The science and applicaton of acountis

THE SCIENCE AND
APPLICATIONS OF
ACOUSTICS


THE SCIENCE AND
APPLICATIONS OF
ACOUSTICS
SECOND EDITION

Daniel R. Raichel
CUNY Graduate Center
and
School of Architecture, Urban Design and Landscape Design
The City College of the City University of New York

With 253 Illustrations


Daniel R. Raichel
2727 Moore Lane
Fort Collins, CO 80526
USA


Library of Congress Control Number: 2005928848
ISBN-10: 0-387-26062-5
ISBN-13: 978-0387-26062-4

eISBN: 0-387-30089-9

Printed on acid-free paper.

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To Geri, Adam, Dina, and Madison Rose


Preface

The science of acoustics deals with the creation of sound, sound transmission
through solids, and the effects of sound on both inert and living materials. As a
mechanical effect, sound is essentially the passage of pressure fluctuations through
matter as the result of vibrational forces acting on that medium. Sound possesses
the attributes of wave phenomena, as do light and radio signals. But unlike its
electromagnetic counterparts, sound cannot travel through a vacuum. In Sylva

Sylvarum written in the early seventeenth century, Sir Francis Bacon deemed sound
to be “one of the subtlest pieces of nature,” but he complained, “the nature of sound
in general hath been superficially observed.” His accusation of superficiality from
the perspective of the modern viewpoint was justified for his time, not only for
acoustics, but also for nearly all branches of physical science. Frederick V. Hunt
(1905–1967), one of America’s greatest acoustical pioneers, pointed out that “the
seeds of analytical self-consciousness were already there, however, and Bacon’s
libel against acoustics was eventually discharged through the flowering of a clearer
comprehension of the physical nature of sound.”
Modern acoustics is vastly different from the field that existed in Bacon’s time
and even 20 years ago. It has grown to encompass the realm of ultrasonics and
infrasonics in addition to the audio range, as the result of applications in materials science, medicine, dentistry, oceanology, marine navigation, communications,
petroleum and mineral prospecting, industrial processes, music and voice synthesis, animal bioacoustics, and noise cancellation. Improvements are still being made
in the older domains of music and voice reproduction, audiometry, psychoacoustics, speech analysis, and environmental noise control.
This text—aimed at science and engineering majors in colleges and universities,
principally undergraduates in the last year or two of their programs and graduation
students, as well as practitioners in the field—was written with the assumption that
the users of this text are sufficiently versed in mathematics up to and including the
level of differential and partial differential equations, and that they have taken the
sequence of undergraduate physics courses that satisfy engineering accreditation
criteria. It is my hope that a degree of mathematical elegance has been sustained
here, even with the emphasis on engineering and scientific applications. While
the use of SI units is stressed, very occasional references are made to physical
vii


viii

Preface

parameters expressed in English (or Imperial) units. It is strenuously urged that
laboratory experience be included in the course (or courses) in which this text
is being used. The student of acoustics will thus obtain a far keener appreciation
of the topics covered in “recitation” classes when he or she gains “hand on”
experience in the use of sound—level meters, signal generators, frequency
analyzers, and other measurement tools.
Many of the later chapters in the text are self-contained in the sense that an
instructor may skip certain segments in order to concentrate on the agenda most
appropriate to the class. However, mastery of the materials in the earlier chapters,
namely, Chapters 1–6, is obviously requisite to understanding of the later chapters.
Chapters such as those dealing with musical instruments or underwater sound
propagation or the legal aspects of environmental noise can be skipped in order to
accommodate academic schedules or to allow concentration on certain topics of
greater interest to the instructor (and, hopefully, his or her class) such as ultrasound,
architectural acoustics, or other topics. Problems of different levels of difficulty
are included at the end of nearly all of the chapters. Many of the problems entail
the theoretical aspects of acoustics, but a number of “practical” questions have
also been included.
As an author, I hope that I have successfully met the challenge of providing
a modern, fairly comprehensive text in the field for the benefit of both students
and practitioners, whether they are scientists or engineers. In using parts of this
book in prepublication editions in teaching acoustics classes, I have benefited from
feedback and suggestions from my students. A number of them have proven to be
quite eagle-eyed, as they have supplied a continuous stream of recommendations
and corrections, even after the publication of the first edition. It is impossible to
acknowledge them all, but Gregory Miller and Jos´e Sinabaldi come to mind as
being among the most assiduous. A number of my colleagues and friends have
gone through the chapters of the first edition. The real genesis of the first edition
occurred when Harry Himmelblau saw the prepublication copy when I was a summer visiting professor at Caltech’s Jet Propulsion Laboratory, and he urged me to
consider publication. In particular I must acknowledge Paul Arveson, now retired

from the Naval Surface Warfare Center, Carderock of Bethesda, Maryland, who
went through the first three chapters with a fine-toothed comb, M. G. Prasad of
Stevens Institute of Technology who made a number of extremely valuable suggestions for Chapter 9 in instrumentation, and Edith Corliss who greatly encourage
me on Chapter 10 dealing with the mechanism of hearing. Dr. Zouhair Lazreq,
who did his postdoctorate under my tutelage, also looked over some of the chapters, Martin Alexander has been helpful in obtaining illustrations for Chapter 9 in
both editions from Br¨uel and Kjær; Dr. Volker Irmer of Germany’s Federal Environmental Agency introduced me to the European Union’s noise regulations and
other international codes, and Armand Lerner arranged to have materials forwarded
from Eckel Corporation of Cambridge, Massachusetts. James E. West, formerly
of Lucent Bell Laboratories (and now at the Johns Hopkins University) and past
president (1998–1999) of the Acoustical Society of America, was instrumental


