If you are interested in room acoustics, you should definitely give this reverberation time calculator a try. You can use it to define one of the most critical acoustical characteristics of any enclosed space: the reverberation time, which describes how sound is reflected and propagates in a room.
This article will give you an overview of how sound behaves in enclosed spaces and provide you with a handy RT60 equation (also known as the reverberation time formula). We will also give you examples of optimum reverberation times in rooms with various purposes.
How to use the reverberation time calculator
To use the reverberation time calculator, follow these easy steps:
- Choose your unit system — metric (meters) or imperial / US (feet).
- Enter the room's dimensions — length, width, and height.
- Enter the dimensions and the number of any doors in the room.
- Do the same for any windows in the room.
- If you want to fine-tune the absorption coefficients of the room's surfaces (including the doors and windows), expand the absorption coefficients section to do so.
- The reverberation time calculator will show you the room's reverberation time (RT60).
What is the reverberation time?
Every sound is nothing but a wave propagating in space — either in the air or a different medium. When a sound wave is generated in a room, it travels forward until it reaches an obstacle. Every boundary of the room, such as the wall, the ceiling, or the window, is an obstacle to the sound wave.
Once the wave comes in contact with the obstacle, two things happen. Firstly, a part of the wave energy gets absorbed by the barrier, usually resulting in an exchange of heat. Secondly, the remaining portion of the wave gets reflected from the obstacle and begins to travel in an opposite direction.
After some time, the sound gets reflected so many times that most of the energy becomes absorbed. As this happens, the sound pressure level (SPL) decreases. The time between sound emission and the moment when the drop in SPL reaches 60 dB is called the reverberation time.
💡 Reverberation is strongly dependent on the frequency of sound, so it must be carefully considered while preparing an architectural design.
RT60 equation
There are two ways to measure the reverberation time. The first one is to use a dedicated device — a level recorder that can plot the SPL against time. The other way is to calculate it based on the parameters of the given room.
The RT60 equation is an empirically found formula that establishes the relation between the reverberation time and the absorptive properties of the room. It can be written as:
RT60 = 0.163 × V / A
where:
- RT60 — reverberation time;
- V — total volume of the room, expressed in m³; and
- A — effective absorbing area of the room, given in m² (sabins).
In the imperial (US) system, where the volume is in cubic feet and the area in square feet, the constant changes and the formula becomes RT60 = 0.049 × V / A.
How to calculate the effective absorbing area
As you probably noticed, the RT60 equation looks deceptively simple. All you have to do is divide the volume of the room by its area and multiply it by a known coefficient. If you give the formula another look, though, you will discover that, in reality, it is much more complicated.
The main idea behind the reverberation time formula is applying the effective absorbing area instead of the regular room surface area. This number can be calculated according to the following equation:
A = S₁·α₁ + S₂·α₂ + … + Sₙ·αₙ = Σ Sᵢ·αᵢ
where each Sᵢ is the area of an individual surface (walls, ceiling, floor, doors, windows) and each αᵢ is the sound absorption coefficient of that surface. The absorption coefficient ranges from 0 (a perfect reflector) to 1 (a perfect absorber, like an open window).
This calculator automatically works out each surface area for you. The net wall area is the perimeter of the room multiplied by its height, minus the area taken up by the doors and windows:
- Wall area = 2 × (length + width) × height − doors − windows
- Ceiling area = length × width
- Floor area = length × width
- Doors area = number of doors × door width × door height
- Windows area = number of windows × window width × window height
Optimal reverberation time formulas
There is no single "correct" reverberation time — the optimum value depends on the volume of the room and, above all, on its purpose. A space used for speech (a classroom or a lecture hall) needs a short reverberation time so that words remain intelligible, while a concert hall benefits from a longer reverberation that makes music sound rich and full.
As a rule of thumb, the optimal reverberation time grows with the logarithm of the room volume. Typical recommended RT60 values (at mid frequencies, around 500–1000 Hz) are:
- Speech / lecture rooms: 0.3 – 0.6 s
- Classrooms: 0.4 – 0.7 s
- Offices and conference rooms: 0.4 – 0.8 s
- Cinemas: 0.6 – 1.0 s
- Chamber music / drama theaters: 1.0 – 1.5 s
- Concert halls (symphonic music): 1.5 – 2.2 s
- Churches and cathedrals (organ music): 2.0 – 3.0 s and above
If your calculated reverberation time is much longer than the recommended value for your room's purpose, you can reduce it by adding absorptive materials — carpets, curtains, upholstered furniture, or dedicated acoustic panels — which increase the effective absorbing area A and therefore shorten RT60.