Abstract
This investigation was undertaken to provide some data on the sound absorption properties of ground surfaces and to develop a method for field measurement of the acoustic impedance of the ground surface. Initial tests with an impedance tube showed that although results were satisfactory for sand with no living vegetation present, the tube was quite ineffective when used for tests on living material, placing unacceptable restrictions on the environment of the material under test. In these tests it was found that there was a slight increase in the sound absorption of granular materials such as sand when a little moisture is present. When vegetable roots are present in the soil it appears that sound absorption is greater than without them, and the attenuation is a maximum at zero moisture content. For field measurements a free wave reflection method has been developed. The specific normal acoustic impedance ratio of the ground surface measured for a range of frequencies from 200 to 1000 Hz was found to vary considerably for different ground surfaces—for example, from 3−3 i for a soft grass patch to 7−17 i for an area of rock covered with chippings.
Published Version
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