ULTRASONICS

FUNDAMENTALS AND APPLICATIONS

UL TRASONICS

FUNDAMENTALS AND APPLICATIONS

HEINRICH KUTTRUFF

Professor of Technical Acoustics, Technische Hochschule Aachen, Germany

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British Ubrary cataloguing in Publication Data

Kuttruff, Heinrich. Ultrasonics. 1. Ultrasonic waves I. Title II. [Physik und Technik des Ultraschalls. English) 534·55

ISBN 1-85166-553-6

Ubrary of Congress Cataloging-in-Publication Data

Kuttruff, Heinrich. [Physik und Technik des Ultraschalls. English) Ultrasonics fundamentals and applications/by Heinrich Kuttruff. p. em. Translation of: Physik und Technik des Ultraschalls. Includes bibliographical references and index. ISBN 1-85166-553-6 1. Ultrasonics. I. Title.

aC244.K8713 1991 534.5'5--dc20 90-1396

CIP

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PREFACE

This book is a translation of 'Physik und Technik des Ultraschalls', originally published in 1988 by S. Hirzel Verlag, Stuttgart. As in the German edition, it is based on lectures on ultrasound which the author has given over the past fifteen years to students of electrical engineer­ing and physics at the Rheinisch-Westfiilische Technische Hochschule Aachen, Germany. Its purpose is to explain and describe the peculiarities of high frequency sound with general acoustics as a foundation. It is these peculiarities which have led to the development of specific methods of sound generation and sound detection on the one hand and are relevant to the way ultrasound propagates in various materials, and which are the origin of a wide range of technical applications on the other.

The first part of the book is devoted to the fundamentals of ultrasonics. Since the reader is not expected to have a knowledge of general acoustics, introductory chapters survey the basic ideas and laws of acoustics without systematically deriving the formulae pre­sented. Likewise, the third chapter, which deals with the radiation and diffraction of sound, is still fairly general, although it is somewhat more adapted to the specific requirements of ultrasound. In the three subsequent chapters, the generation and detection or measurement of ultrasound is dealt with. The seventh chapter is a digression on the peculiarities of the hypersonic range. the following chapter is on the most prominent causes of ultrasound absorption and also a description of the basic methods of measuring sound velocities and attenuation in the ultrasonic range.

In the second part of the book a series of typical and important applications of ultrasound is covered. Firstly, mention is made of the application of ultrasound in signal processing and measuring tech­niques which have gained particular importance in the use of Rayleigh waves during the past decade. Then follow two chapters on diagnostic

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PREFACE

methods employing ultrasound, namely in non-destructive material testing and medical diagnostics. The subsequent Chapter XII can be considered as still belonging under this heading since it deals with special methods of ultrasonic imaging. A separate chapter is devoted to cavitation, which plays a supporting-if not the decisive-role in some of the applications of high intensity ultrasound as described in Chapter XIV. The book concludes with a short chapter on possible health hazards related to ultrasound.

For a complete understanding of the book the reader needs some elementary knowledge of physics as well as relevant mathematical techniques, in particular he should be familiar with the fundamentals of differential and integral calculus and also with the most common mathematical functions. But even if he does not meet these prerequis­ites totally, the major content of the book should be comprehensible.

The units employed are those of the International System of Units (metre, kilogram, second, ampere, etc.). As far as the symbols in mathematical expressions are concerned it is not possible to avoid the situation where some symbols have several different meanings. In general, however, it will become clear from the context which quantity is referred to by a particular symbol and therefore there is little danger of confusion. Furthermore, the list of symbols included at the end of the book should prove helpful in this respect.

References to relevant literature are made only when the author has adopted results from a particular publication, or to enable the reader to obtain more detailed information on a new and particular topic. Thus, no attempt has been made to present in total all representative publications. The same holds for the list of books and journals compiled at the end of the book. Its only purpose is to facilitate access to more special studies or to enable the reader to follow current developments in the field of ultrasonics.

The author is indebted to many people who have assisted him in one way or another. Thus, Professor Dr W. Eisenmenger has checked through Chapters VII and VIII, whereas Professor Dr G. Rau has critically read Chapter IX. To both of them lowe many valuable comments. Diagrams and photographic material have been given to the author by Professor Dr-Ing H. Ermert, Bochum; Dr-Ing H. Giilhan, Aachen; Professor Dr W. Lauterborn, Darmstadt; Professor Dr-Ing A. Troost and his co-workers, Aachen; and also by the companies Ernst Leitz in Wetzlar, Scientific Medical Systems GmbH in Wuppertal and VG Microscopes Inc. in East Grinstead (UK).

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PREFACE

Furthermore, the author is grateful to the firms KLN Ultraschall GmbH in Heppenheim, Schoellerschall-Ultraschallanlagen GmbH in Morfelden-Walldorf and Krautkramer GmbH in Cologne for giving him the chance to collect information on modem ultrasonic methods employed in their factories. Finally, he would like to thank Dipl.­Phys. M. Vorlander and Mr Pietzonka for preparing the drawings, either by hand or computer.

