NASA Long Range Down Wind Propagation of Low Frequency Sound
NASA Technical Memorandum, William L. Willshire, 1985
The propagation of low-frequency noise outdoors was studied using as the source a large (803 diameter) 4-megawatt horizontal axis wind turbine. Acoustic measurements were made with low-frequency microphone systems placed on the ground at five downwind sites ranging from 300m to 10,000m (6.3 mile) away from the wind turbine. The wind turbine fundamental was 1 Hz and the wind speed was generally 12-15 m/s at the hub height (80 m).
The harmonic levels, when plotted versus propagation distance, exhibit a 3 dB per doubling of distance divergence. Two plausible explanations identified for this cylindrical spreading behavior were propagation of the low frequency wind turbine noise via a surface wave and downwind refraction. Surface wave amplitude predictions were found to be more than 20 dB smaller than the measured levels.
Kay-tracing results were used to qualitatively explain measured trends. Anormal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.