Preface

ix

in providing photographs of the anechoic chamber. I am also indebted to Caleb
Cochran of the Boston Symphony Orchestra, Steve Lowe of the Seattle Symphony
Orchestra, Elizabeth Canada of the Kennedy Center, Sandi Brown of the Minnesota
Orchestral Association, Rachelle B. Roe of the Los Angeles Philharmonic, Thomas
D. Rossing of Northern Illinois University, Ann C. Perlman of the American Institute of Physics, Karen Welty of Abbott Laboratories, Tom Radler of Hohner, Inc.,
and others, too many to list here, for their help in providing photographs, certain
figures, and/or permission to reproduce the figures.
I regarded the preparation of this second edition as a splendid opportunity to
update The Science and Applications of Acoustics. A number of features have
been added to this new edition. Besides the obvious updating of information on
acoustic research and applications throughout the text, a section on prosthetic
hearing devices was added to Chapter 10; and the original Chapter 17 was split
into two chapters, one covering music and music instrumentation and the other
dealing with audio processors and sound reproduction. The topic of ultrasound

has also been expanded to the extent that two chapters became necessary, with the
latter chapter treating the increasingly important topic of medical and industrial
applications. An introduction to nonlinear acoustics is provided in Chapter 21.
I also must take this opportunity to thank many of my fellow acousticians for
their comments and suggestions for the second edition. It is hoped that all of
the errors in the first edition has been weeded out and there are precious few, if
any, in this volume. Suggestions for improving the text have come from M. G.
Prasad, Stevens Institute of Technology; Yves Berthelot, Georgia Institute of
Technology; Mark Hamilton, University of Texas at Austin; Neville H. Fletcher,
Australian National University; Uwe Hansen, Indiana State; Frank J. Fahy,
University of Southhampton; Carleen M. Hutchins, Violin Family Association; and
others.
Springer-Verlag’s Dr. Hans Koelsch and Ronald Johnson served ably as the
editor and acquisitions editor, respectively. Komila Bhat supervised the editing process and Natacha Menar proved to be instrumental in expediting this publication;
their contribution surely helped to improve this second edition. It was a pleasure
to work with them. I am still grateful for the past contributions of Dr. Thomas von
Foerster and Steven Pisano, who both worked with me at Springer-Verlag on the
first edition. Dr. Robert Beyer, the editor of this AIP series dealing with acoustics,
provided a great deal of encouragement and inspiration. He has my unbounded
admiration (and that of virtually every acoustician) for the range of his knowledge
and extraordinary wisdom. I deem it a rare privilege to know such a person.
In the preparation of the second edition, my chief source of inspiration and
support continues to come from my wife, Geri. My past and present works were
stimulated by the radiance of her presence.
Daniel R. Raichel
Fort Collins, Colorado


x

Preface

References
Bacon, Sir Francis (Lord Veralum). 1616 (published posthumously). Sylva Sylvarum. In
The Works of Sir Francis Bacon, vol. 2. 1957. Spedding, Ellis, R. L., Heath, D. D., et al.
(eds.). London: Longman and Co. 1957.
Hunt, Frederick Vinton. 1992. Origins in Acoustics. Woodbury, NY: Acoustical Society of
America.


Contents

Preface

vii

1.

A Capsule History of Acoustics

1

2.

Fundamentals of Acoustics

13

3.

Sound Wave Propagation and Characteristics

31

4.

Vibrating Strings

71

5.

Vibrating Bars

89

6.

Membrane and Plates

111

7.

Pipes, Waveguides, and Resonators

131

8.

Acoustic Analogs, Ducts, and Filters

151

9.

Sound-Measuring Instrumentation

173

10.

Physiology of Hearing and Psychoacoustics

213

11.

Acoustics of Enclosed Spaces: Architectural Acoustics

243

12.

Walls, Enclosures, and Barriers

281

13.

Criteria and Regulations for Noise Control

319

14.

Machinery Noise Control

357

15.

Underwater Acoustics

409

xi


xii

Contents

16.

Ultrasonics

443

17.

Commercial and Medical Ultrasound Applications

479

18.

Music and Musical Instruments

509

19.

Sound Reproduction

569

20.

Vibration and Vibration Control

585

21.

Nonlinear Acoustics

617

Appendix A.

Physical Properties of Matter

629

Appendix B.

Bessel Functions

633

Appendix C.

Using Laplace Transforms to Solve Differential
Equations

637

Index

649


1
A Capsule History of Acoustics

Of the five senses that we possess, hearing probably ranks second only to sight
in regular usage. It is therefore with little wonder that human interest in acoustics
would date to prehistoric times. Sound effects entailing loud clangorous noises
were used to terrorize enemies in the course of heated battles; yet the gentler aspects

of human nature became manifest through the evolution of music during primeval
times, when it was discovered that the plucking of bow strings and the pounding of
animal skins stretched taut made for rather interesting and pleasurable listening.
Life in prehistoric society was fraught with emotion, just as in the present time,
so music became a medium of expression. Speech enhanced by musical inflection
became song. Body motion following the rhythm of accompanying music evolved
into dance. Animal horns were fashioned into musical instruments (the Bible
described the ancient Israelites’ use of shofarim, made from horns of rams or
gazelles, to sound alarms for the purpose of rousing warriors to battle). Ancient
shepherds amused themselves during their lonely vigils playing on pipes and reeds,
the precursors of modern woodwinds.
Possibly the first written set of acoustical specifications may be found in the Old
Testament, Exodus XXVI:7:
And thou shalt make curtains of goats’ hair for a tent over the tabernacle . . . . The length of
each curtain shall be thirty cubits and the breadth of each curtain shall be four cubits. . . .

Additional specifications are given in extreme detail for the construction and
hanging of these curtains, which were to be draped over the tabernacle walls to
ensure that the curtains would hang in generous sound-absorbing folds. More fine
details on the construction of the tabernacle followed. Absolutely no substitution
of materials nor deviation from prescribed methods was permitted.
With the advent of metal forming skills, newer wind instruments were constructed of metals. The march evolved from ceremonial processions, on grand
military and ceremonial occasions. Patriotic fervor often was elevated to a state of
higher pitch by the blare of martial music, indeed to the point of sheer madness on
the part of the citizenry, even in modern times as epitomized during the 1930s by
the grandiose thunder of Nazi goose-stepping marches through Berlin’s boulevards
to the accompaniment of the crowds’ roar.
1

