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The near-ground structure of the nocturnal sound field
Roger Waxler1, Carrick L Talmadge, Shantharam Dravida
1National Center for Physical Acoustics, University of Mississippi, University, Mississippi 38677, USA.
Abstract:
The near-ground behavior of the low-frequency (100 Hz to 500 Hz) sound field in the nocturnal sound duct is studied theoretically and experimentally. In the first few meters of the atmosphere, narrow-band sound fields are found to have a characteristic vertical structure. The sound field is the superposition of a "surface mode," whose magnitude decreases monotonically with altitude, with a sum of "higher modes," each of whose magnitudes has a pronounced minimum a few meters from the ground at approximately the same height. The surface mode attenuates to negligible levels after a few hundred meters from the source. Consequently, more than a few hundred meters from a narrow-band source, there is a "quiet height" at which the sound level is reduced by 10 to 15 dB relative to its value on the ground. The narrow-band quiet height is shown to be a robust feature of nocturnal sound propagation.
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