The calculation model

The calculation model in

uses Sabine’s relationship between the total amount of sound absorption in a room and the room’s reverberation time. In essence, the relationship states that the reverberation time is lowered if you introduce more absorption.

Calculations are made in

the octave bands 125, 250, 500, 1000, 2000 and 4000 Hz. This means that the total sound absorption from the room itself (including an absorbing ceiling) and from all furniture and wall absorbers is summed in each octave band. Sabine’s relationship is then used to calculate the corresponding reverberation time in each octave band. These values can be seen for your individual rooms by pressing the graph button.

However, it requires a lot of knowledge and experience

in room acoustics to interpret a reverberation time spectrum. To simplify the interpretation we have chosen to use the technique that is used in Sweden to qualify normal rooms in offices, schools, hotels etc to the building regulations (for more information see SS 252 68:2007).

The Swedish requirement uses a single number for the reverberation time, but this actually means three different reverberation time requirements:

  1. The values in each individual octave band 250, 500, 1000, 2000 and 4000 Hz can be maximum 0.1 s above the required single number value

  2. The mean value of the octave bands 250, 500, 1000, 2000 and 4000 Hz cannot exceed the required single number value

  3. The value in the 125 Hz octave band can be maximum 0.2 s above the required single number value

The single number value

that is calculated for your room follows all three requirements, so you don’t have to think about them. When choosing products for your room it can however be helpful to check the reverberation graph to see which frequency band that needs additional sound absorption at the moment. This is particularly important for the requirement in the 125 Hz octave band.

The implementation

of the calculation model used here follows the international standard EN 12354-6:2004. However, there are some limitations and assumptions to this model. Among the most important are:

  • The model assumes that the diffusion and absorption is reasonably equally distributed over the room’s surfaces. This is fulfilled if the room is normally furnished and/or has diffusive elements. Large, non-flat surfaces (such as walls with windows, bookshelves etc) are effective diffusors.

  • The room needs to be reasonably regular in shape, i e it should not be very long and narrow.

  • The calculated values are valid for normal indoor temperature and humidity (20 °C and 50 – 70 %)


If you want to learn more about acoustics, calculation model and how to achieve a good sound environment etc., Acousticfacts regularly runs courses and seminars. 

All of our courses and seminars is held in English, French or in Swedish. They can also be tailor made for your needs. For inquiries please email us.