2

1. A Capsule History of Acoustics

With sound as a major factor affecting human lives, it was only natural for interest
in the science of sound, or acoustics, to emerge. In the twenty-seventh century
BCE, Lin-lun, a minister of the Yellow Emperor Huangundi, was commissioned
to establish a standard pitch for music. He cut a bamboo stem between the nodes
to make his fundamental note, resulting in the “Huang-zhong pipe”; the other
notes took their place in a series of twelve standard pitch pipes. He also took
on the task of casting twelve bells in order to harmonize the five notes, so as
to enable the composing of regal music for royalty. Archeological studies of the
unearthed musical instruments attested to the high level of instrument design and
the art of metallurgy in ancient China. Approximately 2000 bce, another Chinese,
the philosopher Fohi, attempted to establish a relationship between the pitch of a
sound and the five elements: earth, water, air, fire, and wind. The ancient Hindus
systematized music by subdividing the octave into 22 steps, with a large whole
tone containing four steps, a small tone assigned three, and a half tone containing
two such steps. The Arabs carried matters further by partitioning the octave into
17 divisions. But the ancient Greeks developed musical concepts similar to those
of the modern Western world. Three tonal genders—the diatonic, the chromatic,
and the enharmonic—were attributed to the gods.
Observation of water waves may have influenced the ancient Greeks to surmise
that sound is an oscillating perturbation emanating from a source over large distances of propagation. It cannot have failed to attract notice that the vibrations of
plucked strings of a lute can be seen as well as felt. The honor of being the earliest
acousticians probably falls to the Greek philosopher Chrysippus (ca. 240 bce),
the Roman architect-engineer Vitruvius (also known as Marcus Vitruvius Pollio,
ca. 25 bce), and the Roman philosopher Severinus Boethius (480–524). Aristotle
(384–322 bce) stated in rather pedantic fashion that air motion is generated by
a source “thrusting forward in like movement the adjoining air, so that sound

travels unaltered in quality as far as the disturbance of the air manages to reach.”
Pythagoras (570–497 bce) observed that “air motion generated by a vibrating body
sounding a single musical note is also vibratory and of the same frequency as the
body;” and it was he who successfully applied mathematics to the musical consonances described as the octave, the fifth and the fourth, and established the inverse
proportionality of the length of a vibrating string with its pitch. The forerunner of
the modern megaphone was used by Alexander the Great (400 bce) to summon
his troops from distances as far as 15 km.
The principal laws of sound propagation and reflection were understood by
the ancient Greeks, and the echo figured prominently in a number of classical
tales. Quintillianus demonstrated with small straw segments the resonance of a
string in air. Vitruvius, after making use of the spread of circular waves on a
water’s surface as an example, went on to explain that true sound waves travel
in a three-dimensional world not as circles, but rather as outwardly spreading
spherical waves. He also described the placement of rows of large empty vases
for the purpose of improving the acoustics of ancient theaters. While there may be
some question if such vases have actually been employed in these theaters (since
archeological excavations have failed to disclose their shards), it does presage


1. A Capsule History of Acoustics

3

knowledge of room acoustics on Vitruvius’s part. These vases would have the effect
of low-frequency absorption, similar to that of special panels that are used today
as absorbers. As these amphitheaters were constructed in stony recesses which
provide little or no low-frequency absorption, such vases would definitely improve
the acoustics of the ancient theaters. There is evidence of Lucius Mummius, who,
after destroying Corinth’s theater, brought its bronze vessels to Rome and made
a dedicatory offering from the proceeds of their sale to the goddess Luna in her

temple at Rome.
Aristotle’s eschewal of experiments (which he deemed unworthy of a scientist)
to establish the validity of hypotheses essentially caused the stagnation of all
natural sciences, including acoustics, such was the sway of his authority until the
end of the Middle Ages.
Leonardo da Vinci (1452–1519) knew as the ancients did that “there cannot be
any sound when there is no movement or percussion of the air.” His observations led
him to correlate the waves generated by a stone cast into water with the propagation
of sound waves as similar phenomena. He also ascertained that wave motion of a
sound has a definite value of velocity, and he noted that “the stroke of one bell is
answered by a feeble quivering and ringing of another bell nearby; a string sounding
on a lute, compels to sound on another lute, nearby, a string of the same note,”
thus anticipating by nearly a century Galileo Galilei’s discovery of sympathetic
resonance.
Almost no further progress in acoustics was made until the seventeenth century
when a relationship was established between pitch and frequency. Marin Mersenne
(1588–1648), a French natural philosopher and Franciscan friar, may be considered
to be the “father of modern acoustics.” In Harmonie universelle, published in
1636, he rendered the first scientifically palpable description of an audible tone
(84 Hz), and he demonstrated that the absolute frequency ratio of two vibrating
strings, radiating a musical note and its octave, is of the frequency ratio 1:2. An
analog with water waves is drawn: the belief was registered that the air motion
generated by musical sounds is oscillatory in nature, and it was observed that
sound travels with a finite speed. Sound is also known to bend around corners,
suggestive of diffraction effects which are also commonly observed in water waves.
Mersenne measured the velocity of sound by counting the number of heart beats
during the interval occurring between the flash of a shot and the perception of the
sound.
Independently of Mersenne, Galileo Galilei (1564–1642), in his Mathematical
Discourses Concerning the New Sciences (1638), supplied to date the most lucid

statement and discussion of frequency equivalence. It is interesting to note that
the wave viewpoint was not accorded unanimous acceptance among the early
scientists. Pierre Gassendi (1582–1655), a contemporary of Galileo and Mersenne,
argued for a ray theory whereby sound is attributed to a stream of atoms emitted
by the sounding body; the velocity of sound is the speed of atoms in motion, and
the frequency is the number of atoms emitted per unit time. He also attempted to
demonstrate that sound velocity was independent of pitch by comparing results of
the crack of a rifle with those for the deep roar of a cannon.


4

1. A Capsule History of Acoustics

Robert Boyle (1626–1691) with the help of his assistant Robert Hooke (1635–
1703) performed a classic experiment (1660) on sound by placing a ticking watch
in a partially evacuated glass chamber. He proved that air is necessary, either for the
production or emission of sound. In this respect he disproved Athenasius Kircher’s
(1602–1680) negative experiment in which the latter enclosed a bell in a vacuum
container and excited the bell magnetically from the exterior. Kirchner’s results
were erroneous because he did not take the precaution to prevent the conduction of
sound through the bell’s supports to the surroundings. Francis Hausksbee (1666–
1713) repeated the Boyle experiment (in a modified form) before the Royal Society.
Mention should be made here of Joseph Sauveur (1653–1713) who suggested
the term acoustics (from the Greek word for sound) for the science of sound. In
describing his research on the physics of music at the College Royal in Paris, he
introduced terms such as fundamental, harmonics, node, and ventral segment.1 It
is also an interesting footnote to history that Sauveur may have been born with
defective hearing and speaking mechanisms; he was reported to have been a deaf–
mute until the age of 7. He took an immense interest in music even though he had

