Recent studies indicate that the major sources of wind noise (under 50 Hz) measured on outdoor microphones is due to turbulent eddies moving across the microphone [S. Morgan and R. Raspet, J. Acoust. Soc. Am. 92, 1180–1183 (1992)]. Taylor’s frozen turbulence hypothesis states that these eddies retain their shape over a distance large compared to the size of the eddy. Thus it may be reasonable to assume that each eddy presents a specific acoustic signature that remains somewhat stable over a finite distance and time. If this is true, the cross correlation time for signals from two microphones a known distance apart should correspond to the travel time for an eddy to traverse the distance between the microphones. Assuming that the speed and direction of the eddy is the same as the ambient wind, enough information exists to calculate the wind speed and direction.

5-71

An atmospheric sound wave can couple with the poroelastic ground, resulting in ground (or seismic) motion. This is the phenomenon of acoustic to seismic coupling. Microphones are used to measure the sound pressure level at the surface and geophones are used to measure the resulting seismic motion at or below the surface. The seismic:acoustic transfer function (SATF) characterizes a particular site. SATF measurements are reported as a function of the angle of incidence of the sound wave for frequencies 15 Hz to 1 kHz. The angle of incidence varied from 50° to 80°, and normal incidence. Surface vertical and radial-horizontal geophones were used. In addition six vertical geophones at depth intervals of 10 cm, starting at 10 cm, were used. A seismic p-wave survey indicated a first layer depth of 44 cm having a wave velocity of 159 m/s overlying a layer of velocity 379 m/s. This gives a critical angle of incidence of 65°. It was anticipated and confirmed that the SATF would increase in magnitude for certain frequency bands as the angle of incidence approached and passed over the critical angle. Comparisons of experimental results and multilayered elastic and poroelastic media theory will be addressed as time permits. [Work supported by ONR.]