1968 IEEE symposium on sonics and ultrasonics

components could be satisfactorily added to monolithic circuits. Where minimum size was essential, hybrid circuits that comprise both active monolithic devices and passive film devices should be used. CushingE described the enumeration of hake and the prediction of hake catches by the Kelvin Hughes ‘Humber’ echosounder. He also mentioned briefly the use of the ‘Lowestoft’ 1OOkHzecho-sounder which can resolve individual sprats and herrings. Neither of these counters has yet been incorporated in a fully-developed counting equipment attached to a computer. The problems that remain are: (a) to determine the nature of the variance of received signals in time; (b) the treatment of received signals in the counting equipment to count and size all the fish. When these problems have been overcome, the resulting equipment will be invaluable in the study of such aspects of fish population dynamics as volume distribution of fish, size distribution of fish and the ratio of fish catches to total fish population in any area. Burfordg and Johnston9 showed the results of ocean tests on the Marquardt Doppler sonar navigation system that has overcome the problems of systems accuracy and heightabove-bottom limitations that have troubled previous sonar navigation systems. The new pulsed Doppler system uses two receiving and two transmitting transducers-a system borrowed from radar navigation. The operational frequency is 3OOkHz,which was chosen as a compromise between the transducer size and the height-above-bottom requirement. The new system may be attached to the bottoms of large tankers for navigation; it may also be used in deep-submergence applications and for geophysical surveys, when it is used in conjunction with a could velocimeter. In severe sea states, in which aeration occurs, the system was found to be inoperative, but this difficulty has been overcome by simply lowering the transducer array lo-12in below the hull of the tanker.

25-27

1968

September

1968,

Conference proceedings will be published by BPS Exhibitions Ltd, 6 London Street, London W2 A. H. Crawford

Brooke, J. P. N. and Mason, C. S. (Atlantic Oceanographic Laboratory, Canada) ‘Some instruments for monitoring the performance of undersea mechanical devices’ Hutchins, R. W. (Huntec Ltd, Canada) ‘Broadband electroacoustic sources for high-resolution sub-bottom profiling’ Kroebel, W. (University of Kiel, Germany) ‘On the problem of absolute sound-velocity measurements in seawater ’ Hearn, P. J. (White Fish Authority, UK) and Berktray, H. 0. (University of Birmingham, UK) ‘Underwater telemetry for fisheries applications-a review’ Burrows, D. W. (Ball Brothers Research Corporation, USA) ‘Cableless underwater television link design and test results’ Heuter, T. F. and Lagoe, J. A. (Honeywell Inc, USA) ‘Acoustic command telemetry for deep offshore installations’ Creasey, D. J. (University of Birmingham, UK) ‘Sonar systems in the microelectronic era’ Cushing, D. H. (Fisheries counting’

Laboratory, UK) ‘Acoustic fish

Burford, W. H., Jr and Johnston, J. M. (The Marquardt Corporation, USA) ‘Development and testing of Doppler sonar navigation systems’

New York, USA

IEEE Symposium

on sonics

and ultrasonics

During the three days of the symposium, 108 papers were presented and the total attendance was about 350. The 14 Sessions covered: resonators; surface-wave phenomena and devices; signal processing; scattering and measurement techniques; ultrasonics in metals, liquids and gases; attenuation; holography and visualisation; transducer evaluation; medical ultrasonics; material testing and processing; delay lines and transducers; the acoustoelectric effect; and magnetic materials and phase transitions. Staff of Bell Telephone Laboratories alone contributed 14 papers- which says much for the vol; ume of fundamental work going on there. There were very few contributions from overseas. Invited papers, of which there were about 2 per session, were allotted 30min each. Because of the very large programme, the contributed papers, about 6 or 7 per session, were usually restricted to 15 minutes each. Many contributors handled their time allocation well; but many tried to cover too much ground in too much detail. In most cases, authors were addressing their remarks to relatively few specialists in their particular area, even though their work could have been made to appeal to a wider audience if their approach had been different. It seems that the only satisfactory way to handle large, highly-technical meetings of this sort is to ask authors to submit a text of their contribution, which is printed and circulated in advance. Then the speaker need only emphasise the highlights, leaving plenty of time for discussion. At present, only a very brief abstract is available to members. Some of the sessions, those covering: acoustic holography; ultrasonic processing; acoustic cavitation; and transducer