to rely on the help of his assistants to compensate for his lack of keen musical
discernment in conducting acoustic experiments.
Franciscus Mario Grimaldi (1613–1663) published Physicomathesis de lumine,
coloribus et tride, which dealt with experimental studies of diffraction, much of
which was to apply to acoustics as well as to light, and in 1678 Hooke announced his
law relating force to deformation, which established the foundations of vibration
and elasticity theories.
Kircher’s publication Phomugia, die neue Hall- und Tonkunst (The New Art of
Sound and Tone), issued in 1680, provides us a rather amusing insight into the world
of misconception, nostrums, and plain scientific hokum that were prevalent at the
time. While delving into the phenomena of echoes and whispering galleries, the
text recommended music as the only remedy against tarantula bites and provided
a discourse on wines. In the chapter on wines, Kircher claimed that old wine has
purified itself and acquired a deep soul. If old wine is poured into a glass, which
is then struck, a sound will emanate. On the other hand, new wine was deemed
to be “jumpy” as a child and bereft of a sound. Hence, recent wine in a glass
will not sound. Another misconception widely believed at the time was that sound
could be trapped in a little box and preserved indefinitely, the idea of attenuation
or absorption of sound being completely alien then. It was even proposed by a
Professor Hut of the music academy at Frankfurt that a communications tube be
constructed to transmit speech over long distances.
Ernst F. F. Chladni (1756–1827), author of the highly acclaimed Die Akustik, is
often credited for establishing the field of modern experimental acoustics through
1

Nearly 20 years earlier, in 1683, Narcissus Marsh, then the Bishop of Ferns and Leighlin in the
Protestant Church, published an article “An Introductory Essay to the Doctrine of Sounds, Containing
Some Proposals for the Improvement of Acousticks” in the Philosophical Transactions of the Royal
Society of London. He was using the term “acousticks” to denote direct sound as distinguished from
reflected and diffracted sound.

1. A Capsule History of Acoustics

5

his discovery of torsional vibrations and measurements of the velocity of sound
with the aid of vibrating rods and resonating pipes. The dawn of the eighteenth
century saw the birth of theoretical physics and applied mechanics, particularly
under the impetus of archrivals Isaac Newton (1642–1726) and Gottfried Wilhelm
Leibniz (1646–1716). Newton’s theoretical derivation of the speed of sound (in
the Principia) motivated a spate of experimental measurements by Royal Society members John Flamsteed (1646–1719), the founder of the Greenwich Observatory and the first Astronomer Royal, and his eventual successor (in 1720),
Edmund Halley (1656–1742); also by Giovanni Domenico Cassini (1625–1712),
Jean Picard (1620–1682), and Olof R¨omer (1644–1710) of the French Acadˆemie
des Sciences; and nearly half century later in 1738 by a team led by C´esar Fran¸cois
Cassini de Thury (1714–1762), a grandson of the aforementioned G. D. Cassini
who headed the earlier 1677 measurement team.
Newton’s estimate was found to be in error, for in his observations he had erred
by assuming an isothermal (rather than an isentropic) process as being the prevalent
mode for acoustic vibrations.2 Temperature was found to influence the speed of
sound in independent separate experiments by Count Giovanni Lodovico Bianoni
(1717–1781) of Bologna and Charles Marie de la Condamine (1701–1773). Other
acoustic developments included the evolution of the exponential horn by Richard
Helsham (1680–1758); this device loads the sound source heavily, thus causing
the source to concentrate its energy more than it could without the horn and directs
the output more effectively. Real understanding of this phenomenon did not come
about until John William Strutt, Lord Rayleigh (1845–1919) treated the problem
of source loading, and Arthur Gordon Webster (1863–1923) the theory of horns.
Each of the optical phenomena of refraction, diffraction, and interference was
elucidated during the seventeenth century. But all of these phenomena were soon

realized to apply to acoustics as well as to light. Willbrod Snell (or Snellius)
(1591–1626) composed an essay in 1620 treating the refraction of light rays in a
transparent medium such as water or glass, but he somehow neglected to publish
his manuscript which was later unearthed and used by Christian Huygens (1629–
1695) in his own works, which secured posthumous fame for Snell, in spite of a
publication of the same law by the stellar Ren´e Descartes (1596–1650) who, it
turned out, had made two erroneous assumptions, which were corrected by Pierre
de Fermat (1601–1665). Fermat’s principle derives from the assumption that the
light always travels from a source point in one medium to a receptor point in the
second medium by the path of least time. Diffraction was first observed by the Jesuit
mathematician Francesco Maria Grimaldi (1618–1663) of Bologna. His experiments were repeated by Newton, Hooke, and Huygens; and soon this phenomenon
that light does not always travel in straight lines but can diffuse slightly around corners constituted a core issue in the controversy between the wave and corpuscular
theories of light. But it took nearly 200 years following Newton’s era to resolve the
2

Actually, what Newton really did was to assume that the “elastic force” of the fluid is proportional
to its condensation, which is now realized, in the context of modern thermodynamics, to be the
equivalence of the isothermal process.


6

1. A Capsule History of Acoustics

conflict by embracing elements of both theories. Newton essentially squelched the
wave theory until its revival by Thomas Young (1773–1829) and Augustin Jean
Fresnel (1788–1817), both of whom, independently of each other, elucidated the
principle of interference. On his analysis of diffraction, Fresnel drew heavily on
Huygen’s principle in which successive positions of a wavefront are established
by the envelope of secondary wavelets.

