Page 23 - Winter2021
P. 23

on the order of Uc /f, so that lower frequency sources are found at greater heights (Yu, 2009).
Aeroacoustic Sources
Wind does produce sounds, even if they are not signifi- cant sources of microphone noise. These aeroacoustic sources, sound generated by unsteady wind, commonly depend on the interaction of the turbulent wind with a surface. Lighthill’s acoustic analogy provides the basis for our understanding of sound propagating from a tur- bulent fluid. Conceptually, turbulent eddies radiate as acoustic sources with a quadrupolar directivity (Light- hill, 1952, 1954). Jet noise is a common example, where the acoustic intensity scales according to the flow speed raised to the eighth power. Extensions of the acoustic analogy by Curle (1955) and Doak (1960) consider sound radiation from stationary surfaces embedded within tur- bulence. Pressure fluctuations on a surface radiate sound with dipole directivity. In this case, acoustic intensity scales according to the flow speed raised to the sixth power. Later, Ffowcs Williams and Hawkings (1969) extended the theory to account for moving surfaces.
A cylindrical body in the wind, such as a pole or a wire, will periodically shed turbulent eddies from its leeward side. The unsteady forces load the surface, causing sound radiation known as an Aeolian tone. Careful observa- tions by Lord Rayleigh showed that a wire oscillates in a plane perpendicular to the direction of flow such that the sound radiates most strongly along the path of oscil- lation (Strutt, 1879).
Flow through outdoor vegetation generates a common source of natural noise. Trees generate sound aerody- namically, primarily as Aeolian tones from their leaves and branches and mechanically through unsteady con- tact between branches (Fégeant, 1999; Bolin, 2009). Low-frequency contributions in both coniferous and deciduous trees are due to mechanical contact between branches and unsteady aerodynamic forces. Fully leafed deciduous trees generate high-frequency noise through the unsteady contact of leaves with other leaves and branches (Fégeant, 1999).
Wind Noise Reduction in the
Audible Band
In outdoor audio recordings, the stagnation pressure is typically the dominant wind noise contribution. For
this reason, reduction methods focus on the wind flow around the microphone, typically with a covering known as a windscreen. Although varied in shape, size, and material, the intent of all windscreens is the same: reduce the stagnation pressure received at the microphone with minimal insertion loss for acoustic signals. Windscreens only mitigate turbulent pressure and generally cannot improve the ratio of any acoustic signal to acoustic noise because they will attenuate both equally. Distinct from windscreens, pop screens or pop filters are used in voice recording to shield a microphone from a burst of air generated in speech or singing, not to mitigate sustained turbulent flow (Wuttke, 1992).
As an alternative to windscreens, streamlined nose cones are commonly placed over microphones in indoor air flows to reduce stagnation pressure. However, the largest eddies of turbulence indoors, such as those generated by a fan or an exhaust duct, are several orders of magnitude smaller than those of atmospheric turbulence. The inten- sity of turbulence in such flows is also typically lower. As a consequence, when used in wind outdoors, these nose cones are not as effective (Webster et al., 2010).
Windscreen material properties that affect wind noise include porosity and tortuosity. Porosity is the amount of open space within the material. Tortuosity is an average measure of how twisted the network of pores is through the material. Permeability, a measure of resistance to flow through the open pores, is determined by porosity and tortuosity. Highly permeable materials have no effect on the flow or wind noise. As permeability decreases, the flow is impeded and diverted, resulting in wind noise reduction. If permeability is too low, however, the porous interface restricts flow to a degree that turbulent wake noise increases (Xu et al., 2011; Zhao et al., 2017). Poros- ity plays a role in dissipating turbulent vortices incident to a windscreen (Geyer, 2020).
Polyurethane foam windscreens are often used for wind noise reduction in outdoor acoustic monitoring appli- cations. They are most appropriate for omnidirectional pressure transducers (Wuttke, 1992). Typical shapes include cylinders, spheres, and ellipsoids, which are chosen to accommodate to the shape of the microphone and its enclosure. Some of these shapes are seen in Figure 5. Char- acteristic dimensions are normally on the order of 10 cm, with larger windscreens selected for noise reduction at
Winter 2021 • Acoustics Today 23

   21   22   23   24   25