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Über dieses Buch

The purpose of this book is to j.ir€'~ 0l'l\' a new technique for the experimental investigation of the free wave model sound field of acoustics. The technique is based on the use of spherical harmonic functions of angle. Acousticians frequently encounter random sound fields whose properties may be closely modelled by use of the "free wave" field. This model field is defined by two basic statistical properties: stationarity in time, and homogeneity in space. Stationarity means that any single order statistic measured by a microphone in the field will be independent of the time at which the recording is taken, while homogeneity means that the measurement will also be independent of the mic- phone's position in the field. Furthermore, second order statistics obtained from the measurements of two microphones will depend only on the time lapse between the two recordings, and the relative spatial separation of the micro­ phones, and not on the microphones' absolute positions in space and time. The free wave field may also (equivalently) be pictured as a collection of plane sound waves which approach an observation position from all angles. These are the "free waves" of the title, with no correlation between waves at different angles and frequencies, although there may exist an angle-dependant plane wave density function. This is a measure of the density of sound energy arriving from different angles. The free wave field has proved to be a simple but remarkably powerful model.

Inhaltsverzeichnis

Frontmatter

Chapter One. Introduction

Abstract
The purpose of this book is to present a new technique for the experimental investigation of the free wave model sound field of acoustics. The technique is based on the use of spherical harmonic functions of angle.
S. M. Baxter, C. L. Morfey

Chapter Two. The Free Wave Sound Field

Abstract
The purpose of this book is to present an experimental method to investigate the spatial characteristics of a certain type of complex, random wave field. A wave field qualifies as suitable for study using the new method provided it can reasonably be approximated by a “free wave” model field. The details of the investigation technique are given in later chapters. The present chapter is a description of the properties and importance of the free wave concept.
S. M. Baxter, C. L. Morfey

Chapter Three. Inference of the Plane Wave Weighting Function from Spectral Density Measurements

Abstract
The last chapter was a description of the free wave idea. In whatever context a free wave field is encountered, it will always have certain general characteristics: in particular, the complex cross-spatial density (CCSD) taken between two observation points in the field may always be expressed as a weighted integral of uncorrelated plane wave contributions. The plane wave weighting function occurring in this integral completely characterises the field, and its interpretation was outlined in Chapter Two.
S. M. Baxter, C. L. Morfey

Chapter Four. The Spherical Harmonic Analysis of Free Wave Fields in Practice

Abstract
As we saw in Chapter Two, a free wave field used to model a spatially complex, random wave field may be completely described by its plane wave weighting function. Associated with the weighting function are such quantities as the energy flow directivity in the field. However, the weighting function is not directly observable, and must be inferred from measurements of complex cross-spectral density (CCSD).
S. M. Baxter, C. L. Morfey

Chapter Five. Summary

Abstract
This monograph has been a presentation of a new experimental technique for the spatial characterisation of complex, random sound fields. Such fields are encountered, for example, in the study of acoustic fields in ducts, enclosures and in the ocean, and in many other contexts.
S. M. Baxter, C. L. Morfey

Backmatter

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