Abstract
It is found that it is possible to simulate the influence of a constant sound speed gradient on outdoor sound propagation by curving the ground instead of the sound rays. The verification of the method was performed in two steps. (1) A ray-theoretical study was made to check that the errors in the angle of incidence towards the ground, the distance to the shadow zone in the case of a negative gradient, and the phase difference between the ground reflected and the direct ray are small. (2) Measured sound pressure relative to free field from a spark source above a cylindrically curved sheet of Perspex were compared to the pressure calculated by using the theory of Pridmore-Brown, Pierce, and Rasmussen for a temperature-stratified medium. The agreement is good both for negative and positive gradients. In the Nordic countries, noise prediction and immission measurement of environmental noise from industries shall be performed with the receiver downwind of the source or with a positive temperature gradient, since the reproducibility of outdoor measurements then is better and the measured sound level represents the most disturbing case. The cause of this is analyzed in the paper.
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