Armed with the analytical tools afforded by the advent of calculus by Newton and
Leibniz, the French mathematical school treated problems of theoretical mechanics. Among the major contributors were Joseph Louis Lagrange (1736–1813), the
Bernoulli brothers James (1654–1705) and Johann (1667–1748), G. F. A. l Hˆopital
(Marquis de St. Mesme) (1661–1704), Gabriel Cramer (1704–1752), Leonhard
Euler (1707–1783), Jean Le Rond d’Alembert (1717–1783), and Daniel Bernoulli
(1700–1783). And the next generation provided a further flowering of genius:
Joseph Louis Lagrange (1736–1813), Pierre Simon Laplace (1749–1827), Adrian
Marie Legendre (1736–1833), Jean Baptiste Joseph Fourier (1768–1830), and
Sim´eon Denis Poisson (1781–1840). The nineteenth century was also dominated
by discoveries in electricity and magnetism by Michael Faraday (1791–1867),
James Clerk Maxwell (1831–1879), Heinrich Rudolf Hertz (1857–1894), and by
the theory of elasticity, principally developed by Clause L. M. Navier (1785–
1836), Augustin Louis Cauchy (1789–1857), Rudolf J. E. Clausius (1822–1888),
and George Gabriel Stokes (1890–1909).
These developments constituted the foundation for understanding the physical
and eventually the physiologcial aspects of acoustics. In the attempt to grasp the
nature of musical sound, Simon Ohm (1789–1854) advanced the hypothesis that
the ear perceived only a single, pure sinusoidal vibration and that each complex
sound is resolved by the ear into its fundamental frequency and its harmonics.
Hermann F. L. von Helmholtz (1821–1894) arguably deserves the credit for laying
the foundations of spectral analysis in his classic Lehre von den Tonempfindungen
(Sensation of Sound). The monumental two-volume Theory of Sound, released in
1877 and 1878 by the future Nobel laureate, Lord Rayleigh, laid down in a fairly
complete fashion the theoretical foundations of acoustics.
When the newly constructed Fogg Lecture Hall was opened in 1894 at Harvard
University, its acoustics was found to be so atrocious so as to render that facility almost useless. This prompted Harvard’s Board of Overseers to request the
physics department that something be done to rectify the situation. The task was
assigned to a young Harvard researcher, Wallace Clement Sabine (1868–1919),
and he discovered soon enough that excessive reverberations tend to mask the lecturer’s words. In a series of papers (1900–1915) evolving from his studies of the
lecture hall, he almost single-handedly elevated architectural acoustics to scientific

status. Sabine helped establish the Riverbank Acoustical Laboratories3 at Geneva,
Illinois. Just prior to his scheduled assumption of his duties at Riverbank, Sabine
succumbed at the young age of 50 to cancer. His distant cousin, Paul Earls Sabine

3

Riverbank is possibly the first research facility set up specifically for study and research in acoustics.


1. A Capsule History of Acoustics

7

(1879–1958), also a Harvard physicist, took on the task of running the laboratory.
The development of test procedures, methodology, and standardization in testing
the acoustical nature of products arose from the pioneering efforts of the younger
Sabine. A third member of the family, Paul Sabine’s son Hale Johnson Sabine
(1909–1981), began his career in architectural acoustics at the tender age of 10
by assisting his father at Riverbank, and his efforts centered on control of noise
in industry and institutions. Both father and son, Paul and Hale, served terms as
president of the Acoustical Society of America.
The genesis of ultrasonics occurred in the nineteenth century with James P.
Joule’s (1818–1889) discovery in 1847 of the magnetostrictive effect, the alteration of the dimensions of a magnetic material under the influence of a magnetic
field, and in 1880 with the finding by the brothers Paul-Jacques (1855–1941) and
Pierre (1859–1906) Curie that electric charges result on the surfaces of certain
crystals subjected to pressure or tension. The Curies’ discovery of the piezoelectric electric effect provided the means of detecting ultrasonic signals. The inverse
effect, whereby a voltage impressed across two surfaces of a crystals give rise
to stresses in the materials, now constitutes the principal method of generating
ultrasonic energy.
The study of underwater sound stemming from the necessity for ships to avoid

dangerous obstacles in water supplied the impetus for development of ultrasonic
applications. Until the early part of the twentieth century ships were warned of
hazardous conditions by bells suspended from lightships. Specially trained crew
members listened for these bells by pressing microphones or stethoscopes against
the hulls. In the effort to counteract the German submarine threat during World
War I, Robert Williams Wood (1868–1955) and Gerrard in England and Paul
Langevin (1872–1946) in France were assigned the task of developing counter
surveillance methods.
The youthful Russian electrical engineer, Constantin Chilowsky (1880–1958),
collaborated with Langevin in experiments with an electrostatic (condenser)
projector and a carbon-button microphone placed at the focus of a concave mirror.
In spite of troubles encountered with leakages and breakdowns due to the high
voltages necessary for the operation of the projectors, Langevin and Chilowsky
were able by 1916 to obtain echoes from the ocean bottom and from a sheet
of armor plate at a distance of 200 m. A year later Langevin came up with the
concept of using a piezoelectric receiver and employed one of the newly developed
vacuum-tube amplifiers—the earliest application of electronics to underwater
sound equipment—and Wood constructed the first directional hydrophone geared
to locate hostile submarines. The first devices to generate directional beams of
acoustic energy also constitute the first use of ultrasonics. Reginald A. Fessenden
(1866–1932), a Canadian engineer, working independently, developed a moving
coil transducer operating at frequencies in the range of 500–1000 Hz to generate
underwater signals. In the course of their underwater sound investigations, Wood
and his co-worker Alfred L. Loomis (1887–1975), who also was a trained lawyer,
and Langevin observed that small water creatures could be stunned, maimed, or
even destroyed by the effects of intense ultrasonic fields.


8

1. A Capsule History of Acoustics

World War I ended before underwater echo-ranging could be fully deployed to
meet the German U-boat threat. The years of peace following World War I witnessed a slow but nevertheless steady advance in applying underwater sound to
depth-sounding by ships. Improvements in electronic amplification and processing, magnetostrictive projectors, piezoelectric transducers provided refinements in
echo-ranging. The advent of World War II heightened research activity on both
sides of the Atlantic, and most of the present concepts and applications of underwater acoustics traced their origins to this period. The concept of target strength,
noise output of various ships at different speeds and frequencies, reverberation in
the sea, and evaluation of underwater sound through spectrum analysis were quantitatively established. It was during this period that underwater acoustics became
a mature branch of science and engineering, backed by vast literature and history
of achievement.
The invention of the triode vacuum tube and the advent of the telephone and radio
broadcasting served to intensify interest in the field of acoustics. The development
of vacuum tube amplifiers and signal generators rendered feasible the design and
construction of sensitive and reliable measurement instruments. The evolution of
the modern telephone system in the United States was facilitated by the progress
of communication acoustics, mainly through the remarkable efforts of the Bell
Telephone Laboratories.
The historic invention of the transistor (1949) at the Bell Laboratories in Murray
Hill, New Jersey, gave rise to a whole slew of new devices in the field of electronics,
including solid state audio and video equipment, computers, spectrum analyzers,
electric power conditioners, and other gear too numerous to mention here.
Experiments and development of theory in architectural acoustics were conducted during the 1930s and the 1940s at a number of major research centers,
notably Harvard, MIT, and UCLA. Vern O. Knudsen (1893–1974), eventually the
chancellor of UCLA, carried on Sabine’s work by conducting major research on
sound absorption and transmission. The most notable of his younger associates
was Cyril M. Harris (b. 1917), who was to become the principal consultant on
the acoustics of the Metropolitan Opera House in New York, the John F. Kennedy
Center in the District of Columbia, the Powell Symphony Hall in St. Louis, and a
number of other notable edifices.