The author wishes to express his most sincere thanks to his colleague, Professor Peter Lord for his careful and sensitive translation of the German text, and also to the publishers for complying with the author's wishes and for the excellent production of the book.

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CONTENTS

Preface. . . . . . v I Introduction. . . . 1

1.1 What is ultrasound? 1 1.2 A few historical remarks 3 1.3 Ultrasound in the living world . 5 1.4 Upper frequency limit of sound 7

n Basic Concepts of Acoustics. . 11

11.1 Sound fields and the physical quantities describing them 11 11.2 Sound propagation in gases and liquids. . . . . . .. 15

11.2.1 Basic relations between acoustical quantities, the wave equation. . . . . . . . 15

11.2.2 Plane waves and spherical waves . 17 11.2.3 Energy density and intensity level. 22 11.2.4 Nonlinear effects 24

11.3 Sound waves in solids . 30 11.4 Reflection and refraction 38 11.5 Doppler effect. . . . . 46

m Sound Radiation and Sound Diffraction 49

111.1 Signals in time and frequency representation, linear systems. . . . . . . . . . . . . . . . 50

111.2 The principle of point source synthesis, the moving piston . . . . . . . . . . . . 53

111.3 Radiation from a circular piston 58 111.3.1 Transient radiation. . . 58 III.3.2 Steady-state radiation. . 63 111.3.3 Total power radiated, radiation resistance. 69

111.4 Piston with non-uniform surface velocity 71 111.5 Diffraction and scattering. . . . . . . . . . . . 72

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CONTENTS

IV Generation of Ultrasound-Part I 79

IV.1 The piezoelectric effect. . . . . 80 IV.2 Piezoelectric materials . . . . . 86 IV.3 Basic piezoelectric equations, electro-mechanical cou­

pling factor . . . . . . . . . . . . . . . . . . .. 89 IVA Dynamic characteristics of piezoelectric transducers

operated in their thickness mode. . . . . . . . . .. 94 IVA.1 Transducer loaded on both sides with media of

equal characteristic impedance, Z2 = Z1. .. 97 IVA.2 Transducer unloaded at its rear side, Z2 = 0 . 105 IVA.3 Transducer kept rigid on its rear side, Z2 = 00 106 IVAA Transducer matched at its rear side, Z2 = Zo. 106

IV.S Mechanical and electrical equivalent circuit of a piezo-electric transducer near its resonance. . . . . . .. 109

IV.6 Practical design of piezoelectric ultrasound generators 112

V Generation of Ultrasound-Part II . 119

V.1 Composite piezoelectric transducers 119 V.2 Piezoelectric bending transducers . 122 V.3 Generation of high frequency ultrasound 124 VA Concentration of ultrasound by focusing 126 V.5 Generation of high vibrational amplitudes 130

V.5.1 Stepped transformer. . . 131 V.S.2 Conical transformer. . . . . . . 132 V.5.3 Exponential transformer. . . . . 134

V.6 Generation of shear waves and Rayleigh waves 135 V.7 Magnetostrictive generation of ultrasound. 139 V.8 Electrostatic ultrasound generators. 145 V.9 Mechanical methods . . . . . . . . . . 148

VI Detection and Measurement of Ultrasound 153

VI. 1 Detection of ultrasound with extended piezo trans-ducers, reciprocity . . . . . . . . . 154

VI.2 Electrostatic receivers . . . . . . . 160 VI. 3 Ultrasound microphones, calibration 161 VIA Mechanical detection. . . . . . . . 167 VI.S Thermal ultrasound detectors . . . . 170 VI. 6 Diffraction of light by ultrasound waves 171

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VI.7

VI.6.1 Debye-Sears effect. . . .... V1.6.2 Diffraction of light by shear waves V1.6.3 Bragg diffraction. Visualization of ultrasound . . . . . . .

CONTENTS

171 177 178 180

VII Generation and Detection of Sound with Frequencies above 1 GHz (Hypersound). . . . . . . . . . . . . 187

VII. 1

VII.2 VII.3

VII.4 VII.5

VII.6

VII.7

VIII

VIII. 1 VIII.2 VIII.3 VIII.4

VIII. 5

Coherent methods for the generation and detection of hypersound . . . . . . . . . . . . . . . . . . . . 188 Phonons (sound quanta) in solids . . . . . . . . . . 191 Quantum acoustical interpretation of some effects of ultrasound . . . . . . . . . . . . . . . 193 Generation of hypersound with heat pulses . 195 Detection of hypersound with superconducting bolo-meters . . . . . . . . . . . . . . . . . . . 197 Generation and detection of incoherent hypersound with superconducting tunnel contacts . 199 Detection of 'natural' hypersound 203

Absorption of Ultrasound. . . . 207

Classical sound absorption in gases and liquids 208 Molecular sound absorption in gases 210 Sound absorption in liquids . . . . 217 Sound absorption in solids . . . . 221 VIII.4.1 Polycrystalline materials . 223 VII1.4.2 Sound absorption due to dislocations 226 VIII.4.3 High polymers . . . . . . 229 VIII.4.4 Sound absorption due to interaction with

thermal phonons . . . . . . . . . 231 VII1.4.5 Sound absorption due to interaction with

electrons . . . . . . . . . . . . 232 Experimental methods for the determination of sound velocity and attenuation in the ultrasonic range . . . . 233

IX Applications in Signal Processing and Measuring Techn-

IX.l

iques . ................ .

