Volume 104, Issue 5, November 1998
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
104(1998); http://dx.doi.org/10.1121/1.423852View Description Hide Description
Theoretical calculations for the diffraction of sound by large spheres and cylinders with finite impedance surfaces are reported. The differences between existing two-dimensional and new three-dimensional results are made explicit and are shown to involve a simple correction factor in the case of a large sphere. The results for propagation over an infinitely long cylinder have a bearing on the widely used analogy between sound propagation over a curved surface and sound propagation in a refracting atmosphere above an impedance plane. Specifically, it is found that there is a rigorous analogy between sound propagation above a large circular cylinder and propagation in a medium where the sound speed varies exponentially with height. This differs from the bilinear profile that is often used when exploiting the analogy [see, for example, J. Acoust. Soc. Am. 83, 2047–2058 (1988)]. Predictions for both profiles are found to agree well with each other and with the published data in the shadow zone, but considerable discrepancies are found in the penumbra region.
104(1998); http://dx.doi.org/10.1121/1.423853View Description Hide Description
Point source propagation over a screen located on a finite impedance surface representative of grass-covered ground is investigated under upwind and downwind conditions. The theoretical part of the investigation involves extended use of parabolic equation methods (PE) allowing for the changes in the vertical wind speed profile when the wind field passes the screen. The influence of turbulence is also implemented. The experimental part of the investigation relies on a scale model technique based upon a 1:25 scaling ratio and a triggered spark source. The main results relate to the size of the insertion loss of a screen under windy conditions and to the acoustic importance of the redirection of the flow before and after the screen.