In keeping with the plan of the first edition, the more difficult subjects are segregated at the terminations
of each chapter, so that they need not be assigned if the book is to be used for a beginning course.
The writer is pleased to acknowledge his indebtedness to a large number of friends for valuable
suggestions as to improvements in exposition grammar, arithmetic, and algebra. Some of the most
obvious errors in the first edition have been corrected.
Especial thanks are owing J. R. Pellam, who has expended much time and effort in checking the
manuscript and mathematics, and to Dr. Cyril Harris for his many helpful suggestions concerning subject
matter.
Philip M. Morse
Upton, N.Y., January, 1948
Preface to the First Edition
The following book on the theory of vibrations and sound is intended primarily as a textbook for students
of physics and of communications engineering. After teaching the introductory course in this subject at
the Massachusetts Institute of Technology for several years, the author has become persuaded that
there is need for a new textbook in the field.
There are, of course, many other books on the theory of sound. The author's excuse for adding another
to the list is that in the past ten years the rapid growth of atomic physics has induced a complete
reorganization of the science of acoustics. The vacuum tube and the other applications of electronics
have provided immensely powerful tools for the measurement, recording, and reproduction of sound;
tools which have revolutionized acoustic technique. Another useful tool, perhaps not so obvious, is the
new mathematical technique which has been developed for the working out of quantum mechanics, and
which is capable of throwing light on all problems of wave theory. The last chapter of this book is an
example of the utility o these methods. In it the mathematical methods developed for the study of the
radiation of light from an atom are applied to the theory of the acoustic properties of rooms.
During the recent rapid change in the science of sound, certain parts of the subject have gained and
other parts have lost importance. The present book attempts to follow this change in emphasis and to
discuss the new development as well as those portions of the older theory which are still important.
The book has been planned as a textbook with a twofold aim in view. The first aim, of course, is to give
the student a general introduction to the theory of vibration and sound. An introductory course in this
subject must of necessity be more theoretical than practical. In no other branch of physics are the
fundamental measurements so hard to perform, and the theory relatively so simple; and in few other
branches are the experimental methods so dependent on a thorough knowledge of theory. Since this is
so, the student must first be given a physical picture of the fundamental theory of the vibration of solid
bodies and the propagation of sound waves before he can appreciate the techniques used in the
measurements of sound, and before he can begin to design acoustical apparatus.
The second aim is to give the student a series of examples of the method of theoretical physics; the way
a theoretical physicist attacks a problem and how he finds its solution. This subject is too often
neglected, especially in engineering courses. The student is usually given a series of formulas to use in
standardized cases, the formulas sometimes introduced by a cursory derivation and sometimes with no
derivation at all. After such a course the student is capable of using the formulas on standard problems,
but he is unable to devise a new formula to use in unusual cases.
In this book the author has tried to derive every formula from the fundamental laws of physics (there are
a few exceptions to this procedure) and to show in some detail the steps in these derivations and their
logical necessity. This does not mean that the mathematical machinery is given in excessive detail, but
that the steps in the physical reasoning are brought out. Often generality and mathematical rigor have
been sacrificed to make the chain of logic more distinct. It has been the author's experience that once
the student can grasp the physical picture behind a mathematical derivation, he can himself add what
extra generality and rigor he may need. Often, too, the author has supplemented or replaced the rigid
and esoteric technical vocabulary by more colloquial phrases, in order to make vivid a concept, or to
suggest a new point of view.
It is assumed that the student ha a thorough knowledge of calculus, and some acquaintance with the
fundamental laws of mechanics. A knowledge of differential equations is helpful but is not necessary, for
the solutions of the various differential equations encountered are worked out in the text. Tables of the
functions used are given in the back of the book.
Although the book is designed primarily as a textbook, a certain amount of material of an advanced
nature has been introduced. In this way, it is hoped, the volume will be useful as a fairly complete
reference work for those parts of the theory of sound which seem at present to be most important for the
acoustical scientist. The advanced material has been included in the form of extra sections placed at the
end of various chapters. The instructor may assign the first few sections of these chapters for the
introductory course, ad the student may refer to the other sections for further details when he needs
them.
The author wishes to express his gratitude to Professor R.D. Fay and to Dr. W. M. Hall, whose help in
choosing subject matter and methods of presentation has been invaluable. He is also indebted to Dr. J.B.
Fisk for his willing and painstaking aid in correcting proof, and to many other colleagues in the
Department of Physics at the Massachusetts Institute of Technology, for their many helpful suggestions.
Philip M. Morse
Cambridge, Mass., August, 1936