Ultrasonic delay lines. . . . . . . . . . . IX.l.l Lines with monomode propagation . IX.1.2 Lines with multimode propagation .

241

242 245 247

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CONTENTS

IX.2 Rayleigh wave filters . 250 IX.2.1 The interdigital transducer as a transversal filter 251 IX.2.2 Real Rayleigh wave filters. 254 IX.2.3 Delay lines 256 IX.2.4 Further SA W devices . 259 IX.2.5 Fabrication of SAW devices . 260

IX.3 Light modulation and light deflection. 262 IX.4 Other small-signal applications 264

X Non-destructive Testing of Materials . 269

X.l Survey of various testing methods 270 X.2 Impulse echo method. 273 X.3 Frequencies and wave types . 277 X.4 Transducers for flaw detection. 279 X.5 Types of display . 284 X.6 Suitability of materials for testing 287 X.7 Practical examples of ultrasonic flaw detection. 290

XI Application of Ultrasound in Medical Diagnostics 297

XLI Acoustic properties of biological tissue 298 XI.2 Impulse echo method. 300

XI.2.1 Time gain control 301 XI.2.2 Transducers . 302 XI.2.3 Modes of display . 306 XL2.4 Scanning 308

XI.3 Typical applications of the impulse echo method in sonography . 312 XI.3.1 Internal medicine 312 XI.3.2 Gynaecology and obstetrics 315 XI.3.3 Cardiography 317 XI.3.4 Encephalography. 318 XI.3.5 Opthalmology . 320 XI.3.6 Ultrasound-guided aspiration 321

XL4 Doppler sonography . 322

XII Special Methods of Ultrasonic Imaging . 325

XII. 1 Ultrasonic microscopy 326 XII. 1.1 Microscopes with extended sensors 327

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CONTENTS

XII.1.2 Scanning acoustic microscopes (SAM) 328 XII.1.3 Transducer and lens 330 XII.1.4 Coupling liquid 332 XII. 1.5 Applications . . . 334

XII.2 Acoustic holography . . . . 337 XII.2.1 Ultrasonic holography with optical reconstruc-

tion. . . . . . . . . . . . 341 XII.2.2 Computer reconstruction. . . 345 XII.2.3 Estimation of axial resolution . 348 XI1.2.4 Wide-band"methods . . 350

XII. 3 Ultrasonic tomography . . . . . . . . 351 XI1.3.1 Algebraic reconstruction . . . 355 XI1.3.2 Reconstruction by Fourier transformation 357 XI1.3.3 Reflection tomography. 358

xm Cavitation........... 363

XIII. 1 Basic types of sonically induced cavitation 364 XIII.2 Dynamics of a single cavity . . . . 365

XIII.2.1 Implosion of a gas-free cavity at constant external pressure . . . . . . . . . . . . . 367

XIII.2.2 Bubble oscillation in a stationary sound field. 370 XIII.2.3 Transition to 'hard' cavitation . . 376

XIII. 3 Cavitation nuclei and cavitation thresholds . 377 XIII.4 Real cavitation and some effects caused by it 384

XIII.4.1 Erosion of solid materials 389 XII1.4.2 Sonoluminescence . . 392 XIII.4.3 Sonochemical reactions . 393

XIV Applications of High Intensity Ultrasound 395

XIV. 1 Ultrasonic cleaning. . . . . . . . . . . 397 XIV. 1. 1 Cleaning tanks . . . . . . . . 399 XIV.1.2 Sound generation, operating frequencies and

powers . . . . . . 402 XIV. 1.3 Cleaning liquids . . 403 XIV.1.4 Cleaning procedure. 404

XIV.2 Joining with ultrasound. . . . 406 XIV.2.1 Mechanism of welding plastics 406 XIV.2.2 Welding equipment. . . . . 410 XIV.2.3 Applications of ultrasonic welding of plastics. 412

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CONTENTS

XIV.2.4 Riveting and insertion of metal parts 413 XIV.2.5 Welding of metals . 414 XIV.2.6 Ultrasonic soldering. 417

XIV. 3 Machining . . . . . . . 417 XIV.4 Production of dispersions . 420 XIV.5 Further applications 426 XIV.6 Medical therapy. . . . . 428

XV On the Possibility of Health Risks Caused by Ultra-sound . . . . . . . . . . . . . . . . . . . 431

XV. 1 Damage to tissue caused by diagnostic ultrasound 431 XV.l.1 Heating . . . . 432 XV.l.2 Mechanical stress . . . . . . 433 XV.l.3 Cavitation . . . . . . . . . 433

XV.2 Damage caused by airborne ultrasound. 435 Notation . . 437 Bibliography 440

Index . . . 443

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