The materials
Introduction to auditorium acoustics

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: $R=\frac{Z_{\text{m}}\text{cos}\left(\theta \right)-Z_{\text{c}}}{Z_{\text{m}}\text{cos}\left(\theta \right)+Z_{\text{c}}}$