An impulse method of measuring normal impedance at oblique incidence

An experimental method of determining the oblique incidence behaviour of sound absorbing materials is given. The method involves the measurement of acoustic impulses at the surface of an absorbing material and comparing the complex frequency components with those of a reference signal recorded remote from the surface. Amplitude and phase characteristics of the reflected signal are thus obtained and hence the normal impedance of the material is found. Comparison between measurements made by the “surface” method and standing wave tube measurements shows good agreement for normal incidence. However, at very oblique angles of incidence the method is in obvious error, the accuracy being limited by a sphericity effect.