evaluation, are likely to be of special interest to readers of ULTRASONICS, and are reviewed below. The authors of some of these papers, as well as some from other sessions, will be invited to submit detailed summaries for publication. Acoustic Holography There are two main directions in which acoustic holography is being developed. In liquid surface holography the acoustic beam, intercepted by the object, is directed to the liauid surface, where it interferes with a reference beam. This forms a ripple-pattern, which is scanned from above with coherent light in the usual way. In practical systems, the liquid in which the surface pattern is formed differs from the transmitting medium. It is contained in a separate shallow vessel coupled to the transmitting liquid through a thin diaphragm. This arrangement minimizes acoustic streaming which would otherwise interfere with the record. The physical properties of the display liquid (for example, surface tension) may be chosen so as to optimise the oresentation. The other technique, scanned holography, is analogous to optical holography; but in the acoustical case, improved resolution and image size can be obtained by using simultaneous scanning, in which the transmitter and receiver are located at the same point and scanned together. An interesting paper comparing these methods, was presented by R. B. Smith and B. B. Brendenl. Thev concluded that while liquid surface holography is fulfilling some of its potentialities, it is not yet fully practical. Simultaneous scanned holography is more promising, but is limited by the permissible scanning rates. ULTRASONICS April 1969

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A. F. Metherel12 discovered that a useful reconstruction can be obtained by retaining only the phase information in the waves scattered by the object, the spatial amplitude variations being ignored; this simplifies the processing. A papers by members of the Department of Mechanical Engineering University of California reported a computer study of this technique. These holograms are of interest if only details of the shape, and especially the sharp edges, of the object are required, as distinct from internal and other details. Metherell also points out4 that in an acoustic hologram, the need for any reference beam can theoretically be avoided if the forming wave potential is sampled instantaneously within one cycle, instead of using the normal time-averaged hologram. In this case, the record is made by reference to the time of the measurement, with no need for a complete reference beam. E. Marom5 in a critical review, stressed the difficulties caused by the large wave-length change inherent in the transfer of spatial modulation due to the sould field into a visible light field. He also discussed the limitations of image distortion, resolution and depth and field of view. Of course, many of the difficulties of conventional imaging carry over into holography. Ultrasonic Processing Processing seems to be the one important division of applied ultrasound which has not been covered satisfactorily at IEEE meetings. It is hoped to remedy this at the 1969 Symposium, to be held at St Louis, Missouri, September 1969, where a full session on Sonic Processing will be arranged (Organisers: Dr C. Jones and W. L. Nvbora, Dent of Phvsics. Universitv of Vermont, Burlington, Vermont, USA). At the present meeting there were just three contributed papers in this area, all in the field of metallurgy. It is well-known that high intensity ultrasound can be used to promote flow of metal melts, improve grain structure and get rid of gas inclusions. But the method becomes hopelessly uneconomical when applied to large masses of metal. A paper from the Waldemar Medical Research Foundations reported recent application of this technique to very small castings, as used in dentistry and jewellery technology. In these cases, the ultrasonic method is convenient and economic to apply. Only a short pulse of ultrasonic energy (a few seconds) is needed. The method has some advantages over conventional centrifugal casting, For one thing, the grain size is reduced and dendritic growth eliminated. Interesting results on the effect of intense ultrasonics at 39kHz on deformation kinetics of aluminium were reported7 by J. D. Wood and J. C. Remley of Lehigh University. These workers used an equipment similar to the ultrasonic fatigue testers previously described in the literature, with the resonant specimen placed under compression-bias. Ultrasonic vibration had no effect on the yield stress, but caused a large increase in the deformation from a given applied stress in the inelastic and plastic regions, caused clearly, by activation of anchored dislocations, as also previously reported by Langenecker. The hardness of the metal is appreciably increased for a given annealing time if the anneal is done under vibration. High intensity treatment at low ultrasonic frequencies increases the corrosion rate of steels immersed in salt solutions. Recent measurements reported by H. V. Fairbanks* at West Virginia University confirmed this and also showed that insonation shifts the electrode potential of the corroding steel towards the region of anodic protection. Another sigriificant result of the work was that when heated specimens of the steel were vibrated in air, or irradiated through the air, a thinner and more compact oxide film was produced than when vibration was absent. This implies an increased resistance to corrosion for this condition of treatment.

