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Medical acoustics can be subdivided into diagnostics and therapy. Diagnostics are further separated into auditory and ultrasonic methods, and both employ low amplitudes. Therapy (excluding medical advice) uses ultrasound for heating, cooking, permeablizing, activating and fracturing tissues and structures within the body, usually at much higher amplitudes than in diagnostics. Because ultrasound is a wave, linear wave physics are generally applicable, but recently nonlinear effects have become more important, even in low-intensity diagnostic applications.

This document is designed to provide the nonmedical acoustic scientist or engineer with some insights into acoustic practices in medicine. Auscultation with a stethoscope is the most basic use of acoustics in medicine and is dependent on the fields of incompressible (circulation) and compressible (respiration) fluid mechanics and frictional mechanics. Detailed discussions of tribology, laminar and turbulent hemodynamics, subsonic and supersonic compressional flow, and surfactants and inflation dynamics are beyond the scope of this document. However, some of the basic concepts of auscultation are presented as a starting point for the study of natural body sounds. Ultrasonic engineers have dedicated over half a century of effort to the development of ultrasound beam patterns and beam scanning methods, stretching the current technical and economic limits of analog and digital electronics and signal processing at each stage. The depth of these efforts cannot be covered in these few pages. However, the basic progression of progress in the fields of transducers and signal processing will be covered. The study of the interaction of ultrasound with living tissues is complicated by complex anatomic structures, the high density of scatterers, and the constantly changing nature of the tissues with ongoing life processes including cardiac pulsations, the formation of edema and intrinsic noise sources. A great deal of work remains to be done on the ultrasonic characterization of tissues. Finally, the effect of ultrasound on tissues, both inadvertent and therapeutic will be discussed.

Much of the medical acoustic literature published since 1987 is searchable online, so this document has included key words that will be helpful in performing a search. However, much of the important basic work was done before 1987. In an attempt to help the reader to access that literature, Denis White and associates have compiled a complete bibliography of the medical ultrasound literature prior to 1987. Under Further Reading in this chapter, the reader will find a link to a complete compilation of 99 citations from which list the thousands of articles on medical acoustics written prior to 1987.

The academically based authors develop, use and commercialize diagnostic ultrasonic Doppler systems for the benefit of patients with cardiovascular diseases. To translate ultrasonic and acoustic innovation into widespread clinical application requires as much knowledge about the economics of medicine, the training and practices of medical personnel, and the pathology and prevalence of diseases as about the diffraction patterns of ultrasound beams and signal-to-noise ratio of an echo. Although a discussion of these factors is beyond the scope of this chapter, a few comments will help to provide perspective on the likely future contribution of medical acoustics to improved public health.

This chapter is devoted to vibrations of structures and to their coupling with the acoustic field. Depending on the context, the radiated sound can be judged as desirable, as is mostly the case for musical instruments, or undesirable, like noise generated by machinery. In architectural acoustics, one main goal is to limit the transmission of sound through walls. In the automobile industry, the engineers have to control the noise generated inside and outside the passenger compartment. This can be achieved by means of passive or active damping. In general, there is a strong need for quieter products and better sound quality generated by the structures in our daily environment.

Structural acoustics and vibration is an interdisciplinary area, with many different potential applications. Depending on the specific problem under investigation, one has to deal with material properties, structural modifications, signal processing and measurements, active control, modal analysis, identification and localization of sources or nonlinear vibrations, among other hot topics.

In this chapter, the fundamental methods for the analysis of vibrations and sound radiation of structures are presented. It mainly focuses on general physical concepts rather than on specific applications such as those encountered in ships, planes, automobiles or buildings. The fluid–structure coupling is restricted to the case of light compressible fluids (such as air). Practical examples are given at the end of each section.

After a brief presentation of the properties of the basic linear single-degree-of-freedom oscillator, the linear vibrations of strings, beams, membranes, plates and shells are reviewed. Then, the structural–acoustic coupling of some elementary systems is presented, followed by a presentation of the main dissipation mechanisms in structures. The last section is devoted to nonlinear vibrations. In conclusion, a brief overview of some advanced topics in structural acoustics and vibrations is given.

Noise is discussed in terms of a source–path–receiver model. After an introduction to sound propagation and radiation efficiency, the quantities measured for noise control are defined, and the instruments used for noise measurement and control are described.

The noise emission of sources is discussed with emphasis on the determination of the sound power level of a variety of sources. The properties of two very significant sources of environmental noise, aircraft and motor vehicles, are presented. Tire noise is identified as a major noise source for motor vehicles. Criteria for the noise emission of sources are given, and the basic principles of noise control are presented. A section on active control of noise is included.

The path from the source to the receiver includes propagation in the atmosphere, noise barriers, the use of sound-absorptive materials, and silencers. Guidance is given on the determination of sound pressure level in a room when the sound power output of the source is known.

At the receiver, the effects of noise are presented, including both hearing damage and annoyance. A brief section is devoted to sound quality.

Finally, noise regulations and policies are discussed. Many activities of the US government are discussed, and information on both state and local noise policies and regulations are presented. The activities of the European Union are included, as are the noise policies in many countries.

