Acoustics has entered a new age—the era of precision engineering. In the 19th century, acoustics was an art. The primary measuring instruments used by engineers in the field were their ears. The only controlled noise sources available were whistles, gongs, sirens, and gunshots. Microphones consisted of either a diaphragm connected to a mechanical scratcher that recorded the shape of the wave on the smoked surface of a rotating drum or a flame whose height varied with the sound pressure. About that time the great names of Rayleigh, Stokes, Thomson, Lamb, Helmholtz, König, Tyndall, Kundt, and others appeared on important published papers. Their contributions to the physics of sound were followed by the publication of Lord Rayleigh's two-volume treatise Theory of Sound in 1877/1878 (revised in 1894/1896). In the late 19th century, Alexander Graham Bell invented the magnetic microphone and with it the telephone. Thomas Edison created the carbon microphone, which was the transmitter used in standard telephone handsets for almost 100
years. The next big advance was Edison's phonograph, which made it possible for the human voice and other sounds to be preserved for posterity.
In a series of papers published between 1900 and 1915, W. C. Sabine advanced architectural acoustics to the status of a science. He measured the duration of reverberation in rooms using organ pipes as the source of sound and a chronograph for the precision measurement of time. He showed that reverberation could be predicted for auditoria from knowledge of room volume, audience size, and the characteristics of the sound-reflecting surfaces—sidewalls and ceiling.
Although the contributions of these earlier workers were substantial, the greatest acceleration of research in acoustics followed the invention of the triode vacuum tube (1907) and the advent of radio broadcasting (1920). When vacuum tube amplifiers and loudspeakers became available, loud sounds of any desired frequency could be produced. With the invention of moving coil and condenser microphones, the intensity of very faint sounds could be measured. Above all, it became feasible to build acoustical measuring instruments that were compact, rugged, and insensitive to air drafts, temperature, and humidity.
The progress of communication acoustics was hastened through the efforts of the Bell Telephone Laboratories (1920ff), which were devoted to perfection of the telephone system in the United States. During the First World War, the biggest advances were in underwater sound. In the next two decades (1936ff) architectural acoustics strode forward through research at Harvard, the Massachusetts Institute of Technology, the University of California at Los Angeles, and several research centers in England and Europe, especially Germany. Sound decay in rectangular rooms was explained in detail, the impedance method of specifying acoustical materials was investigated, and the computation of sound attenuation in ducts was put on a precise basis. The advantages of skewed walls and use of acoustical materials in patches rather than on entire walls were demonstrated. Functional absorbers were introduced to the field, and a wider variety of acoustical materials came on the market. Morse, Stenzel, Mast, Rdzanek, and many others have helped to develop the mathematical theory of sound radiation and diffraction, not to mention the “Dutch school” of Zernike, Bouwkamp, Streng, Aarts, and Janssen, contributions from all of whom are found in the latter part of this book.
The science of psychoacoustics was rapidly developing. At the Bell Telephone Laboratories, under the leadership of Harvey Fletcher, the concepts of loudness and masking were quantified, and many of the factors governing successful speech communication were determined (1920–40). Acoustics, through the medium of ultrasonics, entered the fields of medicine and chemistry. For example, ultrasonic diathermy was being tried, and acoustically accelerated chemical reactions were reported.
Then came the Second World War with its demand for the successful detection of submerged submarines and for highly reliable speech communication in noisy environments such as in armored vehicles and high-flying nonpressurized aircraft. Government financing of improvements in these areas led to the formation of great laboratories in England, Germany, and France, as well as in the United States at Columbia University, Harvard, and the University of California. During this period, research in acoustics reached proportions undreamed of a few years before and has continued unabated since.
In the last 50
years, the greatest revolution has undoubtedly been the vast increase in computing power accompanied by a rapid rate of miniaturization, which has lead to a previously unimaginable plethora of hand portable products, including cell phones, palmtop computers, and measuring devices. Size has presented new challenges for the acoustical designer as the pressure to reduce dimensions is ever increasing. Contrary to popular expectations, electroacoustic transducers do not obey Moore's law
[1], so it cannot be assumed that a reduction in size can be achieved without sacrificing performance, although new materials such as polysilicon membranes for microphones and neodymium magnets for loudspeakers have helped preserve performance to some extent. Reducing the size of loudspeakers usually compromises their maximum sound power
output, particularly at low frequencies, and in the case of microphones, the signal-to-noise ratio in their output deteriorates. Therefore, the ability of the acoustical engineer to optimize the design of transducers and electronics has never been more important.
In addition to the changes in products in which electroacoustic transducers are employed, computers have revolutionized the way in which the transducers themselves are modeled
[2]. The first wave of tools came in the 1960s and early 1970s for simulating electrical circuits. Acoustical engineers were quick to adapt these for modeling loudspeakers and microphones using lumped mechanical and acoustical circuit elements analogous to electrical ones, as given in
Acoustics. However, simulation by this method was largely a virtual form of trial-and-error experimentation, albeit much faster than actual prototyping, until Thiele and Small applied filter theory to the transfer function so that the designer could choose a target frequency response shape for a loudspeaker and engineer the electromechanoacoustic system accordingly. Finite element modeling (FEM) and boundary element modeling (BEM) both followed quickly. Unlike lumped element simulation, the range was no longer limited to that where the acoustical wavelength is much greater than the largest dimension of the device. With the wide availability of modern tools for acoustical simulation, it is perhaps tempting to neglect more traditional analytical methods, which are a focus of this text, especially in
Chapters. 12 and
13. However, analytical (mathematical) methods can offer some distinct benefits:
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• According to Richard Hamming [3], “The purpose of computation is insight not just numbers.” By examining the mathematical relationships, we can gain a better understanding of the physical mechanisms than when the calculations are all “hidden” in a computer. This helps us to create improved systems, especially when we can manipulate the equations to arrive at formulas that enable us to design everything correctly first time such as those given in Chapter 7 for loudspeaker systems. By contrast, a simulation tool can only simulate the design we load into it. It cannot tell us directly how to design it, although it may be possible to tweak parameters randomly in a Monte Carlo optimization or “evolve” a design using a Darwinian genetic algorithm. Even then, a global optimum is not necessarily guaranteed.
