Low-frequency wind noise reduction by spherical windscreens

Abstract

Spherical windscreens are surprisingly effective at reducing wind noise at frequencies for which the turbulence scale is much larger than the windscreen. In the 1930s, Phelps postulated that the low-frequency reduction of windscreens could be explained by assuming that the dc flow pressure distribution around a sphere also applied to low-frequency fluctuations and wind noise. The area average of the dc pressure distribution was then calculated and shown to be smaller than the stagnation pressure. Morgan extended this idea by measuring the pressure distribution around a porous foam windscreen. Recent measurements [Webster et al., J. Acoust. Soc. Am. 118, 2009 (2005)] have shown that the pressure fluctuations at low frequency do not follow the dc distribution and that the velocity and pressure fluctuation correlations are drastically reduced by the presence of the windscreen. A simple calculation demonstrates that the decorrelation of the pressure fluctuations is sufficient to explain the observed low-frequency reduction afforded by the 9.0 cm diameter windscreen. [Research supported by the U.S. Army TACOM-ARDEC at Picatinny Arsenal, NJ.]