Sound decay, in terms of reverberation times, was discovered to be a decisive
factor in gauging the suitability of enclosed areas for use as listening chambers.
The impedance method of rating acoustical materials was established to predict the
radiative patterns of sonic output, and prediction of sound attenuation in ducts was
established on a scientific footing. The architectural acoustician now has a wide
array of acoustical materials to choose from and to tailor the walls segmentwise
in order to effect the proper acoustic environment.
Acoustics also engendered the science of psychoacoustics. Harvey Fletcher
(1884–1990) led the Bell Telephone Laboratories in describing and quantifying
the concepts of loudness and masking, and there, many of the determinants of
speech communication were also established (1920–1940). Fletcher, now regarded
as “the father of psychoacoustics,” worked with the physicist Robert Millikan at


1. A Capsule History of Acoustics

9

the University of Chicago, on the determination of the electron charge. Fletcher
indeed performed much of the famed oil drop experiment, to the extent that many
physicists feel that the student should have shared the 1923 Nobel Prize in physics
with his professor who received the award for this effort. At Bell Labs, Fletcher also
developed the first electronic hearing aid and invented stereophonic reproduction.
Sound reproduction also constituted the domain of Harry F. Olson (1902–1982),
who directed the Acoustical Laboratory at RCA and developed modern versions
of loudspeakers. Warren P. Mason’s (1900–1986) major work in physical acoustics essentially laid down the modern foundations of ultrasonics, and Georg von
Bek´esy (1849–1972) earned the Nobel Prize for his research on the mechanics of
human hearing. Acoustics penetrated the fields of medicine and chemistry through
the medium of ultrasonics: ultrasonic diathermy became established and certain
chemical reactions were found to become accelerated under acoustic conditions.

The outbreak of World War II served to greatly intensify acoustics research at
major laboratories in Western Europe and in the United States, particularly in view
of the demand for sonar detection of stealthily moving submarines and for reliable
speech communication in cacophonous environments such as propeller aircraft
and armored vehicles. This research not only has reached great proportions, it has
continued unabated to this day, at major universities and government institutions,
among them being the U.S. Naval Research Laboratory, Naval Surface Warfare
Center, MIT, Purdue University, Georgia Institute of Technology, and Pennsylvania
State University.
Prominent among the researchers were Richard Henry Bolt (1911–2002) and
Leo L. Beranek (b. 1914) who teamed up after World War II to found a major research corporation, Bolt, Beranek & Newman (now BBN Technologies);
Phillip M. Morse of MIT [who authored and co-authored with Karl Uno Ingard
(b. 1921) major texts in physical acoustics]; R. Bruce Lindsay (1900–1985) of
Brown University; and Robert T. Beyer, who contributed to nonlinear acoustics,
also at Brown. In 1947 Eugen Skudrzyk (1913–1990) began research in nearly all
areas of acoustics at the Technical University of Vienna and went on to Pennsylvania State University in the United States, he wrote possibly the best comprehensive
text on physical acoustics since Lord Rayleigh’s Theory of Sound.
Karl D. Kryter (b. 1914) of California dealt with the physiological effects of noise
on humans, and Carleen Hutchins (b. 1911) is still providing great insight into the
design and construction of musical string instruments, in her dual role as investigating acoustician and craftsperson seeking to emulate the old Cremona masters in her
hometown of Montclair, New Jersey. Laser intereferometry was applied by Karl H.
Steson (b. 1937) and by Lothar Cremer (1905–1990) to visualize vibrations of the
violin body. Sir James Lighthill (1924–1998), who held the Lucasian chair (once
occupied by Newton) in mathematics at Cambridge University, laid down the foundations of modern aeroacoustics, building on the foundations of Lord Rayleigh’s
earlier research. UCLA’s Isadore Rudnick (1917–1997) performed major experiments in superfluid hydrodynamics, involving sound propagation in helium at cryogeneic temperatures and also conducted studies of acoustically induced streaming
modes of vibrations of elastic bodies and attenuation of sound in seawater. At


10

1. A Capsule History of Acoustics

the Applied Physics Laboratory at the University of Washington, Lawrence A.
Crum (b. 1941) directs major research on sonofluorescence as well as the development of ultrasound diagnostic and therapeutic medical devices. Kenneth S.
Suslick (b. 1952) and his co-workers at the University of Illinois are making major
contributions in the field of sonochemistry. Whitlow W. L. Au at the University of
Hawaii is conducting studies on the characteristics of cetacean acoustics, including
the target discrimination capabilities of dolphins and whales.
With acoustic research continuing apace, the number of great acoustcians living
surely exceeds that of deceased ones.
It can truly now be said that the U.S. Navy has done more (and is still doing
more) than any other institution to further acoustics research at its widespread facilities, including Naval Research Laboratory (NRL) and the Naval Surface Warfare
Center (NSWC). Much magnificent work was done under the cloak of security
classification during the days of the Cold War, with the consequence that many
deserving researchers do not bask in the glory that have been publicly accorded
professional societies’ medal honorees and Nobel Prize laureates.
Robert J. Bobber (b. 1918) of NRL facility in Orlando paved the way in underwater electroacoustics measurements. Acoustics radiation constituted the domain of
Sam Hanish, late of the NRL in the District of Columbia. At NSWC’s David Taylor
Basin in Bethesda, Maryland, Murray Strausberg (b. 1917) continues to make major contributions in the field of propeller noise, which entails the study of cavitation
and hydroacoustics as he did for the past three decades; David Feit (b. 1937) ranks
as a leading expert in the field of structural acoustics; and William K. Blake reigned
preeminent in the category of aero-hydroacoustics (Blake, 1964). Herman Medwin
(b. 1920) of the Navy Postgraduate School at Monterey, California, conducted major research in acoustical oceanography. As a senior research physicist at the U.S.
Naval Surface Weapons Center, headquarters in Silver Spring, Maryland, Robert
Joseph Urick (1916–1996) elucidated the characteristics of underwater acoustical
phenomena, including sonar effects. He later taught the principles of underwater
sound at the Catholic University of America in Washington, DC.
Acoustics is no longer the esoteric domain of interest to a few specialists in
the telephone and broadcasting industries, the military, and university research
centers. Legislation and subsequent action have been demanded internationally to

provide quiet housing, safe and comfortable work environments in the factory and
the office, quieter airports and streets, and protection in general from excessive
exposure to noisy appliances and equipment.
The wiser architects are increasingly using acoustical engineers to ensure environmental harmony with the esthetic aspects of their designs. Acoustic instrumentation is being used in industry to facilitate manufacturing processes and to
ensure quality control. Acoustics has even invaded the living room through the
medium of high fidelity reproduction, giving rise to a spate of new equipment such
as Dolby processors, digital processors, compact disc (and more lately DVD) players, multi-speaker “Surround-Sound” environment conditioners, music synthesizer
circuit boards for personal computers. The escalating applications of ultrasound
provide new diagnostic and therapeutic tools in the medical field, more reliable


