Abstract:

In outdoor sound propagation; refraction, diffraction, and scattering may all affect the properties of the propagated signal. The wind and temperature profiles characterize the refractive effects, the boundary conditions determine the effects of diffraction, and the presence of atmospheric turbulence drives the scattering. Signal measurements recently conducted at Penn State Univ.'s Rock Springs micrometeorological field site were focused on measuring the properties of the received complex acoustic signal. A source was configured to radiate continuous tones at five frequencies ranging from 90 to 660 Hz. Ten- to fifty-min recordings were made at distances ranging from 46 to 460 m. Both the magnitude and phase of the received signal were evaluated. The results suggest that use of the complex signal, rather than considering only the sound pressure levels, is valuable in characterizing the propagation. Recorded phase differences exhibit strong coherence between the source frequencies, suggesting refraction dominated propagation. Recorded fluctuations in magnitude exhibit much weaker coherence and appear to be independent of source frequency. Observed travel time fluctuations were on the order of 0.2%. A model is presented that explains these fluctuations by considering the influence of atmospheric turbulence over a fixed propagation path.