Propagation of sound above a curved surface

Abstract

There is a useful analogy between flat ground and curved ray paths in the atmosphere, and a curved ground surface above which there is no refraction. Many features of the sound field in the refractive shadow can be studied under controlled conditions indoors over a carefully constructed curved surface. Preliminary measurements, and comparison with simple theory restricted to the ideal cases of rigid and pressure‐release boundaries, have been presented at an earlier meeting [J. Acoust. Soc. Am. Suppl. 1 79, S20 (1986)]. The measurements have been completed in the frequency range between 0.3–10 kHz, above a rigid boundary and a surface of finite impedance. Particular attention is given to the region across the shadow boundary. In the shadow, the theory has been extended by removing restrictive approximations and by calculating the creeping wave to higher‐order terms. A numerical algorithm allows the extension to the case of a finite impedance. Above the shadow boundary the sound field is calculated using geometrical theory that accounts for reflection from a curved surface. At the shadow boundary both theories typically agree to within 0.5 dB. Good agreement between measured results and theory is obtained in general, except across the shadow boundary where discrepancies between 2 to 5 dB are observed. The theory may also predict a deeper shadow than measured at higher frequencies above the surface of finite impedance.