Page 20 - Winter2021

FEATURED ARTICLE
As the Wind Blows: Turbulent Noise on Outdoor Microphones
Gregory W. Lyons, Carl R. Hart, and Richard Raspet
Introduction
What does wind sound like? It certainly makes all kinds of noise. Wind is heard in the rustling of trees and whis- tling inside buildings. It can cause wires to sing a tone. Wind has been recorded as the rumbling in outdoor sound recordings for as long as microphones have been used. The motion picture industry has had to contend with it for at least a century. Infrasound recordings are routinely limited by noise from wind that goes mostly unnoticed by people. You may be surprised to hear, how- ever, that not all these noises are sounds at all. This article explores the various sounds and noises produced by wind, their physical principles, and how these principles are used in reducing unwanted noise.
The Signal Is “Blowin’ in the Wind”
Wind noise becomes unwanted whenever it overwhelms sounds of interest. Anyone who has struggled to under- stand a phone call made outside on a windy day has experienced the essential issue. Wind noise is also a common problem in video recordings made outdoors.
Although a nuisance in these cases, for example, in users of digital hearing aids, wind noise is a serious limitation to hearing outdoors (Launer et al., 2016).
Wind noise occurs wherever there is wind, including on other planets. The Venera 13 and 14 probes that were landed on Venus by the former Soviet Union in 1982 (see go.nasa.gov/2VYTl0f) each included a microphone that recorded signals that Soviet scientists inferred to be wind noise. These recordings were used to estimate the wind speed at each landing site (Ksanfomaliti et al., 1982).
In addition, the NASA Mars 2020 Perseverance rover entry, descent, and landing camera (EDLCAM) system includes an omnidirectional microphone (Maki et al., 2020). One of the first recordings released from this
microphone was of Martian wind noise (Figure 1) (National Aeronautics and Space Administration/Jet Propulsion Laboratory-Caltech, 2021).
In physical acoustics, wind noise refers to a distinct type of noise made by the wind. In this definition, wind noise is the limiting noise recorded on microphones during windy conditions. Although a wide range of whistling, rustling, and rumbling noises are produced by and are readily associated with the wind, only a few are domi- nant sources of wind noise on microphones. This article focuses on microphone wind noise, with a limited discus- sion of other “wind sounds.”
Atmospheric Turbulence
Before examining wind noise, it is helpful to first consider the wind itself. The atmosphere immediately above the ground is the planetary boundary layer (PBL). Although
Figure 1. Time series (a) and spectrum (b) of a wind noise recording from the entry, descent, and landing camera (EDLCAM) of the NASA Mars 2020 Perseverance rover. Wind noise contributions to the spectrum unaffected by microphone frequency response or rover noise fall between the vertical dashed lines at 20 and 400 Hz. Audio recording courtesy of National Aeronautics and Space Administration/ Jet Propulsion Laboratory-Caltech (2021), available at go.nasa.gov/2WTC9t2.
20 Acoustics Today • Winter 2021 | Volume 17, issue 4
https://doi.org/10.1121/AT.2021.17.4.20