.

Can I estimate RT60 myself? You can get a rough idea with the Sabine equation and published absorption coefficients, as in the example above. For compliance, you need a measurement to BS EN ISO 3382-2 and a calculation per octave band.

Does your free survey include a measurement? The free site survey gives you a quotation and an AI-assisted acoustic report. Before-and-after acoustic measurement is available as a paid add-on. Compliance testing is carried out with our testing partner, EMC Acoustics.

Working out what your room needs

See our guide to acoustic wall panels to understand how individual products are tested, or book a free survey to have your room assessed properly rather than estimated.

Related product guides: acoustic ceilings: tiles, baffles or rafts, acoustic wall panels.