Page 46 - Spring 2015

D. Keith Wilson
Postal:
US Army Engineer Research and Development Center 72 Lyme Road Hanover, New Hampshire 03755-1290 USA
Email:
D.Keith.Wilson@usace.army.mil
Chris L. Pettit
Postal:
Aerospace Engineering Department US Naval Academy 590 Holloway Road Annapolis, Maryland 21402-5042 USA
Email:
pettitcl@usna.edu
Vladimir E. Ostashev
Postal:
US Army Engineer Research and Development Center 72 Lyme Road Hanover, New Hampshire 03755-1290 USA
Email:
Vladimir.Ostashev@noaa.gov
sound Propagation in the Atmospheric boundary layer
Advances in computational methods have helped to reveal the complex impacts of the atmosphere on sound propagation, and provide
new approaches for quantifying these impacts.
Introduction
Sound propagation in the atmosphere impacts noise emissions from factories and highways, detection and location of sound sources, and even remote sensing of atmospheric turbulence. Scientific study of the subject can be traced back to the 1700s (Embleton, 1996; Ostashev, 1997; Beyer, 1999). A number of excellent review articles (e.g., Piercy et al., 1977; Embleton, 1996; Attenborough, 2002) describe the present state of understanding. Most of these reviews are structured as follows. Geometrical spreading of sound waves from a point source is first introduced, which causes the sound intensity to diminish as 1/R2, where R is the distance from the source (that is, a 6-dB loss per doubling of distance). The exponential attenua- tion of sound amplitude, resulting from heat conduction, viscosity, and molecular relaxation processes in air, is then described. Next, reflections from the ground are discussed, which may be conveniently conceptualized as emanating from an image source, the magnitude and phase of which is determined by the ground surface impedance. Refraction by vertical gradients of wind and temperature in the atmo- sphere are typically discussed thereafter.
Last, complications such as those produced as a result of the scattering of sound by atmospheric turbulence as well as from interactions with natural and man-made terrain features such as hills, vegetation, walls, and buildings may be introduced. Recent reviews also discuss the advances in numerical methods for outdoor sound propagation. Like many fields, progress during the past several decades has been coupled to advances in computational capabilities and numerical methods. In par- ticular, the late 1980s saw the introduction of numerical techniques that do not depend on the high-frequency approximations inherent to ray tracing, namely the fast-field program (FFP) and the parabolic equation (PE). Recently, finite-differ- ence time-domain (FDTD) methods, capable of handling complications such as multiple reflections from trees and buildings, have also been introduced.
The preceding ordering of topics is natural in the sense of introducing the subject as a sequence of increasingly complex phenomena. It also happens to correspond, loosely, to the historical progress of research in outdoor sound propagation. But, intentionally or not, the ordering of topics may convey the sense that sound propagation outdoors is primarily a deterministic phenomenon (i.e., geometrical spreading, ground interactions, refraction by mean gradients of wind and tem- perature) while deemphasizing propagation phenomena that are fundamentally random or predictable only with substantial uncertainty (i.e., interactions with turbulence and complex terrain features). This article attempts to reverse that per- spective. Hopefully, by the end, the reader will have gained an appreciation for
44 | Acoustics Today | Spring 2015 , volume 11, issue 2 ©2015 Acoustical Society of America. All rights reserved.