References

11

characterization of materials, better surveillance methodologies, and improved
manufacturing techniques.
And what does the future hold in acoustics? The continuing miniaturization of
electronic circuitry is now resulting in digitized hearing aids that can circumvent
the “cocktail party effect” (the tendency of background noise to make it difficult
for the sensorneurally impaired listeners to focus on a conversation). Even newer
diagnostic and therapeutic processes entailing acoustical signals are being developed and tested at major medical centers. More sensitive and versatile transducers
that can withstand harsher environments lead to new acoustical devices such as
sonic viscometers, undersea probes, and portable voice-recognition devices. And
if we can gain a greater understanding of how cetaceans make use of their natural
sonars to assess the submarine environment and perhaps to communicate with
each other, we could be well on the way to constructing far more sophisticated
megachannel acoustical analyzers. The generation of acoustical waves in the gigahertz range can rival or exceed the optical microscope for resolution with greater
penetrating power. The repertoire of what is to come should truly constitute an
amazing cornucopia of beneficence to humanity.

References
Beranek, Leo J. 1995. Harvey Fletcher: Friend and scientific critic, Journal of the Acoustical
Society of America 97(5):3357.
Beyer, Robert T. 1995. Acoustic, acoustics. Journal of the Acoustical Society of America
98(1): 33–34.
Beyer, Robert T. 1999. Sounds of Our Times. New York: Springer-Verlag. [A fascinating
history of acoustics over the past 200 years, with many allusions to even earlier history.
This text picks up where Frederick Vinton Hunt left off in his unfinished, meticulously
researched work which was published posthumously (seen below).]
Blake, William K. 1964. Aero-hydroacoustics for Ships, 2 Vols. Bethesda, MD: David
Taylor Basin publication DTNSRDC-84/010, June 1964.
Bobber, Robert J. 1970. Underwater Electroacoustic Measurements. Washington, DC:
Naval Research Laboratory.
Chladni, E. F. F. 1802. Die Acustik. Leipzig: Breitkopf & Hartel.
Clay, Clarence S. and Medwin, Herman. 1977. Acoustical Oceanography: Principles and
Applications. New York: John Wiley & Sons.
Fletcher, Steven Harvey. 1995. Harvey Fletcher: A son’s reflections. Journal of the Acoustical Society of America 97(5 Pt. 2): 3356–3357.
Galileo, Galilei. 1638 (translation published in 1939). Dialogues Concerning Two New Sciences, Translated by Crew, H. and De Salvio, A. Evanston, IL: Northwestern University
Press.
Hanish, Sam. 1981. A Treatise on Acoustic Radiation. Washington, DC: Naval Research
Laboratory.
Harris, Cyril M. 1995. Harvey Fletcher: Some personal recollections. Journal of the Acoustical Society of America 97(5 Pt. 2): 3357.
Helmholtz, Hermann F. L. von. 1877. Lehre con den Tonempfindungen. Braunschweig,
Wiesbaden: Vieweg.


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1. A Capsule History of Acoustics

Hertz, J. H. (ed.). 1987. The Pentateuch and Haftorahs. London: Soncino Press.
Hunt, Frederick V. 1992 (reissue). Origins in Acoustics. Woodbury, NY: Acoustical Society
of America. (Although left incomplete by the author at the time of his death, this text is
one of the most definitive accounts by one of the great modern acoustical scientists of
the history of acoustics leading up to the eighteenth century.)
Junger, Miguel C., Feit, David. 1986. Sound, Structure, and Their Interaction. Cambridge,
MA: The MIT Press.
Kopec, John W. 1994. The Sabines at Riverbank. Proceedings, Wallace Clement Sabine
Centennial Symposium. Woodbury, NY: Acoustical Society of America, pp. 25–28.
Lindsay, R. Bruce. 1966. The story of acoustics. Journal of the Acoustical Society of
America 39(4): 629–644.
Lindsay, R. Bruce (ed.). 1972. Acoustics: Historical and Philosophical Development.
Benchmark Papers in Acoustics. Stroudsburg, PA: Dowden, Hutchinson & Ross, Inc.
(A most interesting compendium of selected papers by major contributors to acoustical
science, ranging from Aristotle to Wallace Clement Sabine. A must-read for the serious
student of the history of acoustics).
Lindsay, R. Bruce. 1880. Acoustics and the Acoustical Society of America in Historical
Perspective. Journal of the Acoustical Society of America 68(1): 2–9.
Mersenne, Marin. 1636. Harmonie universelle. Paris: S. Cramoisy; English translation:
Hawkins, J. 1853. General History of the Practice and Science of Music. London: J. A.
Novello, pp. 600–616, 650 ff.
Newton, Sir Isaac. 1687. Philosophiae Naturalis Principia Mathematica. London: Joseph
Streater for the Royal Society.
Pierce, Allan D. 1989 (reissue). Acoustics: An Introduction to its Physical Principles and
Application. Woodbury, NY: Acoustical Society of America.
Raman, V. V. 1973. Where credit is due: Sauveur, the forgotten founder of acoustics.
Physics Teacher pp. 161–163.
Shaw, Neil A., Klapholz, Jesse, Gander, Mark R. 1994. Books and Acoustics, especially Wallace Clement Sabine’s Collected Papers on Acoustics. Proceedings, Wallace
Clement Sabine Centennial Symposium. Woodbury, NY: Acoustical Society of America,

pp. 41–44.
Skudrzyk, Eugen. 1971. The Foundations of Acoustics—Basic Mathematics and Basic
Acoustics. New York: Springer-Verlag. (A text of classic proportions. Nearly one quarter
of this volume lays the mathematical foundations requisite to analysis of acoustical
phenomena.)
Strutt, John William (Lord Rayleigh). 1877. Theory of Sound. London: Macmillan & Co.
Ltd. 2nd edition revised and enlarged 1894, reprinted 1926, 1929. Reprinted in two
volumes, New York: Dover, 1945. (These volumes should be in every acoustician’s
library.)
Wang, Ji-qing. 1994. Architectural Acoustics in China, Past and Present. Proceedings,
Wallace Clement Sabine Centennial Symposium. Woodbury, NY: Acoustical Society of
America, pp. 21–24.
Webster, Arthur G. 1919. Proceedings of the National Academy of Science 5:275.


