- Email: [email protected]
78th Meeting of the Acoustical Society of America
Toronto, Canada, 6-8 October 1969
International
Electronics Conference
The Conference, sponsored by the Canadian Region of the Institute of Electrical and Electronics Engineers, is a regular event taking place every second year. The Conference is combined with an Exposition of electronic equipment and is the largest conference in Canada dealing with electronics, with the total registration of well over 10,000. The technical program comprises over 100 papers treating a wide variety of electronics subjects. At the 1967 conference, a session on Ultrasonics was introduced with five papers: ‘UltrasonicsGeneral Review’, T. S. Hutchison, RMC, ‘Recent developments in piezoelectric transducer materials,’ S. R. Burlage, Clevite Corp, ‘Instrumentation for Ultrasonic studies in solids.’ 0. M. M. Mitchell. Bell Telenhone Laboratories. ?Laser modulation by acoustic waves,’ A. J. De Maria, United Aircraft Research Laboratories and Ultrasonics and the physics of normal and superconducting metals,’ J. M. Perz, University of Toronto. A session of five review papers dealing with ultrasonics was included in the 1989 program. Two papers were on high intensity sound and ultrasound. The first paper, by A. J. Last, Ontario Research Foundation, dealt with a variety of high intensity sources operating in gases and in liquids, some of them developed by the author and his group. Large whistles up to 1OOkwof acoustic energy, pulsating combustion sources and electro-acoustic horns were described in detail. Of sources operating in liquids, focused beam generators deserve special attention. They make it possible to generate very high intensities at some distance from the radiating surface. Professor R. C. McMaster, who is a well known authority on applications of high intensity ultrasound, gave an amazing array of examples in various fields of engineering and industry: civil, ceramics and metallurgical .engineering in particular. In metal working operations the ultrasonically assisted metal deformation is of great interest and is under intensive investigation. Incidentally, at the first International Symposium on high intensity ultrasonics to be held in Graz, Austria, the emphasis will be on forming and processing of metals and plastics. Professor McMaster is also known to everybody using ultrasonics as a nondestructive technique, as the editor of the classic two volume set of books on Nondestructive Testing, American Society for Nondestructive Testing (1959) and we hear the second edition of this work is in preparation.
Ultrasonic inspection methods were the subject of the paper Material Evaluation by Ultrasonics by G. D. Martens, Automation Industries Inc, Sperry Division. After dealing with inspection concepts, including the delta configuration, the advances in the equipment were described. Gating, ultrasonic multiplexing, multi-channel instrumentation and C-Scan recording were explained. Ultrasonic Holography was the subject of the paper by E. Marom, Bendix Research Laboratories. Dr Marom, who is well known for his contribution in acoustic# holography, first outlined the theory of light and acoustical holography and then discussed various detection configurations used in ultrasonic holography in particular. Acoustic holograms have been generated by point-by-point scanning of a detector in both air and water. Fast hologram generators can be obtained by either electronic or light scanning, utilizing a TV monitor to display the holograms. The possibility of real-time spatial modulation of a laser beam was discusses.. Finally, an underwater imaging system based on holographic principles was described. Numerous holograms mainly from the authors laboratory were shown. The last paper dealt with Physical Measurements with Lasers-Ultrasonics by A. A. Gundjian and S. Sizgoric, McGill University, Montreal. The paper gave a review of the different experimental methods utilizing the interaction of laser radiation with ultrasonic structure in order to obtain information about the acoustic and electro-mechanical properties of materials. Methods for determining the velocity and attenuation of ultrasonic and hypersonic waves in transparent materials were described. Laser beam can be used also in the study of the propagation characteristics of ultrasonic surface waves. Further, measurements of small mechanical deformations can be made either by modulation of the laser frequency by perturbation of the resonant cavity length, or by the use of an external Michelson-type interferometer where mechanical deformations modulate in amplitude the resulting, superposted laser beam. Two page abstracts of papers are available in the PreConference Digest (IEEE Cat 89C40-Reg 7). W. J. Bratina
San Diego, California, 4-7 November 1969
78th Meeting of the Acoustical Society of America
An attendance .__ of 1012 gathered at San Diego to hear a record number of 427 papers including special sessions on interaction of light with sound and urban noise. Probably the session of greatest interest to readers of Ultrasonics was one containing a group of five papers on ultrasonic cavitation. CAVITATION Robert E. Apfel of the Acoustics Research Laboratory, Harvard discussed the role of vapour-cavity dynamics in cavitation threshold determinations (Paper 6Ml). Following his work on nucleation of cavities from cracks on motes (solid impurities) Apfel has now shown by numerical solutions of a nondimensional equation that a small spherical cavity will only grow in a few cycles to many times its initial radius if a certain threshold value of acoustic pressure amplitude is exceeded. Viscosity as well as surface tension is shown to play an important role. 126
ULTRASONICS April 1970
H. Scott Fogler of the University of Michigan (Paper 6M2) presented some studies of the stability and collapse of spherical cavities in visco-elastic fluids. Taking a linear rheological model for the liquid, a non-linear integro-differential equation of motion for the cavity was obtained. Analytical solutions were derived for limiting values of the parameters, and numerical results for other values. One of the more interesting results of this work is the finding that elasticity in the liquid can significantly retard the collapse of a void and produce prolonged oscillatory motion whenever the relaxation time is moderately large in comparison with the Rayleigh collapse time. F. A. Angona of Mobil Research and Development Corporation described measurements (Paper 6M3) of the threshold of cavitation in water at ambient pressures from 1 to 15atm. A 3OkHz beam was focused by an aluminum parabolic reflector to a small volume in the interior of the pressure chamber, thus minimizing wall effects and erosion damage. The
