Models and measurements of sound propagation from a point source over mixed impedance ground

Measurements of the excess attenuation of sound from a point source over mixed impedance ground in an anechoic chamber are compared with predictions obtained from models based on (a) the semiempirical theory due to De Jong, (b) Nyberg’s theory, (c) a Fresnel-zone approximation, and (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 impedance surfaces, the De Jong semiempirical model, the Fresnel-zone model and boundary element code are found to give satisfactory agreement with measured data. The frequency of the first maximum in attenuation is found to be highest at approximately 70% hard surface cover rather than at the 100% expected. It is argued that this is a result of edge diffraction. When extended to multiple impedance discontinuities, the De Jong semiempirical model performs poorly. However, both Nyberg’s theory and the boundary element code give good agreement with measured results, and the Fresnel-zone model gives qualitative agreement with the measured data. The measurements confirm that, under certain constraints given by Nyberg’s theory, the ground effect due to mixed impedance may be determined from that predicted by using the area-average impedance.