Volume 119, Issue 2, February 2006
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
119(2006); http://dx.doi.org/10.1121/1.2146085View Description Hide Description
To reduce the effect of wind on a microphone, wind screens are used, usually made of foam or cloth. Although this measure to reduce wind noise is well known and widely used, it lacks a theoretical explanation. One possible explanation was the turbulent wake in the air flow behind the wind screen, another the pressure variations due to turbulence inherent in outdoor wind. In the present paper it is shown that atmospheric turbulence is indeed the cause of outdoor microphonewind noise, and the knowledge that is available in atmospheric boundary layer physics can be used to model the effects of atmospheric turbulence on a bare or screened microphone. A wind screen can be seen as a first order low pass filter for pressure variations due to atmospheric turbulence. Induced wind noise is thus determined by (average) wind speed and wind screen diameter, but also by factors related to frictional and thermal turbulence (roughness length and atmospheric stability). This approach leads to predicted spectral values of the induced “sound” pressure that match experimental data. The result can be used to obtain proper outdoor ambient wind related sound levels by correcting measured sound levels with calculated wind-induced pressure levels.
119(2006); http://dx.doi.org/10.1121/1.2146113View Description Hide Description
Morgan and Raspet [J. Acoust. Soc. Am.92, 1180–1183 (1992)] performed simultaneous wind velocity and wind noisemeasurements and determined that the wind noise spectrum is highly correlated with the wind velocity spectrum. In this paper, two methods are developed for predicting the upper limits of wind noisepressure spectra from fluctuatingvelocity spectra in the inertial range. Lower limits on wind noise are estimated from two theories of the pressurefluctuations that occur in turbulence when no wind screen or microphone is present. Empirical results for the self-noise of spherical and cylindrical windscreens in substantially nonturbulent flows are also presented. Measurements of the wind velocity spectra and wind noise spectra from a variety of windscreens are described and compared to the theoretical predictions. The wind noise data taken at the height of the anemometer lies between the upper and lower limits and the predicted self-noise is negligible. The theoretical framework allows windscreen reduction to be evaluated in terms of the turbulent inflow properties and establishes practical upper limits on wind noise reduction for varying wind conditions.