Introduction To Acoustics

Focusing on the systems and engineering aspects of acoustics, this book emphasizes the importance of speech and hearing in our lives. Organized from simple to complex, enabling readers to apply concepts and explore issues, while also offering detailed illustrations and explanations. Examines key concepts of real life situations and features examples in music, speech, hearing, architecture, and other recent developments in acoustics. For anyone interested in learning more about acoustics; as a reference for practicing engineers.

The biological evolution of hearing is believed to have begun while our progenitors were still in the sea. Fish hear with an organ similar to the human cochlea, and it is believed that the external and middle ears evolved in amphibians and mammals as a means of matching the acoustic impedances of the atmosphere and the cochlea (or inner ear). This evolution would have had to occur when amphibians first emerged from the sea onto the land. On the other hand, while many animals use sound for communication, speech is a uniquely human activity, and presumably recent on the evolutionary time scale. One of the major anatomical differences between ourselves and the great apes is that we have an elaborate vocal tract above the larynx, and can articulate with great versatility, but they cannot. Because of this difference there has been speculation recently that it was the evolution of the voice as well as improvement in brain capacity that were the crucial factors in the ascendancy of humans over apes. It has even been suggested that perhaps the Neanderthals could not speak any better than the apes, and that the Cro-Magnons were the first humans to show this unique trait. Regardless of how science assesses this speculation in the future, it does make us realize just how important the acoustic system of speech and hearing is in our lives, and that to be deprived of speech or hearing is a major impediment.

Perhaps as old as our speaking abilities is our inclination to make music. Unwanted sounds can intrude on our speech and music, interfere with sleep, or cause deafness if loud enough. Noise control is therefore one of the main tasks of the acoustical engineer. The industrial revolution of the eighteenth century ushered in an age of high-power machinery, and along with it the unwanted by-product: vibration. This unwanted by-product led to the recognition of vibration as a field of serious study for mechanical engineers. In turn, the study of vibration led to the development of a discipline sometimes called "automatic" controls, which became especially important in aeronautical engineering. The pioneering work of Bell and Edison on the telephone and in broadcasting resulted in the growth of two new industries that have vastly extended our abilities to communicate. The use of submarines to destroy Allied shipping during World War I led to the invention of sonar, since electromagnetic waves will not travel very far under water. Sonar is now used for many peaceful purposes. The interest in underwater sound led to the development of practical ultrasonic transducers, which it was discovered could be used in medical diagnosis and treatment as well as in nondestructive evaluation and processing in industry. Starting in the 1920s, it becam4 common for oil companies to use sound in seismic prospecting. We are also gaining an understanding of how to deal with earthquakes, another form of acoustic wave. Currently, significant efforts are underway to develop systems for machine recognition of speech. In addition, mechanical engineers are increasingly using machines for the acoustic and vibrational monitoring of machinery.

To some people, the word "acoustics" brings to mind orchestras and concert halls. Others think of hearing aids or noise control. Some engineers realize that mechanical oscillations may occur in discrete (lumped) systems, but fail to understand that they may also occur in continuous (distributed) systems. An example of a discrete system is the motion of a mass on a spring, and the study of such phenomena is usually termed "vibrations." An example of a continuous system is the motion of the air during the playing of music, and the term "sound" is employed in this case. Sound can also propagate at frequencies beyond the range of human hearing; at the high end, we speak of ultrasonics, and of infrasonics at the low end. Ultrasonics have become part of our lives through medical imaging and sonar, while earthquakes are an example of infrasonic waves. This wide variety of phenomena and applications is linked by a definition from the Acoustical Society of America (1965): Acoustics: "The Science of mechanical radiation in all its aspects and applications, including origin (vibration of material media, etc.), transmission through material media, at all frequencies and under the most diverse conditions, and reception."

