Electroacoustics
The Analysis of Transduction and Its Historical Background

Frederick V. Hunt

Originally published in 1954; Reprinted in 1981


TABLE OF CONTENTS

Preface to the Reprint edition

Preface to the Original book

Chapters

  1. Introduction--Historical Context

  2. Electromechanical Coupling--General

  3. Reciprocity and Symmetry Considerations in Electromechanical Coupling

  4. Electrical-Impedance Analysis of Transducer Performance

  5. Moving-Conductor (Dynamic) Transducer Systems

  6. Electrostatic Transducer Systems

  7. Moving-Armature (Magnetic) Transducer Systems

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PREFACE TO THE REPRINT EDITION

Electroacoustics was published in 1954, the year the Acoustical Society of America celebrated its 25th anniversary. Now, more than another quarter-century celebration later, the Society is happy to be able to republish F.V. Hunt's monograph. The Society is grateful to Thomas K. Hunt, son of F.V. Hunt, and to Harvard University Press for granting permission to republish the book.

The only change in the book made with this reprinting is the addition of a list of Errata (p. iii), which had been compiled by the author himself.

Hunt's lucid and flowing style of writing, which old readers will enjoy and new readers discover, was a trademark. I am told that the easy reading of the finished product is due in part to Hunt's practice of running the drafts by his wife Kay and, in some cases their son Tom. Besides being able to write well himself, Hunt had a knack for teaching good writing to his students. The student's pain and embarrassment on seeing how badly he had done with his first draft later turned to appreciation as, under Hunt's tutelage, his critical eye developed.

Another of Hunt's hallmarks was his attention to detail. Chapter 1, the historical introduction, is practically a book in itself. Hunt's dogged determination to record not just the players in the drama but their birth and death dates as well (see the Index of Names, pp. 247-253) produced more than one anecdote recalled by family, students and colleagues. For example, it is related that Hunt elected to drive to an Acoustical Society Meeting in Washington (probably the one in 1951) rather than to fly so that he and his wife Kay might visit a cemetery or cemeteries along the way. He had discovered that in some cases the only available source of an inventor's birth and/or death date was the person's headstone.

Of course, cemeteries were good primary sources only for those who had died. For those still living, Hunt turned to the telephone directory. In Chapter 1 we learn that the idea of using high frequency sound to detect submarines was first proposed by a Russian engineer. Constantin Chilowsky, in Switzerland in 1914. The story of how Chilowsky satisfied a unique test in order to get a hearing for his proposal is recounted by Hunt in a footnote on p. 46. Almost as bizarre as the story itself is the tale of how Hunt dug up the material for the footnote. While in New York to attend a meeting, he decided to see whether the New York telephone directory listed any Chilowsky's. It did, in fact a "C. Chilowsky," and when Hunt called the number, he found the main he was looking for. Hunt then visited Chilowsky and obtained from him the story related in the footnote.

F.V. Hunt died in 1972. It is a tribute to him that 28 years after it was first published, his book is still in demand.

David T. Blackstock
June 1982
The University of Texas at Austin


PREFACE TO THE ORIGINAL BOOK

This monograph is concerned with three topics selected from the wide range of subject matter embraced by the general field of electroacoustics. The first of these is a long introduction devoted to the placement of the allied arts and to the basic sciences from which it derives. This is followed by three chapters that include the description of a new scheme for the analysis of both electrostatic and electromagnetic systems of electromechanical coupling in a single homogeneous frame of reference. This method of analysis is then illustrated in the succeeding chapters by examples of its application to three representative transducer systems.

Electric-circuit analogs have been widely exploited as a tool for the study of acoustical and mechanical systems. They have been less widely used, however, for the representation of electromechanical transducers owing the disparity in the symmetry conditions pertaining to electromagnetic and electrostatic coupling. It has been standard practice to say that one type of analog recommends itself for use with one type of coupling, and that the "other" type must be used with the other--but never the twain could be connected back to back!

I have been experimenting pedagogically since about 1937 with a method for resolving this dilemma by using a space operator to import analytical symmetry into the electromechanical-coupling equations for the antireciprocal cases involving magnetic fields. After the war, when there was an opportunity to reexamine the question, it became possible to resolve the basic issues more clearly and to establish, on sound physical grounds, the validity of using such a space operator to restore symmetry in the analysis of electromagnetic coupling. As a consequence, it now becomes possible to give a unified discussion of all types of electromechanical coupling, including magnetic, electric, and mixed transduction fields. I hope that the novelty of this unified approach will justify in part the publication of this material in advance of the completion of the textbook in whose context it was first drafted.

