Sound and Sources of Sound

Abstract

Reviewed by M. M. Sevik 1 Sound and Sources of Sound by A. P. Dowling and J. E. Ffowcs-Williams is based on a series of lectures given to undergi aduate engineering students at the University of Cambridge. The book is masterfully written and its use far transcends classroom instruction. In fact, it is recommended reading for all practicing engineers whose major concern is the control of sources of vibrations and sound that are induced by unsteady fluid flow. The book's major contribution to the literature of acoustics begins with Chapter 7. Here, sources of sound which are not due to the vibration of boundary surfaces are described. These include combustion noise, sound generated by the "creation of matter," by externally applied forces on a fluid, and by sound due to turbulent flow. Lighthill's equation is derived and its solution for the sound field is given. Lighthill's famous V s law for jet noise follows from dimensional analysis and its range of validity is discussed. Chapter 8 begins with the derivation of the reciprocal theorem. The ingenious application of this theorem leads to easy solutions of what would otherwise be very difficult problems. Examples are the sound scattered by a bubble entrained in a turbulent flow of water and the sound generated by turbulent quadrupoles near a sharp edge, such as the trailing edge of an airfoil. In many practical cases, sources of sound move relative to an observer. This class of problems is particularly significant in aeroacoustics, where Mach numbers are generally high. The sound field generated by moving sources is described in Chapter 9 in considerable detail. The Fourier decomposition of a wave field in Chapter 10 leads to useful clarifications of the physics of sound generation. It is demonstrated early in the chapter, for example, that "a sound field is generated by elements of the source with sonic phase velocity." In order to deal with the effects of turbulence, a discussion of the statistical analysis of random signals is provided. Cross correlation functions involving both separations in space and delays in time are defined and are used to establish some general characteristics of turbulent flows. The final chapter of the book deals with the response of structures to a spatially and temporally unsteady flow. Fluid loading-which is important in hydroacoustics-is clearly explained. The criteria for temporal or spatial instabilities of

Observations of an Acoustic Aerosol Particle Transducer

“…Small PSL spheres used in this study can be considered moving &&compact'' spheres and radiate a sound power proportional to ;d, where d is the diameter and ; is the r.m.s. particle velocity [8]. The radiated sound is then ampli"ed at the fundamental frequency of the inlet tube, to be discussed in the text below.…”

Practical Ranges of Loudness Levels of Various Types of Environmental Noise, Including Traffic Noise, Aircraft Noise, and Industrial Noise

“…The strong frequency dependence of façade insulation can be understood from the expression for the transmission loss R for a solid wall (and normal sound wave incidence): $R(\omega )=10\text{lg}\left[1+{\left(\frac{\omega m}{2Z}\right)}^2\right]$which is commonly referred to as the mass law for sound transmission [28,45,46]. Here, m is the mass of the wall per unit area, Z is the impedance of air, and ω is the angular frequency of the transmitted sound wave.…”

Acoustic energy output and coupling effect of a pair of thermoacoustic lasers

“…Theories and models have been developed for TA lasers of simple design and configuration 14, 35, 36. These theories and models and fundamentals of acoustic waves (37, 38) related to TA lasers are summarized below.…”

“…The sound pressure level and energy density can be computed from the following equations 37: where is the root mean square value of p ′ .…”