Application of Ray Theory to Downward Propagation of Low Frequency Noise

date February 19, 1986
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Wednesday, February 19, 1986 4:00 p.m.

James A. Hawkins, Jr.
Applied Research Laboratories
The University of Texas at Austin

With the advent of large megawatt sized wind turbines has come an interest in the low frequency noise from the turbines . NASA has reported measurements of downwind propagation for very low frequency noise from a large wind turbine generator . The source height was 40 m , and ground measurements were taken out to 10,000 m . The noise was found to spread spherically near the source (out to 450 m) but cylindrically thereafter. We present ray theory calculations of the propagation loss for this experiment. Our computer program is adapted to the atmosphere from MEDUSA , a ray theory program developed for underwater sound. We assume a realistic ground impedance to calculate reflections at the ground and a logarithmic wind velocity profile to account for sound speed variation. Results for 8 Hz show that channeling caused by downwind refraction is responsible for the cylindrical spreading observed downrange of the source. Near the source, spherical spreading occurs because channeling has not yet been established. The results are in quantitative agreement with NASA’s data.