Random Incidence Absorption

The most useful quantity in most room acoustics applications is the random incidence absorption coefficient. With this you can predict the reverberation time of the room or the sound level for a known sound powwer level. Sabines equation is the most well known relationship between absorption and reverberation.

The random incidence absorption differs from the normal incidence absorption for a number of reasons, firstly the absorption coefficient will vary with different angles of incidence, secondly the absorption coefficient will depend on the area of the absorber due to difraction effects at the edge of the material. Zorba can calculate the effect of these influences and estimate the random incidence absorption coefficient for the system. Note that at present the area of the absorber is assumed to be 12 m² which is the standard test area for measuring absorption coefficients to ISO 354.

For most materials which are so called locally reacting, the absorption coefficient varies in a predictable way with the angle of incidence of the sound wave. Paris developed an equation for averaging the absorption coefficient over all angles of incidence, unfortunately this does not agree with measurements because of the effects of diffraction around the edges of the sample. In certain cases the measured absorption coefficient can exceed unity, a fact which is intuitively difficult to understand. For normal test samples the diffraction effects are strongest between 100 Hz and 1000 Hz.

An early attempt at predicting diffractive effects was made by Northwood, which involved predicting the diffraction for an infinitely long strip. More recently Thomassen has developed a theory for predicting the diffraction of square patches of material which is relatively easy to apply and which appears to be reasonably successful for a useful range of material arrangements.