Reverberation time

Reverberation Time

Typical sequence of sound level during the measurement of reverberation time T. Initially the measurement signal, a very loud pink noise, defines the level. After switching off the noise, the level decreases until it reaches the background noise.

The most important physical quantity used to describe the acoustical properties of a room is its reverberation time. It is a measure for the reverberance of a room, and is defined as the length of time required for the sound level in a room to drop by 60 dB. The duration of the reverberation time in a given room mainly depends on the sound absorption properties of its walls, floors and ceilings, and its furnishings as well as on the volume of the room. The reverberation time is frequency-dependent since stones, wood, carpets or textiles absorb sound to a different extent at varying frequencies.

The reverberation time best suited for a certain room mainly depends on its volume and usage. These factors are also taken into account in DIN 18041 and ÖNORM B 8115-3. Both standards set optimum reverberation times and corresponding tolerance ranges for different rooms.

The Room Acoustics Calculator enables the simple application of these complex rules. Using this calculator, all you have to do is select the usage, the applicable standard, the volume of the room and its typical furnishings for a given number of persons. The calculator will then determine all relevant quantities for you.

The reverberation time is the essential acoustic quantity for all rooms described below.

In the case of auditoria one has to decide whether they are only used to play music, or for music and speech purposes alike, or solely for speech purposes. This distinction should be made very carefully as it determines the necessary reverberation time which must be longer for music than for speech.

Lecture halls, conference or meeting rooms, classrooms:
In all these rooms a high level of speech intelligibility is required. In order to achieve optimum reverberation times, it is usually necessary to only partially construct suspended ceilings from absorbent material. The standards contain several examples illustrating the position of absorbent sections. The rule of thumb for these constructions is that the ceiling must be reflective in the middle part of the room. Most manufacturers of acoustic ceilings offer systems that allow the combination of sound absorbent and sound reflective elements to meet the requirements mentioned above.

Classrooms for music, practice and rehearsal rooms:
The acoustic design of classrooms for music lessons and practice rooms necessitates the right balance between two opposing requirements. Often similar acoustic conditions as in a concert hall are desired, i.e. rooms with a long reverberation time. Teachers and pupils should, however, also be able to notice even the faintest nuances of their instruments which would require a minimum reverberation time for the room. A very good discussion of these relations can be found in the book by Meyer on pages 197-199. He particularly recommends to ensure a short enough reverberation time at low frequencies and sufficient diffusion of the sound field.

Chamber music halls, concert halls, theaters, opera houses:
These rooms are amongst the most demanding ones. The standards can only recommend preferred reverberation times. When designing chamber music halls, concert halls, theaters, and opera houses care must be taken to ensure, among other things, that for the audience the acoustic quality is more or less the same irrespective of their seats. This usually requires complex computer simulations to determine the ideal inclination of walls and ceilings as well as a favorable distribution of absorbing, reflecting and sound scattering surfaces (see also Fasold, Sonntag and Winkler or Barron).

Listening rooms, home cinemas:
In order to achieve a well-balanced, lively and not too dry acoustics in hi-fi listening rooms or home cinemas, a reverberation time linear to the frequency should be aspired. The recommendations for the reverberation time given in the Loudspeakers Calculator approximately correspond to the requirements for rooms with multimedial usage according to DIN 18041 and ÖNORM B 8115-3. The tolerance range, however, is more generous.

Recording studios:
DIN 15996, a special standard for recording studios, sets very high requirements for the reverberation time. According to these, control rooms should be as neutral as possible so that the sound engineer only hears the music of the relevant recording. This can only be achieved by means of very short reverberation times. Furthermore, the permitted tolerance range is very narrow. Keeping within its limits requires a lot of effort and constant metrological control during construction.