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Long range downwind propagation of low-frequency sound
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Author and Affiliation:
Willshire, W. L., Jr.(NASA Langley Research Center, Hampton, VA, United States)
Abstract: The propagation of low-frequency noise outdoors was studied using as the source a large (80-m 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 300 m to 10,000 m (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 was amplitude predictions were found to be more than 20 dB smaller than the measured levels. Ray-tracing results were used to qualitatively explain measured trends. A normal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.
Publication Date: Apr 01, 1985
Document ID:
19850018007
(Acquired Nov 13, 1995)
Accession Number: 85N26318
Subject Category: ACOUSTICS
Report/Patent Number: NASA-TM-86409, NAS 1.15:86409
Document Type: Technical Report
Publisher Information: United States
Contract/Grant/Task Num: RTOP 505-31-33
Financial Sponsor: NASA; United States
Organization Source: NASA Langley Research Center; Hampton, VA, United States
Description: 21p; In English
Distribution Limits: Unclassified; Publicly available; Unlimited
Rights: No Copyright
NASA Terms: ACOUSTIC ATTENUATION; ACOUSTIC PROPAGATION; LOW FREQUENCIES; NOISE PROPAGATION; SOUND WAVES; ACOUSTIC IMPEDANCE; NOISE (SOUND); NOISE GENERATORS; SOUND PROPAGATION; WIND TURBINES
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