Sound and Sources of Sound

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 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 mechanical structures subjected to fluid flow are derived from "power flow" considerations. The first six chapters of the book provide the introduction for the ground covered in the latter chapters which is evidently of greatest interest to the authors. The first two chapters give an outline of the basic physics of sound waves and of the fundamental equations that govern these wave fields. Specific applications are provided in Chapters 3 to 6, such as waves in pipes, transmission of sound from one medium to another, ray theory, and various resonators. Frequently, engineering applications of "laboratory" phenomena are cited. For example, the "reheat buzz" which occurs in some jet engines is shown to be analogous in nature to the instabilities which have been observed in a Rijke tube. In summary, Sound and Sources of Sound is both instructive and pleasant to read. The physics is lucidly explained and illustrated by numerous diagrams and examples. Theoretical results are compared with experimental data, whenever possible. The book is not overburdened with detail and the reader never loses sight of the essential information which the authors attempt to convey. The text is generally free of typographical errors, although a few have inevitably passed the authors' scrutiny. An example is the omission of the circumflex in section 11.5 over the symbol representing the power spectral density of the fluctuating pressure. The mathematics is generally clear and straightforward (although a review of generalized functions is recommended before reading Chapters 7 to 11); and the book's usefulness would have been enhanced if bibliographic references had been included.