An old adage characterizes a lecture as the process whereby the notes of the lecturer become the notes of the student. Textbooks tend to procreate in a similar manner. In defense of textbook authors, it should be noted that if our intention is to pass on to the student the most important discoveries and inventions of the past, then it is only to be expected that successive generations of textbooks should change in an evolutionary way. There is little doubt that the ancestor of all modern texts in acoustics is Rayleigh's Theory of Sound, first published in two volumes in 1877 and expanded in several later editions. In the United States, this book was followed by Stewart and Lindsay's Acoustics (1930), and then by Lindsay's Mechanical Radiation (1960). Another descendant in the Rayleigh tradition is Morse's Vibration and Sound (1936) followed by Morse and Ingard's Theoretical Acoustics (1968). In Britain, the most notable contributions are A.B. Wood's A Textbook of Sound (1930) and Stephens and Bates's Wave Motion and Sound (1950), the latter being used in the Physics Department at Imperial College when this author was a student. Historically, vibrations were the first branch of acoustics to enter the engineering curriculum, and Rayleigh's volume one was an early text among engineers. His influence is seen in Timoshenko's text (1928), and subsequently in that by Den Hartog (1934). Later came the well-accepted work by Thomson (1993) and many others, including the recent notable contribution by Dimarogonas (1996). Now, the most widely used general acoustics texts are probably those by Kinder et al., Fundamentals of Acoustics (1982), and Pierce's Acoustics (1981). There are many other fine texts in acoustics influenced by Rayleigh in varying degrees, but those mentioned here are some of the better known and will suffice to make the point.

Sooner or later all educators confront this question of conscience: What it is that I am trying to accomplish? It is clear that acoustics is flourishing, but I would still argue strongly with those representatives of industry who advocate "training" young people for "jobs." Our task is to educate human beings to achieve the potential of which they are capable. Even when it comes to designing a strategy for those who must meet economic necessity, it is a poor teacher who equips the student with the tricks of only one trade. During the 30 years I have been teaching, there have been numerous shifts in employment for the acoustics graduate. Opportunities in seismic exploration are notorious for their ups and downs linked to the price of oil. The Navy's employment in underwater sound depends on national priorities for defense. Vibration and noise control and architectural acoustics tend to follow trends in the overall economy. A certain number of graduates find homes in the telephone and communications industries. Nondestructive testing of industrial components and medical ultrasonics are also important areas for employment. Very few people with backgrounds in acoustics are unemployed for long, but it is a good idea to be prepared for work in more than one industry. The task of the university should therefore be to provide the student with a broad repertoire of basic ideas. As time moves on, the content of this repertoire needs to change with the advance of science and of engineering practice. Recent years have seen the practice of acoustics revolutionized by the digital computer and digital signal processing. In the future, we can expect even greater changes in acoustical engineering as systems theorists better comprehend such functions of the human brain as speech recognition and music appreciation.

Frederick V Hunt, the famous teacher of acoustics and professor of physics at Harvard, recognized as early as 1967 that acoustics had much to offer as a "fusion" subject, and I quote his words: "Some of the subjects now involved in this fusion process acquired their own recognized status only within the last few decades. One would cite in this connection modern information theory, the theory of stochastic processes, . . . modern control theory, . . . and the exploitation of nonlinearity." Today, we would recognize these topics as a part of systems theory. In this book, the intention is to stress the systems and engineering aspects of acoustics. Acoustics is a good introduction to general systems theory, both because of the heavy emphasis on mathematics and because of its interdisciplinary nature.

The book is intended for senior- or graduate-level engineers, with prerequisites in mathematics through differential equations, basic mechanics, and electromagnetism. The readership is expected to be primarily students majoring in mechanical or electrical engineering, although the book could also be of use to practicing engineers as a reference. The courses that might use this book include introductory acoustics, or vibrations, transducers, acoustic radiation, noise control, stress waves, physical acoustics, and acoustic measurements. Also included is some coverage of the effects of sound on man and on speech and hearing, insofar as these subjects define the tasks of the acoustical engineer, as well as an account of acoustic systems and a review of their design. The total length of a continuous course based on this book would be about two years.