MODELS AND MEASUREMENTS OF SOUND-PROPAGATION FROM A POINT-SOURCE OVERMIXED IMPEDANCE GROUND

Measurements of the excess attenuation of sound from a point source ov er mixed impedance ground in an anechoic chamber are compared with pre dictions obtained from models based on (a) the semiempirical theory du e to De Jong, (b) Nyberg's theory, (c) a Fresnel-zone approximation, a nd (d) a boundary element code. The impedance discontinuities studied in this work are perpendicular to the source-receiver line. When there is a single discontinuity between acoustically hard and finite impeda nce surfaces, the De Jong semiempirical model, the Fresnel-zone model and boundary element code are found to give satisfactory agreement wit h measured data, The frequency of the first maximum in attenuation is found to be highest at approximately 70% hard surface cover rather tha n at the 100% expected. It is argued that this is a result of edge dif fraction. When extended to multiple impedance discontinuities, the De Jong semiempirical model performs poorly. However, both Nyberg's theor y and the boundary element code give good agreement with measured resu lts, and the Fresnel-zone model gives qualitative agreement with the m easured data. The measurements confirm that, under certain constraints given by Nyberg's theory, the ground effect due to mixed impedance ma y be determined from that predicted by using the area-average impedanc e. (C) 1997 Acoustical Society of America.