The condenser microphone continues to be the standard against which other microphones are calibrated. A brief discussion of the theory of the condenser microphone, including its open-circuit voltage, electrical transfer impedance, and mechanical response, is given. The most precise method of calibration, the reciprocity pressure calibration method for laboratory standard microphones is discussed in detail, beginning with the principles of the reciprocity method. Corrections for heat conduction, equivalent volume, capillary tube, wave motion, barometric pressure and temperature are necessary to achieve the most accurate open-circuit sensitivity of condenser microphones.

Free-field calibration is discussed briefly, and in view of the difficulties in obtaining more accurate results than those provided by the reciprocity method, references are given for more detailed consideration. Secondary microphone calibration methods by comparison are described. These methods include interchange microphone comparison, comparison with a calibrator, comparison pressure and free-field, and comparison with a precision attenuator. These secondary calibration methods, which are adequate for most industrial applications, are economically attractive and less time consuming.

The electrostatic actuator method for frequency response measurement of working standard microphones is discussed with some pros and cons presented. An example to demonstrate the stability of laboratory standard microphones and the stability of a laboratory calibration system is described.

Appendix A discusses acoustic transfer impedance evaluation, while appendix B contains physical properties of air, which are necessary for microphone calibration.

Sound intensity is a vector that describes the flow of acoustic energy in a sound field. The idea of measuring this quantity directly, instead of deducing it from the sound pressure on the assumption of some idealized conditions, goes back to the early 1930s, but it took about 50 years before sound intensity probes and analyzers came on the market. The introduction of such instruments has had a significant influence on noise control engineering.

This chapter presents the , which is the basis for sound power determination using sound intensity. The concept of reactive intensity is introduced, and relations between fundamental sound field characteristics and active and reactive intensity are presented and discussed.

Measurement of sound intensity involves the determination of the sound pressure and the particle velocity at the same position simultaneously. The established method of measuring sound intensity employs two closely spaced pressure microphones. An alternative method is based on the combination of a pressure microphone and a particle velocity transducer.

Both methods are described, and their limitations are analyzed. Methods of calibrating and testing the two different measurement systems are also described. Finally the state of the art in the various areas of practical application of sound intensity measurement is summarized. These applications include the determination of the sound power of sources, identification and rank ordering of sources, and the measurement of the transmission of sound energy through partitions.

One of the subtle problems that make noise control difficult for engineers is the invisibility of noise or sound. A visual image of noise often helps to determine an appropriate means for noise control. There have been many attempts to fulfill this rather challenging objective. Theoretical (or numerical) means for visualizing the sound field have been attempted, and as a result, a great deal of progress has been made. However, most of these numerical methods are not quite ready for practical applications to noise control problems. In the meantime, rapid progress with instrumentation has made it possible to use multiple microphones and fast signal-processing systems. Although these systems are not perfect, they are useful. A state-of-the-art system has recently become available, but it still has many problematic issues; for example, how can one implement the visualized noise field. The constructed noise or sound picture always consists of bias and random errors, and consequently, it is often difficult to determine the origin of the noise and the spatial distribution of the noise field. Section  of this chapter introduces a brief history, which is associated with “sound visualization,” acoustic source identification methods and what has been accomplished with a line or surface array. Section  introduces difficulties and recent studies, including de-Dopplerization and de-reverberation methods, both essentialfor visualizing a moving noise source, such as occurs for cars or trains. This section also addresses what produces ambiguity in realizing real sound sources in a room or closed space. Another major issue associated with sound/noise visualization is whether or not we can distinguish between mutual dependencies of noise in space (Sect. ); for example, we are asked to answer the question, “Can we see two birds singing or one bird with two beaks?”

Modern optical methods applicable to vibration analysis, monitoring bending-wave propagation in plates and shells as well as propagating acoustic waves in transparent media such as air and water are described. Field methods, which capture the whole object field in one recording, and point measuring (scanning) methods, which measure at one point (small area) at a time (but in that point as a function of time), will be addressed. Temporally, harmonic vibrations, multi-frequency repetitive motions and transient or dynamic motions are included.

Interferometric methods, such as time-average and real-time holographic interferometry, speckle interferometry methods such as television (TV) holography, pulsed TV holography and laser vibrometry, are addressed. Intensity methods such as speckle photography or speckle correlation methods and particle image velocimetry (PIV) will also be treated.

Modal analysis is widely used to describe the dynamic properties of a structure in terms of the modal parameters: natural frequency, damping factor, modal mass and mode shape. The analysis may be done either experimentally or mathematically. In mathematical modal analysis, one attempts to uncouple the structural equations of motion so that each uncoupled equation can be solved separately. When exact solutions are not possible, numerical approximations such as finite-element and boundary-element methods are used.

In experimental modal testing, a measured force at one or more points excites the structure and the response is measured at one or more points to construct frequency response functions. The modal parameters can be determined from these functions by curve fitting with a computer. Various curve-fitting methods are used. Several convenient ways have developed for representing these modes graphically, either statically or dynamically. By substituting microphones or intensity probes for the accelerometers, modal analysis methods can be used to explore sound fields. In this chapter we mention some theoretical methods but we emphasize experimental modal testing applied to structural vibrations and also to acoustic fields.