Pressure variation above a layer of absorbing material and impedance measurement at oblique incidence and low frequencies

Measurements of the ratio of the acoustical pressures at two points above a sample of typical glass wool are performed for various angles of incidence and various distances from the sound source to the sample. The measurements are compared with predictions made using a recent model, and are used to estimate the surface impedance of the material at oblique incidence. It appears that, for the material studied, surface impedance measurements are possible for frequencies as low as 300 Hz and angles of incidence between 0° and 75°, as long as the source–sample distance is larger than half the sound wavelength in air.