Noise White Paper Outline - Minuteman Wind, LLC

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A discussion of complex propagation models that include all these factors is beyond thescope of this paper. More information can be found in Wagner, et al. (1996). Forestimation purposes, a simple model based on the more conservative assumption ofhemispherical sound propagation over a reflective surface, including air absorption isoften used [International Energy Agency, 1994]:Lp= Lw− 10 log 10 22( πR ) −αRHere Lp is the sound pressure level (dB) a distance R from a sound source radiating at apower level, Lw , (dB) and α is the frequency-dependent sound absorption coefficient.This equation can be used with either broadband sound power levels and a broadbandestimate of the sound absorption coefficient (α = 0.005 dB per meter) or more preferablyin octave bands using octave band power and sound absorption data. The total soundproduced by multiple wind turbines would be calculated by summing up the sound levelsdue to each turbine at a specific location using the dB math mentioned above.An example of the sound that might be propagated from a singe large modern windturbine is shown in Figure 11. This example assumes hemispherical sound propagationand uses the formula presented above. In this case the wind turbine is assumed to be on a50 m tower, the source sound power level is 102 dB(A), and the sound pressure levels areat estimated at ground level.60Sound Pressure Level, dB(A)555045403530Turbine sound power = 102 dB(A)Sound absorption coefficient = 0.005 dB(A)/mTower height = 50 m0200400 600Distance from Turbine Tower, meters8001000Figure 11: Example of propagation of sound from a large wind turbineThe location of the receptor is also significant. Upwind of a wind turbine there may belocations where no sound is heard. On the other hand sound may be propagated moreeasily downwind.Ambient Sounds & Wind SpeedThe ability to hear a wind turbine in a given installation also depends on the ambientsound level. When the background sounds and wind turbine sounds are of the samemagnitude, the wind turbine sound gets lost in the background.Ambient baseline sound levels will be a function of such things as local traffic, industrialsounds, farm machinery, barking dogs, lawnmowers, children playing and the interactionWind Turbine Noise Issues Renewable Energy Research Laboratory Page 18
of the wind with ground cover, buildings, trees, power lines, etc. It will vary with time ofday, wind speed and direction and the level of human activity. As one example,background sound levels measured in the neighborhood of the Hull High School in HullMassachusetts on March 10, 1992 ranged from 42 to 48 dB(A) during conditions inwhich the wind speed varied from 5 to 9 mph (2-4 m/s).Both the wind turbine sound power level and the ambient sound pressure level will befunctions of wind speed. Thus whether a wind turbine exceeds the background soundlevel will depend on how each of these varies with wind speed.The most likely sources of wind-generated sounds are interactions between wind andvegetation. A number of factors affect the sound generated by wind flowing overvegetation. For example, the total magnitude of wind-generated sound depends more onthe size of the windward surface of the vegetation than the foliage density or volume[Fégeant, 1999]. The sound level and frequency content of wind generated sound alsodepends on the type of vegetation. For example, sounds from deciduous trees tend to beslightly lower and more broadband than that from conifers, which generate more soundsat specific frequencies. The equivalent A-weighted broadband sound pressure generatedby wind in foliage has been shown to be approximately proportional to the base 10logarithm of wind speed [Fégeant, 1999]:L A, eq ∝ log 10The wind-generated contribution to background sound tends to increase fairly rapidlywith wind speed. For example, during a sound assessment for the Madison (NY)Windpower Project, a project in a quiet rural setting, the background sound was found tobe 25 dBA during calm wind conditions and 42 dBA when the wind was 12 mph (5.4m/s). Background sound generated during sound measurements on a small wind turbineare shown in the Figure 12 [Huskey and Meadors, 2001]. The graph includes alogarithmic fit to that data based on the model mentioned above.( U )Background Sound Pressure, Lp806040200ModelMeasurements0246 8 10Wind Speed, m/sFigure 12: Sample background noise measurementsas a function of wind speedSound levels from large modern wind turbines during constant speed operation tend toincrease more slowly with increasing wind speed than ambient wind generated sound. Asa result, wind turbine noise is more commonly a concern at lower wind speeds [Fégeant,1999] and it is often difficult to measure sound from modern wind turbines above wind1214Wind Turbine Noise Issues Renewable Energy Research Laboratory Page 19
Page 1: Wind Turbine Acoustic NoiseA white
Page 7: human ear's response to changes in
Page 10 and 11: There is no reliable evidence that
Page 12 and 13: nature, but tonal components may oc
Page 14 and 15: sound emission from modern wind tur
Page 16 and 17: Figure 10: Summary of NREL study of
Page 20 and 21: speeds of 8 m/s because the backgro
Page 22 and 23: In the U.S., although no federal no
Page 24 and 25: o Small turbines: national standard
Page 26: Manwell et al., Wind Energy Explain

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