The ability to represent all transducer types with a single form of equivalent circuit makes it relatively more useful to invoke the methods of electric-impedance analysis for the study of transducer performance. These methods, like the use of equivalent circuits, had already been widely exploited but they were still further developed during the war period. The account of this subject appearing in Chapter 4 leans heavily on the work carried out under NDRC auspices at the Harvard Underwater Sound Laboratory during the period 1941-1945. Although the relevant results of these studies are no longer classified, in the military sense, the research reports have not been generally available and their only summary was incorporated in another document that could receive only limited distributed. Most of the novel features of this work originated with Malcolm H. Hebb and Harvey Brooks, but Francis P. Bundy and many other members of the HUSL staff contributed significantly, and the cogency of the summary report owed much to Paul E. Sabine. In marshaling this material, I have attempted to act as spokesman for this group of wartime colleagues. None of them can be held responsible, however, for the form of presentation I have adopted, since substantial changes from the original have been introduced in order to adapt the procedures to the broadened frame of reference.

The primary generic types of electromechanical coupling include two that make use of a magnetic field and one that uses an electric field. These are exemplified by movable conductors in a fixed magnetic field, by fixed conductors linking a variable magnetic field, and by movable conductors bearing fixed or variable electric charges. The last two categories embrace both lumped-constant systems, such as the moving-armature earphone and the electrostatic loudspeaker, and distributed-constant systems, such as magnetostriction and piezoelectric transducers. Although the methods of analysis presented here are equally applicable to all these transducer types, the gamut of analytical procedures can be illustrated adequately, and a good bit more succinctly, in terms of the lumped-constant systems. This is fortunate, since an adequate discussion of magnetostriction and piezoelectric transducers could not have been included in any case without far exceeding the dimensional limitations of the monograph format. As a consequence, the consideration of these distributed systems, in which electromechanical coupling is effected through body forces, is preforce relegated to a later monograph or other publication.

In the original form of these notes, a few "firsts" had been mentioned casually as an introduction to the various sections devoted to specific mechanisms of transduction. The process of recasting the material in the form of a monograph provided an opportunity to broaden the scope of these historical allusions and to draw them together into a coherent introduction designed to exhibit electroacoustical transduction in its relevant historical context. There is a close parallelism between electroacoustics and the science of electrical communication, and the mushroom expansion of the latter industry has provided incentive for the publication of many accounts dealing with the history of its growth. Some of these accounts contain useful material bearing on electroacoustics, but I had been well coached about not relying on such secondary sources except as an auxiliary guide in prospecting for original source material. For better or worse, these historical notes are based on the cited primary sources, and while it may not be fluent history, I can at least guarantee that every bibliographical reference has been verified at first hand.

Fortunately, most of the needed source material was available in the rich collections of the Harvard College Library, but a few items (identified by LC) had to be run to ground in the Library of Congress, a few in the Engineering Sciences Library (New York) NNE, and a few in the Vail Library of the Massachusetts Institute of Technology MCM. I am also indebted to Mr. David P. Wheatland for graciously making available from his private collection the choice items listed in notes 10, 20, 21, and 68 to Chapter 1. The almost complete files of United States, British, and German patents maintained by the Boston Public Library were also invaluable.

Some readers may be surprised by the prominent role played by patent references in the documentation of a history of transduction. However, one may feel about the probity of scientists applying for letters patent, if one wishes to be realistic it is necessary to recognize that electroacoustical transduction is an applied science that presents both electricity and acoustics in their working clothes. It follows that many of the most significant gains in know-how, as well as in basic understanding, have made their earliest public appearance--and some, their only appearance--as publications of the Patent Office. On the basis of my experience in assembling the material for Chapter 1, I am persuaded that a good many scientists and most of the science historians have paid too little attention to this class of source material.

I feel overwhelmed by the inadequacy of any acknowledgment I can record here of my indebtedness to others. Since this material has been accumulating throughout most of my adult life, the list needs to start with Professors G.W. Pierce and E.L. Chaffee, who initiated me into these mysteries a good many years ago. The history chapter presented many problems that were novel, at least to me, but generous help came from many quarters. Professor I Bernard Cohen was always ready with wise guidance, kindly criticism, and warm encouragement. Mr. David Rines, patent expert extraordinary, deepened my long-standing obligation to him by making available documents, briefs, his collection of patents, and much good advice. I am also indebted to several makers of this history who were kind enough to read and criticize relevant portions of the manuscript. Among these were Robert W. Boyle, Willoughby G. Cady, Edward W. Kellogg, Edward C. Wente, Raymond L. Wegel, Hugh S. Knowles, and Harry F. Olson. I owe special thanks to Mr. Fred J. Harbaugh for comments and many helpful leads to the patent literature.

To the students of many classes, and to my colleagues in our Acoustics Research Laboratory, I am deeply grateful for their patient forbearance through many long discussions of points and viewpoints. Professor Philippe Le Corbeiller has been a gracious tutor and a stout foil, and his careful reading and criticism of this manuscript now puts me further in his debt.

I am in no position to deny that completing a first book has its painful moments, alike for the author and for his wife and son. Without their unfailing devotion and indulgence this book probably never would have been finished.

F.V. Hunt
Belmont, Massachusetts
December 1953


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