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Underwater acoustics
BOOK REVIEW
Underwater Acoustics, by Leon Camp. Published by Wiley Interscience, Great Britain, Price: £8.25. This book is based on a short, intensive series of lectures given to mature students already concerned with underwater acoustic systems, at the Engineering and Physical Sciences Extension of the University of California. The material covered and the level of presentation are eminently suitable, however, for degree students and other relative newcomers to the subject. The first three chapters are concerned with what has come to be described as Classical Acoustic Theory and treat such topics as simple, harmonic oscillators, forced vibrators, analysis of vibration of distributed systems, waves and the wave equation, plane and spherical wave solutions and the concepts of specific acoustic impedance and radiation impedance, together with the theory of plane reflection at normal and oblique incidence. Coverage of material in this section is generally adequate. The section on propagation is altogether too brief to be of real help to anyone coming to the subject for the first time. Although vertical refraction due to velocity profiles is treated briefly, more attention should have been paid to this topic which, generally, has a pronounced effect on medium- and long-range propagation and is often a limiting factor on range capability. Similarly, the subject of volume reverberation is only sketchily covered with merely a very brief reference to surface and bottom reverberation. Although of considerable importance to sonar system designers, the basic concepts of echo formation by complex extended targets have, unfortunately, been omitted altogether. A correct understanding of the principles of electroacoustic transduction is of prime importance to the sonar designer and, quite rightly, the author has given this subject a major section of his book. Transducers are treated as 2-port networks and their equivalent circuits derived. The ideas of motional impedance and admittance are introduced, together with motional diagrams and the determination of efficiency from them. The theoretical 149 Applied Acoustics (5) (1972)---O Applied Science Publishers Ltd, England--Printed in Great Britain
150
BOOK REVIEW
and practical aspects of magnetostrictive, piezoelectric and ceramic transducers are discussed in detail. Various basic designs are described, including magnetostrictive scrolls and ceramic rings. The chapter on radiation patterns is less satisfactory. Although it deals with the patterns of point source arrays, both the line array and the planar array, it omits any discussion of the effect of amplitude tapering (shading) on the pattern. This is unfortunate since such tapering is a powerful means of controlling pattern shape. Beam steering, another very important topic, is only briefly mentioned in spite of its incorporation in many acoustic systems. Pattern synthesis is covered in some detail with a worked example and the concepts of directivity index are also included. There is a useful, if short, chapter on transducer evaluation dealing with reciprocity calibration, circle diagrams, equivalent noise pressure levels and hydrophone sensitivity which should serve as a worthwhile introduction to this very important subject. Some discussion of the sonar equation used for the prediction of performance of both active and passive sonar systems is included and forms the introduction to what is perhaps the most useful contribution made by this book--its final chapter on signal processing. This is the first general book on underwater acoustics to deal with this particular topic and its inclusion is very welcome. The basic ideas of matched filtering and correlation techniques are discussed and these are used in the development of a number of signal processing schemes. The chapter on signal processing is complemented by the first two appendices which deal with the statistical properties of waveforms and waveform sampling. A useful feature of this book which will be welcomed by students is the inclusion of nearly 60 problems with their solutions. Apart from the minor criticisms made above the book does have one major failing: there is a considerable number of errors in the equations, many of which can very easily be spotted by applying dimensional techniques, but these errors are bound to undermine the confidence of the reader in the validity of any of the equations presented. Another feature which makes the situation worse is the confusion of symbols resulting from the author's rigid adherence to symbols which are traditionally associated with a particular discipline. Since acoustics cuts across many disciplines, e.g. physics, mechanical and electrical engineering, this policy results in one symbol being used in different parts of the book to refer to two or more quantities. In spite of the criticisms levelled at this book it still represents a valuable addition to the growing volume of literature on the subject of underwater acoustics and, being written in very readable terms, should prove a useful addition to the library of any underwater acoustician. BRIAN GAZEY
Underwater Acoustics, by Leon Camp. Published by Wiley Interscience, Great Britain, Price: £8.25. This book is based on a short, intensive series of lectures given to mature students already concerned with underwater acoustic systems, at the Engineering and Physical Sciences Extension of the University of California. The material covered and the level of presentation are eminently suitable, however, for degree students and other relative newcomers to the subject. The first three chapters are concerned with what has come to be described as Classical Acoustic Theory and treat such topics as simple, harmonic oscillators, forced vibrators, analysis of vibration of distributed systems, waves and the wave equation, plane and spherical wave solutions and the concepts of specific acoustic impedance and radiation impedance, together with the theory of plane reflection at normal and oblique incidence. Coverage of material in this section is generally adequate. The section on propagation is altogether too brief to be of real help to anyone coming to the subject for the first time. Although vertical refraction due to velocity profiles is treated briefly, more attention should have been paid to this topic which, generally, has a pronounced effect on medium- and long-range propagation and is often a limiting factor on range capability. Similarly, the subject of volume reverberation is only sketchily covered with merely a very brief reference to surface and bottom reverberation. Although of considerable importance to sonar system designers, the basic concepts of echo formation by complex extended targets have, unfortunately, been omitted altogether. A correct understanding of the principles of electroacoustic transduction is of prime importance to the sonar designer and, quite rightly, the author has given this subject a major section of his book. Transducers are treated as 2-port networks and their equivalent circuits derived. The ideas of motional impedance and admittance are introduced, together with motional diagrams and the determination of efficiency from them. The theoretical 149 Applied Acoustics (5) (1972)---O Applied Science Publishers Ltd, England--Printed in Great Britain
150
BOOK REVIEW
and practical aspects of magnetostrictive, piezoelectric and ceramic transducers are discussed in detail. Various basic designs are described, including magnetostrictive scrolls and ceramic rings. The chapter on radiation patterns is less satisfactory. Although it deals with the patterns of point source arrays, both the line array and the planar array, it omits any discussion of the effect of amplitude tapering (shading) on the pattern. This is unfortunate since such tapering is a powerful means of controlling pattern shape. Beam steering, another very important topic, is only briefly mentioned in spite of its incorporation in many acoustic systems. Pattern synthesis is covered in some detail with a worked example and the concepts of directivity index are also included. There is a useful, if short, chapter on transducer evaluation dealing with reciprocity calibration, circle diagrams, equivalent noise pressure levels and hydrophone sensitivity which should serve as a worthwhile introduction to this very important subject. Some discussion of the sonar equation used for the prediction of performance of both active and passive sonar systems is included and forms the introduction to what is perhaps the most useful contribution made by this book--its final chapter on signal processing. This is the first general book on underwater acoustics to deal with this particular topic and its inclusion is very welcome. The basic ideas of matched filtering and correlation techniques are discussed and these are used in the development of a number of signal processing schemes. The chapter on signal processing is complemented by the first two appendices which deal with the statistical properties of waveforms and waveform sampling. A useful feature of this book which will be welcomed by students is the inclusion of nearly 60 problems with their solutions. Apart from the minor criticisms made above the book does have one major failing: there is a considerable number of errors in the equations, many of which can very easily be spotted by applying dimensional techniques, but these errors are bound to undermine the confidence of the reader in the validity of any of the equations presented. Another feature which makes the situation worse is the confusion of symbols resulting from the author's rigid adherence to symbols which are traditionally associated with a particular discipline. Since acoustics cuts across many disciplines, e.g. physics, mechanical and electrical engineering, this policy results in one symbol being used in different parts of the book to refer to two or more quantities. In spite of the criticisms levelled at this book it still represents a valuable addition to the growing volume of literature on the subject of underwater acoustics and, being written in very readable terms, should prove a useful addition to the library of any underwater acoustician. BRIAN GAZEY