Vermont. Bubbles can be held and examined even when vibrating violently as stable cavities. Effects caused by the motion of single bubbles are often similar to those reported from uncontrolled transient cavitation, but initiate at lower thresholds. W. L.Nyborg’s paper9 included an account of recent measurements and calculations on the effect of vibration on gas flow from a capillary into a liquid. Bubbles have also been held in holes cut into the vibrator surface. A useful tabulation gave the measured thresholds for some of the well-known effects of cavitation which can be initiated from a single trapped bubble. T. K. Saksena’s paperlo described rather similar work in which streams of bubbles of controlled size were generated in free suspension in viscous liquids by vibrating a fine capillary jet through which the gas was forced. Sonoluminescence was detected in glycerolwater mixtures at excitation levels well below the transient cavitation threshold. There is also evidence that sonochemical effects (free radical production) originates from this stable bubble activity, which appears to be a radial pulsation, not associated with surface activity. Transducer evaluation This turned out to be one of the most interesting of all the sessions. There were three short papers, followed by comments by other experts and a general discussion. Future meetings could, with advantage, include more than one session of this sort. This one resolved itself into a discussion of the value of electromechanical coupling coefficients in assessing transducer performance. Electromechanical coupling may be defined as: the ratio of the mutual elastic-dielectric energy density to the geometric mean of the elastic and dielectric self-energy densities (The terms used in this discussion are those for dielectric-type transducers (piezoelectric and electrostrictive). The statements apply equally to magnetic-field transducers, with appropriate naming of terms.). (An analogy can be seen here with the definition of coupling between purely elastic or purely electric elements). This is the so-called physical definition, and defines a property of the transducer material itself. It leads to the expressions: k2 = (sE - @)/se or (@J - ES)/@, where the symbols have their usual meanings. (That is, the fractional decrease in stiffness in passing from constant-field to constant-charge conditions, or, identically, the fractional decrease in permittivity in passing from constant-stress to constant-strain conditions.). The alternative definition of k2 as the fraction of stored dielectric energy that can be extracted as elastic energy (or, identically, as the fraction of stored elastic energy extractable as dielectric energy) is called the technical definition. This does not necessarily simply define a constant of the material. The two definitions are identical at frequencies well below any resonances; but at r’esonances, where the geometry of the vibrator and the energy-storage qualities of any added electrical or elastic elements must be considered, they may be very different. Then the measured coupling is usually called the effective coupling and a calculation is needed to derive the material coupling. Coupling has traditionally been recognised as a most useful parameter in assessing transducer performance. It is directly involved in estimating bandwidth, power utilisation and potential efficiency. But now an important question has arisen with the development of transducers that can never be considered as free entities: thin-film and fired-on transducers that can only be examined in association with the substrate on which the material has been deposited. Since the substrate stores elastic energy, the usual method of arriving at the material coupling, by measuring resonant and antiresonant frequencies, cannot be used. (ka = (f$ - f;)/fi

= 2(f, - fr)/fa

for the breathing modes (radial and spherical modes) Acoustic cavitation Important work has been going on in the borderline area between physical acoustics and liquid processing. Methods of generating gas bubbles of controlled sizes in liquids and holding them stably in a sound field have been extensively studied at Professor Nyborg’s laboratory at University of 134

ULTRASONICS April 1969

=?fLcot’fr=%

(f;-f;),f;_.ii” 4 (fa - fr)/fa 2fa 8 2 fa for the length-expander and thickness modes, with more complex expressions for other modes.) But the effective coupling can, of course, be obtained in this way, or from measurements taken from the admittance or impedance loops. k2

A. J. Bahrrr dealt with this problem of measuring coupling for thin-film transducers. The main reason why it is

’ Fairbanks, H. V. (West Virginia University, Morgantown, West Virginia, USA) ‘Influence of ultrasonics on the corrosion of steel’ Nyborg, W. L. and Rooney, J. A. (Physics Department, University of Vermont, Burlington, Vermont, USA) ‘Considerations in design of cavitation fields’ Saksena, T. K. (Department of Physics, University of Vermont, Burlington, Vermont, USA) ‘Low-level excitation of sonoluminescence in liquids of high viscosity containing air bubbles’ Bahr, A. J. (Stanford Research Institute, Menlo Park, California, USA) ‘Electrical measurement techniques for obtaining the electromechanical coupling factors of thinfilm piezoelectric transducers’ (See Ultrasonics Vol 6, No 4 (October 1968) p 208-209) Tiersten, H. F. (Bell Telephone Laboratories, Inc, Murray Hill, New Jersey, USA) ‘The relation of electromechanical coupling factors to the fundamental material constants for thickness vibrating piezoelectric plates’ Meeker, T. R. (Bell Telephone Laboratories, Inc, Murray Hill, New Jersey, USA) ‘Methods of evaluating piezoelectric transducers’ Sittig, E. K. and Berlincourt, D. -Contributions discussion

to the

Proceedings of the IRE,Vol 3’7 (1949) p 1378;Vol 45 (1957) p 354;VoI 46 (1958) p 765;Vol 49 (1961) p 1162

ULTRASONICS April 1969

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