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• Although exact analytical formulas are generally limited to simple rectangular, cylindrical, or spherical geometries, many electroacoustic transducers have, or can be approximated by, these simple geometries. (Note this restriction does not apply to lumped elements that can have almost any shape.) It may sometimes take time and effort to derive a formula, but once it is done it can be used to generate as many plots as you like simply by varying the parameters. Furthermore, the right formula will give the fastest possible computation with the least amount of memory space. If a picture paints a 1000 words, it could be said that an equation paints a 1000 pictures. In the words of Albert Einstein, “Politics is for the moment, but an equation is for eternity.”
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• Analytical solutions often yield simple asymptotic expressions for very low/high frequencies or the far field, which can form the basis for elements in circuit simulation programs.
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• It is often useful to have an analytical benchmark against which to check FEM/BEM simulation results. This can tell us much about the required element size and what kind of meshing geometry to use. Of course, having two ways of solving a problem gives us increased confidence in both methods.
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• Analytical formulas are universal and not restricted to a particular simulation tool. They can be written into a wide choice of programming languages.
Another area in which computers have contributed is that of symbolic computation. For example, if we did not know that the integral of cos
x was sin
x, we would have to integrate cos
x numerically, which is a relatively slow and error-prone process compared with evaluating sin
x directly. Modern mathematical tools are capable of solving much more complicated integrals than this symbolically, which has led to new analytical solutions in sound radiation. For example, the previous formulas for radiation from a circular disk in free space were too lengthy to include in the original
Acoustics but a more compact solution, with or without a circular baffle, is given in the new
Chapter 13.
Not only have computers led to the advances mentioned above, but they have fallen dramatically in price so much that many devices such as cell phones, hearing aids, and sound level meters now contain a digital signal processor (DSP) as well as electroacoustical transducers. This enables an acoustical designer to design a complete system including DSP equalization. Although DSP algorithms are beyond the scope of this book,
Chapter 14 has been written with the intention of aiding this part of the design process. It describes state-variable circuit simulation theory, which can be used to obtain a transfer function of the electroacoustical system. The inverse transfer function can then be used as a basis for DSP equalization. However, any form of equalization should come with a health warning because it cannot be used to compensate for a poor acoustical design. On the other hand, a DSP can be used in real time to monitor changes in the electromechanoacoustical parameters and to adjust the drive levels accordingly to extract the maximum possible performance, while avoiding burnout.
In 1962, Sessler and West invented a new kind of capacitor microphone, which contained a permanently stored charge on a metalized membrane as well as a preamplifier, which has become to be known as the foil electret microphone. This device has been followed by microelectromechanical systems (MEMS), now incorporated into microphones and vibration pickups (accelerometers) and gyroscopes, which have dimensions in the order of microns. One embodiment of MEMS is widely used in hearing aids and cell phones, where the trend is to incorporate more microphones for noise cancellation and beam forming. It consists of a freely vibrating diaphragm made from polysilicon, which is spaced from a perforated backplate that is coated with
vapor-deposited silicon nitride. When the device is moved, there is a change in capacitance in the order of femtofarads. The combination of low cost, small size, reliability, and near-studio quality has made the “crackly” carbon microphone obsolete. Hence, the electret and MEMS models are also described in this text. Other new additions include call loudspeakers for cell phones and an improved tube model for very small diameters.
Today, acoustics is no longer a tool of the telephone industry, a few enlightened architects, and the military. It is a concern in the daily life of nearly every person. International movements legislate and provide quiet housing. Labor and office personnel demand safe and comfortable acoustic environments in which to work. Architects in rapidly increasing numbers are hiring the services of acoustical engineers as a routine part of the design of buildings. Manufacturers are using acoustic instrumentation on their production lines. In addition, there has been great progress in the abatement of noise from jet engine propelled aircraft, in which efforts were instigated by the Port of New York Authority and its consultant Bolt Beranek and Newman in the late 1950s and have been carried on by succeeding developments in engine design. Acoustics is coming into its own in the living room, where high-fidelity reproduction of music has found a wide audience. Overall, we witness the rapid evolution of our understanding of electroacoustics, architectural acoustics, structural acoustics, underwater sound, physiological and psychological acoustics, musical acoustics, and ultrasonics.
It is difficult to predict the future with any certainty, although nanotechnologies look as though they will play a steadily increasing role. One can truly say that although over 100
years have passed since the publication of Rayleigh's Theory of Sound, there is still plenty to explore.
This book covers first the basic aspects of acoustics: wave propagation in the air, the theory of mechanical and acoustical circuits, the radiation of sound into free space, and the properties of acoustic components. It is then followed by chapters dealing with microphones, loudspeakers, enclosures for loudspeakers, and horns. The basic concepts of sound in enclosures are treated next, and methods for solving problems related to the radiation and scattering of sound are given. The final chapter describes a computer method for analyzing circuits. Throughout the text we shall speak to you—the student of this modern and exciting field.