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This brief introduction may help to persuade the reader that acoustics covers a wide range of interesting topics. It is impossible to cover all these topics in a single handbook, but we have attempted to include a sampling of hot topics that represent current acoustical research, both fundamental and applied.

is the science of sound. It deals with the production of sound, the propagation of sound from the source to the receiver, and the detection and perception of sound. The word is often used to describe two different things: an auditory sensation in the ear, and the disturbance in a medium that can cause this sensation. By making this distinction, the age-old question “If a tree falls in a forest and no one is there to hear it, does it make a sound?” can be answered.

Although there are certainly some good historical treatments of acoustics in the literature, it still seems appropriate to begin a handbook of acoustics with a brief history of the subject. We begin by mentioning some important experiments that took place before the 19 century. Acoustics in the 19 century is characterized by describing the work of seven outstanding acousticians: Tyndall, von Helmholtz, Rayleigh, Stokes, Bell, Edison, and Koenig. Of course this sampling omits the mention of many other outstanding investigators.

To represent acoustics during the 20 century, we have selected eight areas of acoustics, again not trying to be all-inclusive. We select the eight areas represented by the first eight technical areas in the Acoustical Society of America. These are architectural acoustics, physical acoustics, engineering acoustics, structural acoustics, underwater acoustics, physiological and psychological acoustics, speech, and musical acoustics. We apologize to readers whose main interest is in another area of acoustics. It is, after all, a broad interdisciplinary field.

This chapter deals with the physical and mathematical aspects of sound when the disturbances are, in some sense, small. Acoustics is usually concerned with small-amplitude phenomena, and consequently a linear description is usually applicable. Disturbances are governed by the properties of the medium in which they occur, and the governing equations are the equations of continuum mechanics, which apply equally to gases, liquids, and solids. These include the mass, momentum, and energy equations, as well as thermodynamic principles. The viscosity and thermal conduction enter into the versions of these equations that apply to fluids. Fluids of typical great interest are air and sea water, and consequently this chapter includes a summary of their relevant acoustic properties. The foundation is also laid for the consideration of acoustic waves in elastic solids, suspensions, bubbly liquids, and porous media.

This is a long chapter, and a great number of what one might term classical acoustics topics are included, especially topics that one might encounter in an introductory course in acoustics: the wave theory of sound, the wave equation, reflection of sound, transmission from one media to another, propagation through ducts, radiation from various types of sources, and the diffraction of sound.

Propagation of sound close to the ground outdoors involves geometric spreading, air absorption, interaction with the ground, barriers, vegetation and refraction associated with wind and temperature gradients. After a brief survey of historical aspects of the study of outdoor sound and its applications, this chapter details the physical principles associated with various propagation effects, reviews data that demonstrate them and methods for predicting them. The discussion is concerned primarily with the relatively short ranges and spectra of interest when predicting and assessing community noise rather than the frequencies and long ranges of concern, for example, in infrasonic global monitoring or used for remote sensing of the atmosphere. Specific phenomena that are discussed include spreading losses, atmospheric absorption, diffraction by barriers and buildings, interaction of sound with the ground (ground waves, surface waves, ground impedance associated with porosity and roughness, and elasticity effects), propagation through shrubs and trees, wind and temperature gradient effects, shadow zones and incoherence due to atmospheric turbulence. The chapter concludes by suggesting a few areas that require further research.

It is well established that sound waves, compared to electromagnetic waves, propagate long distances in the ocean. Hence, in the ocean as opposed to air or a vacuum, one uses sound navigation and ranging (SONAR) instead of radar, acoustic communication instead of radio, and acoustic imaging and tomography instead of microwave or optical imaging or X-ray tomography. Underwater acoustics is the science of sound in water (most commonly in the ocean) and encompasses not only the study of sound propagation, but also the masking of sound signals by interfering phenomenon and signal processing for extracting these signals from interference. This chapter we will present the basics physics of ocean acoustics and then discuss applications.

An overview of the fundamental concepts needed for an understanding of physical acoustics is provided. Basic derivations of the acoustic wave equation are presented for both fluids and solids. Fundamental wave concepts are discussed with an emphasis on the acoustic case. Discussions of different experiments and apparatus provide examples of how physical acoustics can be applied and of its diversity. Nonlinear acoustics is also described.

