# Acoustics/Anechoic and reverberation rooms

![](Acoustics_anechoic_reverberation.JPG "Acoustics_anechoic_reverberation.JPG")

## Introduction

Acoustic experiments often require to realise measurements in rooms with
special characteristics. Two types of rooms can be distinguished:
anechoic rooms and reverberation rooms.

## Anechoic room

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:

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Intensimetry: measurement of the acoustic power of a source.

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Study of the source directivity.

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## Reverberation room

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 off the walls many times
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:

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measurement of the ability of a material to absorb a sound

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measurement of the ability of a partition to transmit a sound

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Intensimetry

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measurement of sound power

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