📄️ Reflection Coefficient
The amplitude of a sound wave is reduced with a combination of geometrical spreading (sound intensity being proportional to the inverse of the square of the distance), attenuation (friction and relaxation processes in air), and boundary interactions. In this section we focus on the last point.
📄️ Absorption Coefficient
As an incident wave hits a boundary, a part of it is reflected, as described by the reflection coefficient, and the rest of it is considered to be absorbed by the material at the boundary. Depending on the surface, a part of the energy might be transmitted through it and ideally this energy is accounted for in the absorption coefficients so that the relationship between absorption and reflection can be written as
📄️ Surface Impedance
Similar to the pressure reflection coefficient, the surface impedance can describe the changes in the magnitude and phase of a reflected pressure at the surface of interest. The real part of the surface impedance is called resistance, whereas the imaginary part is called reactance. The surface impedance is a less intuitive quantity, as defined by the following equation.
📄️ Conversion
Most available data are unarguably the Sabine or practical absorption coefficient provided by the manufacturers. For the low frequency prediction using DGM, we need to convert them to pressure-based quantities, i.e., surface impedances or reflection coefficients. This is a challenging task because there are infinitely many surface impedance values that correspond to a single absorption coefficient.