Thermodynamic and fluid-dynamic processes in sound waves in gases can convert energy from one form to another. In these processes [,], high-temperature heat or chemical energy can be partially converted to acoustic power, acoustic power can produce heat, acoustic power can pump heat from a low temperature or to a high temperature, and acoustic power can be partially converted to chemical potential in the separation of gas mixtures. In some cases, the thermoacoustic perspective brings new insights to decades-old technologies. Well-engineered thermoacoustic devices using extremely intense sound approach the power conversion per unit volume and the efficiency of mature energy-conversion equipment such as internal combustion engines, and the simplicity of few or no moving parts drives the development of practical applications.

This chapter surveys thermoacoustic energy conversion, so the reader can understand how thermoacoustic devices work and can estimate some relevant numbers. After a brief history, an initial section defines vocabulary and establishes preliminary concepts, and subsequent sections

explain engines, dissipation, refrigeration, and mixture separation. Combustion thermoacoustics is mentioned only briefly. Transduction and measurement systems that use heat-generated surface and bulk acoustic waves in solids are not discussed.

At high sound intensities or long propagation distances at sufficiently low damping acoustic phenomena become nonlinear. This chapter focuses on nonlinear acoustic wave properties in gases and liquids. The origin of nonlinearity, equations of state, simple nonlinear waves, nonlinear acoustic wave equations, shock-wave formation, and interaction of waves are presented and discussed. Tables are given for the nonlinearity parameter / for water and a range of organic liquids, liquid metals and gases. Acoustic cavitation with its nonlinear bubble oscillations, pattern formation and sonoluminescence (light from sound) are modern examples of nonlinear acoustics. The language of nonlinear dynamics needed for understanding chaotic dynamics and acoustic chaotic systems is introduced.

This chapter deals specifically with concepts, tools, and architectural variables of importance when designing auditoria for speech and music. The focus will be on cultivating the useful components of the sound in the room rather than on avoiding noise from outside or from installations, which is dealt with in Chap. 11. The chapter starts by presenting the subjective aspects of the room acoustic experience according to consensus at the time of writing. Then follows a description of their objective counterparts, the objective room acoustic parameters, among which the classical measure is only one of many, but still of fundamental value. After explanations on how these parameters can be measured and predicted during the design phase, the remainder of the chapter deals with how the acoustic properties can be controlled by the architectural design of auditoria. This is done by presenting the influence of individual design elements as well as brief descriptions of halls designed for specific purposes, such as drama, opera, and symphonic concerts. Finally, some important aspects of loudspeaker installations in auditoria are briefly touched upon.

This chapter describes the theory of subjective preference for the sound field applied to designing concert halls. Special attention is paid to the process of obtaining scientific results, rather than only describing a final design method. Attention has also been given to enhancing satisfaction in the selection of the most preferred seat for each individual in a given hall. We begin with a brief historical review of concert hall acoustics and related fields since 1900.

A neurally grounded theory of subjective preference for the sound field in a concert hall, based on a model of the human auditory–brain system, is described []. Most generally, subjective preference itself is regarded as a primitive response of a living creature and entails judgments that steer an organism in the direction of maintaining its life. Brain activities relating to the scale value of subjective preference, obtained by paired-comparison tests, have been determined. The model represents , relating the autocorrelation function (ACF) mechanism and the interaural cross-correlation function (IACF) mechanism for signals arriving at the two ear entrances. The representations of ACF have a firm neural basis in the temporal patterning signal at each of the two ears, while the IACF describes the correlations between the signals arriving at the two ear entrances. Since Helmholtz, it has been well appreciated that the cochlea carries out a rough spectral analysis of sound signals. However, by the use the of the spectrum of an acoustic signal, it was hard to obtain factors or cues to describe subjective responses directly. The auditory representations from the cochlea to the cortex that have been found to be related to subjective preference in a deep way involve these temporal response patterns, which have a very different character from those related to power spectrum analyses. The scale value of subjective preference of the sound field is well described by four orthogonal factors. Two are temporal factors (the initial delay time between the direct sound and the first reflection, Δ, and the reverberation time, ) associated with the left cerebral hemisphere, and two are spatial factors [the binaural listening level (LL) and the magnitude of the IACF, the IACC] associated with the right hemisphere. The theory of subjective preference enables us to calculate the acoustical quality at any seat in a proposed concert hall, which leads to a seat selection system.

The temporal treatment enables musicians to choose the music program and/or performing style most suited to a performance in a particular concert hall. Also, for designing the stage enclosure for music performers, a temporal factor is proposed. Acoustical quality at each seating position examined in a real hall is confirmed by both temporal and spatial factors.