Vol 21 No 1

CONTENTS

April 1993

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ARTICLES

An Introduction to Cohen's Class of Time-Frequency Distributions
Michael J Harrap and Z L Zhuang

The Instantaneous Sound Intensity in Two-Dimensional Sound Fields - a Finite Element Approach
Qinghui Zhong and Robin Alfredson

REPORTS

Underwater Acoustics Activities at ADFA
Glen A Stewart

R and D in Underwater Acoustic Arrays
Ron J Wyber

Underwater Acoustics Activities at the Australian Maritime College
David R Edwards

Remote Sensing with Underwater Acoustics
John D Penrose, T J Pauly, W R Arcus, A J Duncan and G Bush

News and Notes
Books
New Products
Publications Received
Advertisers Index
AA & Society information
Diary

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An Introduction To Cohen's Class Of Time-Frequency Distributions

M.J.Harrap and Z.L.Zhuang
Acoustics and Vibration Centre
University College
Australian Defence Force Academy

Vol. 21 No. 1 pp 7 - 12 (1993)

ABSTRACT: Time-frequency distributions describe the evolution of a signal's energy in both frequency and time. This paper describes one generic class of time frequency distribution known as Cohen's Class. Better known members of this class include the Spectrogram and Wigner Distributions. Several distributions in this class are compared. The relationship between the properties of these distributions and the shape of their Kernel functions is explained. A portion of a speech waveform is used to illustrate the performance of these distributions.

The Instantaneous Sound Intensity in Two-Dimensional Sound Fields - A Finite Element Approach

Qinghui Zhong and Robin J, Alfredson
Department of Mechanical Engineering
Monash University, Clayton, Vic. 3168

Vol. 21 No. 1 pp 22 - 27 (1993)
ABSTRACT: The acoustic finite element approach is employed for the calculation of the instantaneous sound intensity vectors in a two-dimensional sound field. The sound pressure distribution is first calculated via the acoustic finite element method. The sound particle velocities are then solved for each element from the linearised Euler's equation, and are used to derive the active sound intensity and the reactive sound intensity through a decomposition approach. It is demonstrated that the instantaneous sound intensity vector can be calculated by retaining the time dependence factor. it is also found that the sign, or direction, of the reactive sound intensity vector can be specified in different ways, provided that the instantaneous sound intensity is defined accordingly. An example is provided to simulate sound propagation in a two-dimensional duct with rigid walls, excited by a point mono pole source with constant particle velocity. The results of the active sound intensity field agree well with those given by Fahy [2]. The calculated reactive sound intensity field is very much related to the contour lines of the sound pressure levels, as expected. Two circulatory patterns were observed in the active sound intensity field, each corresponding to a zero pressure point in the sound field. These two circulatory patterns, however, were hardly visible in the instantaneous sound intensity field. It is suggested that the instantaneous sound intensity should be used in complement with the time-averaged active and reactive sound intensities in cases where the acoustic energy transfer is vitally important, e.g. the active noise control for a pure-tone sound field.

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