Page 25 - Winter2021

leeward sides of a windscreen. In measurements on a porous foam sphere, Raspet et al. (2007) instead observed that the surface stagnation pressure decorrelated over distances much shorter than the free atmospheric turbulence. They hypothesized that flow distortion around the windscreen reduces turbulent length scales. As a result, the spatial aver- age over the windscreen surface is far more effective and the wind noise is reduced at lower frequencies than those predicted by spatial averaging alone (Raspet et al., 2019).
Wind Noise Reduction in the Infrasound Band
At frequencies below human hearing in the infrasound band, wind noise becomes more intense due to the increase in the turbulence spectral density with decreas- ing frequency. Infrasound sensors are used for measuring the geophysical and anthropogenic sources of sound, such as volcanoes, tornadoes, meteorites, and large explosions, at frequencies down to hundredths of a hertz. Because these sensors often are placed in sheltered locations on the ground surface or below it, intrinsic pressure will often dominate the wind noise. For long-range infrasonic sensing, such as that conducted by the International Monitoring System (IMS) for monitoring of atmospheric nuclear explosions, wind noise is the primary limitation to detection (Marty, 2019). Many methods exist for reduc- ing infrasonic wind noise. The effective frequency band of these methods principally depends on the dimension of the filter system (Raspet et al., 2019). Because lower frequencies are caused by larger eddies, systems for infra- sonic wind noise reduction tend to be much larger than the audio microphone windscreens.
One method for wind noise reduction for permanent infrasound sensor installments, known as a pipe array, consists of a branching network of interconnected solid pipes with inlets arranged over a wide area. A diagram and photograph of a common pipe array configuration known as a standard multiple rosette is shown in Figure 7, a and b, respectively, which covers an approximately 18×18-m2 ground area. Yet again, the wind noise reduction results from the fact that for length scales less than the array aper- ture, atmospheric turbulence will be incoherent relative to acoustic waves. The acoustical response of pipe arrays is not uniform with frequency, however, because waves with a length near the array diameter or smaller will cause arriv- als at the sensor from different inlets to be out of phase. Pipe arrays also suffer from resonance effects. Similar, but
more portable, lengths of microporous “soaker” hoses can be attached to infrasound sensor ports and spread over the ground to act as a spatial filter (Walker and Hedlin, 2010).
Porous domes and wind fence enclosures are alterna- tive types of wind noise mitigation that are free from the acoustical response and resonance issues affecting pipe arrays. For short-term use, small meter-scale-diameter domes are placed over infrasound sensors on the ground, which are effective for relatively high frequencies, above 2 Hz. Larger hemispherical domes ranging from one to six meters in diameter have been made from porous fabrics (Figure 8a) (Noble et al., 2014) and perforated
Figure 7. a: Diagram of a standard multiple rosette variant of a pipe array wind noise filter system. Diagram courtesy of Thomas Gabrielson. b: Photograph of the same configuration at the Sandia National Laboratories Facility for Acceptance, Calibration, and Testing (FACT) Site Array. Photograph courtesy of John Merchant.
Winter 2021 • Acoustics Today 25