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Long range downwind propagation of low-frequency soundThe 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.
Document ID
19850018007
Document Type
Technical Memorandum (TM)
Authors
Willshire, W. L., Jr.
(NASA Langley Research Center Hampton, VA, United States)
Date Acquired
September 5, 2013
Publication Date
April 1, 1985
Subject Category
Acoustics
Report/Patent Number
NASA-TM-86409
NAS 1.15:86409
Funding Number(s)
PROJECT: RTOP 505-31-33
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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