2
Fundamentals of Acoustics

2.1

Wave Nature of Sound and the Importance of Acoustics

Acoustics refers to the study of sound, namely, its production, transmission through
solid and fluid media, and any other phenomenon engendered by its propagation
through media. Sound may be described as the passage of pressure fluctuations
through an elastic medium as the result of a vibrational impetus imparted to that
medium. An acoustic signal can arise from a number of sources, e.g., turbulence
of air or any other gas, the passage of a body through a fluid, and the impact of a
solid against another solid.
Because it is a phenomenon incarnating the nature of waves, sound may contain

only one frequency, as in the case of a pure steady-state sine wave, or many
frequency components, as in the case of noise generated by construction machinery
or a rocket engine. The purest type of sound wave can be represented by a sine
function (Figure 2.1) where the abscissa represents elapsed time and the ordinate
represents the displacement of the molecules of the propagation medium or the
deviation of pressure, density, or the aggregate speed of the disturbed molecules
from the quiescent (undisturbed) state of the propagation medium.
When the ordinate represents the pressure difference from the quiescent pressure, the upper portions of the sine wave would then represent the compressive
states and the lower portions the rarefaction phases of the propagation. A sine wave
is generated in Figure 2.2 by the projection of the trace of a particle A traveling in
a circular orbit. This projection assumes the pattern of an oscillation, in which the
particle A’s projection or “shadow” A onto an abscissa moves back and forth at a
specified frequency. Frequency f is the number of times the sound pressure varies
from its equilibrium value through a complete cycle per unit time. Frequency is
also denoted by the angular (or radian) frequency
ω = 2π f =

2π
T

(2.1)

expressed in radians per second. The period T is the amount of time for a single cycle to occur, i.e., the length of the time it takes for a tracer point on the
sine curve to reach a corresponding point on the next cycle. The reciprocal of
13


14

2. Fundamentals of Acoustics

Figure 2.1. Plot of a sine wave y(t) = sin 2π sin ft over slightly more than two periods
of T = 1/ f s, where f is the frequency of the sine wave. y(t) may be the displacement
function x/x0 , velocity ratio v/v0 , pressure variation p/ p0 , or condensation variation s/s0 ,
where the subscript 0 denotes maximum values.

Figure 2.2. The oscillation of a particle A in a sinusoidal fashion is generated by the
circular motion of particle A moving in a circle with constant angular speed ω. A is
the projection of Acos ωt = Acos θ onto the diameter of the circle which has a radius
A. The projection of point A to the right traces a sine wave over an abscissa representing
time t. The projections for three points at times t1 , t2 , and t3 are shown here. The amplitude
of the oscillation is equal to the radius of the circle, and the peak-to-peak amplitude is
equal to the diameter of the circle.


2.1 Wave Nature of Sound and the Importance of Acoustics

15

period T is simply the frequency f . The most common unit of frequency used
in acoustics (and electromagnetic theory) is the hertz (abbreviated Hz in the SI
system), which is equal to one cycle per second. An acoustic signal may or may
not be audible to the human ear, depending on its frequency content and intensity.
If the frequencies are sufficiently high (>20 kilohertz, which can be expressed
more briefly as 20 kHz), ultrasound will result, and the sound is inaudible to the
human ear. This sound is said to be ultrasonic. Below 20 Hz, the sound becomes
too low (frequency-wise) to be heard by a human. It is then considered to be
infrasonic.
Sound in the audio frequency range of approximately 20 Hz–20 kHz can be heard
by humans. While a degree of subjectivity is certainly entailed here, noise conveys

the definition of unwanted sound. Excessive levels of sound can cause permanent
hearing loss, and continued exposure can be deleterious, both physiologically and
psychologically, to one’s well-being.
With the advent of modern technology, our aural senses are being increasingly
assailed and benumbed by noise from high-speed road traffic, passing ambulances
and fire engine sirens, industrial and agricultural machinery, excessively loud radio
and television receivers, recreational vehicles such as snowmobiles and unmuffled
motorcycles, elevated and underground trains, jet aircraft flying at low altitudes,
domestic quarrels heard through flimsy walls, and so on.
Young men and women are prematurely losing their hearing acuity as the result of sustained exposure to loud rock concerts, discotheques, use of personal
cassette and compact disk players and mega-powered automobile stereo systems.
In the early 1980s, during the waning days of the Cold War, the Swedish navy
reported considerable difficulty in recruiting young people with hearing sufficiently keen to qualify for operating surveillance sonar equipment for tracking
Soviet submarines traveling beneath Sweden’s coastal waters. Oral communication can be rendered difficult or made impossible by background noise; and
life-threatening situations may arise when sound that conveys information becomes masked by noise. Thus, the adverse effects of noise fall into one or
more of the following categories: (1) hearing loss, (2) annoyance, and (3) speech
interference.
Modern acoustical technology also brings benefits: it is quite probable that the
availability (and judicious use) of audiophile equipment has enabled many of us,
if we are so inclined, to hear more musical performances than Beethoven, Mozart
or even the long-lived Haydn could have heard during their respective lifetimes.
Ultrasonic devices are being used to: dislodge dental plaque; overcome the effects
of arteriosclerosis by freeing up clogged blood vessels; provide noninvasive medical diagnoses; aid in surgical procedures; supply a means of nondestructive testing
of materials; and clean nearly everything from precious stones to silted conduits.
The relatively new technique of active noise cancellation utilizes computerized
sensing to duplicate the histograms of offending sounds but at 180 degrees out
of phase, which effectively counteracts the noise. This technique can be applied
to aircraft to lessen environmental impact and to automobiles to provide quieter
interiors.