![](/assets/images/picto-drapeau-france-3a76576a5d60a512053b4612ab58dae5.png)
4. The materials
As we saw in the first paragraph, a surface can be characterized acoustically by its admittance β. We also use the notion of acoustic impedance Z, which is the inverse of admittance: Z = 1/β. When a sound wave emitted in a room hits a wall, part of it is reflected back into the volume. Another part is absorbed by the material and a third transmitted to adjacent structures (a neighboring room, for example). The reflection coefficient of a plane-wave wall R is defined as the ratio between the amplitude of the reflected wave and the amplitude of the incident wave. It is expressed as a function of the material's impedance Z
m
by the following expression:
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