Sound Attenuation over Simulated Ground Cover

In dealing with wave propagation over land below 1000 Hz, ground attenuation is an important consideration. Very often, the actual ground surface is covered with a thick layer of vegetation that has a very high over‐all void‐to‐volume ratio. Such a composite boundary should be represented as layered media in order to assess the ground attenuation effect correctly. The wave attenuation characteristics under such conditions are studied in this paper. It is found through theoretical analysis that a layered boundary gives rise to strong selective absorption in certain frequency bands. Experimental data has been taken under laboratory conditions over a wide range of normalized parameters, and the results quantitatively confirm many aspects of the predictions. Both theory and experiment indicate that significant attenuation to very low frequencies can be expected for sound propagation over a deep layer of vegetation. Other results include the confirmation that there is a strong sound‐pressure gradient in the vertical direction near the layered boundary, that the excessive attenuation can increase at most 6 dB per doubling distance, and that the effectiveness of ground attenuation depends critically on the elevation angle of the sound source.