Table Of Content

Sonar for Practising Engineers Third Edition This page intentionally left blank Sonar for Practising bngineers Third Edition A. D. Waite 8 JOHN VVILEY & SONS, LTD Copyright 02 002 by Ashley Waite Published by John Wiley & Sons Ltd, Baflhs Lane, Chichester, West Sussex PO19 lUD, England National 0 1243 779777 International (+44) 1243 779777 e-mail (for orders and customer service enquiries): [email protected] Visit our Home Page on http://www.wiley.co.uk This work is based on Sonar for Practising Engineers, Second Edition, published and distributed by Thales Underwater Systems Limited (formerly named Thomson Marconi Sonar Limited), Ocean House, Templecombe, Somerset, BA8 ODH (www.tms-sonar.com), 1998 All Rights Reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, UK W 1P OLE without the permission in writing of the Publisher with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system for exclusive use by the purchaser of the publication. 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British Library Cataloguingi n Publication Dpta A catalogue record for this book is available from the Brisish Library ISBN 0 471 49750 9 Typeset in 10/1 Ipt Times by Keytec Typesetting Ltd., Bridport, Dorset Printed and bound in Great Britain by Antony Rowe Ltd., Chippenham, Wilts This book is printed on acid-free paper responsibly manufactured from sustainable forestry, for which at least two trees are planted for each one used for paper production. To all my past and present colleagues within the sonar community who have made my career in sonar so rewarding and this book possible - This page intentionally left blank Contents Preface xiii About the Author xv Introduction xvii 1 Sound 1 1.1 Wave motion 1 1.2 Sound pressure 1 1.3 Reference intensity 3 1.4 Source level 4 1.5 Radiated power 5 1.6 Limitations to sonar power 5 1.7 Cavitation 6 1.8 Interaction 8 1.9 Changes to arrays 8 1.10 Projector sensitivity 9 I. 1 1 Hydrophone sensitivity 9 I.I 2 Spectrum level 10 1.13 Sound in air and in sea water 11 1.14 Problems 12 2 Arrays 13 2.1 Need for projector arrays 13 2.2 Need for hydrophone arrays 13 2.3 Beam patterns 14 2.4 Directivity of a dipole 15 2.5 The general line array 18 2.6 Line array: beam pattern vs. steer angle 20 2.7 Broadside array: length and spacing 20 2.8 Beam pattern for a continuous line 21 2.9 Shading 24 2.10 Shaded arrays: transmit source levels 27 2.1 1 Directivity index 28 ... Cont ents Vlll 2.12 DI of a simple dipole 29 2.13 DI of a line array 31 2.14 DI of a planar array 32 2.15 DI of a cylindrical array 35 2.16 DI formulae based for simple arrays 36 2.17 Conformal arrays 36 2.18 Spherical arrays 36 2.19 Volumetric arrays 37 2.20 Beamformers 39 2.2 1 Domes and arrays 40 2.22 Problems 41 3 Propagation of Sound in the Sea 43 3.1 Propagation loss 43 3.2 Losses 43 3.3 Spreading losses 44 3.4 Absorption losses 46 3.5 Spherical spreading and absorption 48 3.6 Propagation in the real ocean 49 3.7 The speed of sound 50 3.8 Sound speed profiles 51 3.9 Deep sound channel 52 3.10 Reliable acoustic path 53 3.1 1 Surface duct propagation 54 3.12 Convergence zone propagation 56 3.13 Bottom bounce propagation 57 3.14 Propagation loss models 58 3.15 Ray theory and the Hodgson model 59 3.16 Hodgson example 61 3.17 Performance prediction 64 3.18 Multipath propagation 66 3.19 Problems 66 4 Target Strength 67 4.1 Definition 67 4.2 Formulae 67 4.3 Measurement 68 4.4 Dependence on pulse type and duration 69 4.5 TS of a sphere 69 4.6 TS of some simple shapes 70 4.7 TS of small targets 72 4.8 Mine target strength 72 4.9 Torpedo target strength 73 4.10 Submarine echoes 74 4.1 1 Beam aspect target strength 74 4.12 Bow aspect target strength 75 4.13 Submarine target strengths 75 Cont ents iX 4.14 Towed arrays 77 78 4.15 Target strength reduction 80 4.16 Practical values 4.17 Problems 81 83 5 Noise in Sonar Systems 5.1 Sources of noise 83 5.2 Thermal noise 84 5.3 Noise from the sea 86 89 5.4 Noise from a vessel 90 5.5 The sonar environment 5.6 Self-noise 90 90 5.7 Electrical noise 5.8 Machinery noise 91 5.9 Flow noise 91 5.10 Propeller noise 92 5.1 1 Variation with speed 92 5.12 Variation with frequency 92 5.13 Directivity 93 5.14 Self-noise and radiated noise 93 5.15 Addition of noise levels 93 94 5.16 Receiver noise factor 5.17 Noise factor of a sonar 95 5.18 Acceptable receiver noise level 97 5.19 Alternative calculation 99 5.20 Practical values 100 5.2 1 Problems 101 6 Reverberation 103 6.1 Sources of reverberation 103 6.2 Scattering and reflection 103 6.3 Boundary roughness 104 6.4 Classes of reverberation 105 6.5 Backscattering strength 106 6.6 Reverberation target strength 106 6.7 Volume reverberation 107 6.8 Boundary reverberation 110 6.9 Scattering layers 111 6.10 Volume scattering strength 112 6.1 1 Sea surface scattering strength 113 6.12 Bottom scattering strength 114 6.13 Variation with fi-equency 116 6.14 Reverberation under ice 117 6.15 Problems 11: 7 The Sonar Equations 119 '7.1 What are they? 119