This calculator estimates the absorption coefficients of multi layer porous aborbers. Up to four absorbers can be modelled at once, each of which can have up to four layers. Each layer can be either a porous absorbent or an air gap. It is assumed that there is a rigid backing (such as a solid wall) behind the last layer.

The calculater works as follows. For each layer, the characteristic impedance and complex wavenumber are estimated using an empirical model. The transfer matrix method is then used to calculate the surface impedance of each layer. The pressure reflection coefficient is calculated from the surface impedance of the outermost layer. Finally, the absorption coefficient is calculated from the pressure reflection coefficient. Random incidence absorption coefficients are estimated using Paris's formula.

This calculator makes no attempt to estimate excess absorption at the edges of a finite sample. Random incidence absorption coefficients estimated using the calculator are therefore likely to be lower than those measured in a reverberation chamber.

Graphs are displayed using SVG graphics. Versions of Microsoft Internet Explorer prior to IE9 will require the installation of a plug-in such as the Adobe SVG Viewer. Most other browsers (eg. Firefox, Safari) support SVG directly and do not require a plug-in.

The formulae used by the calculator have been taken from the following sources: