Acoustics/Anechoic and Reverberation Rooms
Acoustic experiments often require to realise measurements in rooms with special characteristics. Two types of rooms can be distinguished: anechoic rooms and reverberation rooms.
The principle of this room is to simulate a free field. In a free space, the acoustic waves are propagated from the source to infinity. In a room, the reflections of the sound on the walls produce a wave which is propagated in the opposite direction and comes back to the source. In anechoic rooms, the walls are very absorbent in order to eliminate these reflections. The sound seems to die down rapidly. The materials used on the walls are rockwool, glasswool or foams, which are materials that absorb sound in relatively wide frequency bands. Cavities are dug in the wool so that the large wavelength corresponding to bass frequencies are absorbed too. Ideally the sound pressure level of a punctual sound source decreases about 6 dB per a distance doubling.
Anechoic rooms are used in the following experiments:
Intensimetry: measurement of the acoustic power of a source.
Study of the source directivity.
The walls of a reverberation room mostly consist of concrete and are covered with reflecting paint. Alternative design consist of sandwich panels with metal surface. The sound reflects a lot of time on the walls before dying down. It gives a similar impression of a sound in a cathedral. Ideally all sound energy is absorbed by air. Because of all these reflections, a lot of plane waves with different directions of propagation interfere in each point of the room. Considering all the waves is very complicated so the acoustic field is simplified by the diffuse field hypothesis: the field is homogeneous and isotropic. Then the pressure level is uniform in the room. The truth of this thesis increases with ascending frequency, resulting in a lower limiting frequency for each reverberation room, where the density of standing waves is sufficient.
Several conditions are required for this approximation: The absorption coefficient of the walls must be very low (α<0.2) The room must have geometrical irregularities (non-parallel walls, diffusor objects) to avoid nodes of pressure of the resonance modes.
With this hypothesis, the theory of Sabine can be applied. It deals with the reverberation time which is the time required to the sound level to decrease of 60 dB. T depends on the volume of the room V, the absorption coefficient αi and the area Si of the different materials in the room :
Reverberation rooms are used in the following experiments:
measurement of the ability of a material to absorb a sound
measurement of the ability of a partition to transmit a sound
measurement of sound power