transducer was driven by a 1Okw amplifier. The system was calibrated with a hydrophone under non-cavitating conditions. The distilled water sample was first degassed by energizing at low level under vacuum and then pressurised with a water pump. The cavitation was observed to occur on a target surface of aluminum or rock wafers. The threshold was measured with decreasing air saturation (loo’& and 353 saturation and degassed). The first two values agreed only qualitatively with the Hsieh-Plesset rectified diffusion theory.1 The threshold for degassed water agreed with Akulichev’s theory2 for degassed water with nucleation on a solid nucleus of 2 x 10-a mm. Even with degassed water at the highest static pressure applied, in that case lOatm, threshold was lower than ambient pressure. A. Masse, M. L. Chu, Jr and R. D. Finch of the University of Houstonc (Paper 6M4) showed movies of cavitation in superfluid helium. The cavitation was generated by a PZT4 cylindrical transducer at 52kHz. The bubbles were found to occur in the form of streaks or streamers which persisted from frame to frame. The streamers were frequently paired, apparently having a common point of origin. The authors were unable to tell if the mechanism of streamer formation in normal liquids was applicable or whether the bubbles originated on vortex filaments. In a further paper from the University of Houston, J.R. Shadley and R. D. Finch (Paper 6M5) discussed nucleation of cavitation in liquid helium by alpha-particle irradiation. They reported that the presence of a 1mCi Pozlo source, an alpha emitter, caused a lowering of the cavitation noise threshold in helium II by as much as 50”& the sound field being generated by a cylindrical PZT4 transducer resonant at 57kHz. The lowest temperature investigated was 1.80% Thresholds corresponding to various specific sound levels increments were determined from graphic records. There was no discernable effect on the threshold in helium I, so that the nucleation mechanism cannot be the formation of micro-bubbles at hot spots as in Seitz’s theory.3 Neither the presence of a 13 mCi tritiated titanium beta source nor electric fields affected the nucleation, so that ions, per se, are also ruled out as a nucleating mechanism. However, several similarities to behaviour of the threshold in the presence of a rotating shaft and bucket were observed. Evidence was obtained indicating that the cavitation noise was localized in the vicinity of the alpha source. It was concluded that the nucleation occurs on vortex filaments created by the alpha particles. R. D. Finch
REFERENCES 1
Hsieh, D-Y and Plesset, M. S. Journal of the Acoustical Society of America, Vol 33 (1961), p 206.
2
Akulichev, V. A., Soviet Physics Acoustics, Vol 11 (1965), p 15.
3
Seitz, F., Physics of Fluids, Vol 1 (1958), p 2.
OTHER SESSIONS There were also papers involving ultrasonics in some of the other sessions. The latest advances in the techniques for observing Brillouin scattering were described. A. Monkewicz reported on a sensitive apparatus for observing Brillouin scattering in gases. A Bendix photomultiplier giving 20 counts per minute dark current, a Fabry-Perot interferometer with a bandwidth of ~OMHZ,and a single mode helium neon laser were used. This apparatus was used to study (by varying the pressure and hence the mean free path) the transition from the kinetic to the hydrodynamic point of view. A clever technique for utilising a multimode laser was described by R. Mohr et al employing a Fabry-Perot etalon whose length was the same as the cheap multimode laser used. Thi% made all the modes degenerate at the detector and gave an effective line width of 2MHz. They used this equipment to observe the Brillouin scattering from SF, near its critical point for angles of 166” and 45”.
Sound velocities deduced from these observations were compared with existing ultrasonic data and shown to be consistent with a single temperature dependent relaxation frequency. Several authors extended the theoretical treatment of sound radiation from plates with H. Davies describing the radiation from a simply supported square plate, into a fluid filled halfspace. The analysis was based on the in-vacua modes of the plate, and the modal coupling coefficients were evaluated below the acoustic critical frequency (where only the corners of the plate radiate). He said that coupling is due to both fluid inertia and radiation damping effects and depends on the widths of the modal resonance peaks and the frequency spacing of the resonances. He finished by showing graphs of the computed spectral density of the radiated acoustic power. D. Feit and H. Saureman were less general in their treatment and gave an expression for the sound radiated from a beam stiffened thin plate, to a point in a liquid. This expression was then used as a means of finding the inter-frame resonances and graphs were shown of the radiated sound pressure with the plates stiffened with one, two and three stiffening beams. The propagation characteristics of Rayleigh waves on LiNb, were described by Lean et al. They used a probing light beam to study the surface deformation which in this case was the only factor contributing to the deformation of the reflected beam. The transmitted beam is diffracted by index of refraction variation also and these are related to the particle displacements of the propagating Rayleigh wave. A graph of attenuation increasing with frequency was shown. Rayleigh waves were also the topic of a paper by D. E. Bray and R. D. Finch who gave the arrival times for Rayleigh, asymmetrical Lamb and symmetrical Lamb waves for different positions of the 4mm diameter receiver around the rim of a l/4 scale model of a railway wheel. The asymmetrical Lamb waves arrived later than predicted by the theory. They pointed out that the pulses propagating around the interior of the rim were similar to modes observed in a rectangular bar. Sound radiation from a collimated source of large amplitude was described by R. H. Mellen and D. G. Browning who pointed out that there were three regions of sound from such a source: The near-field plane-wave region, the intermediate region where secondary-wave divergence balances its growth and the farfield where the collimated volume tends to act as an end fire radiator for the secondary radiation. Sound vibration visualisation techniques were much in evidence at the conference, and Willard Wells described an acoustic holographic system using N linearly arranged receivers listening simultaneously to N transmitters arranged linearly at right angles to them sending in rapid sequence the virtual hologram obtained contains three aberrations: an image field rotation proportional to pulse rate, a distortion with uncorrelated optical reconstruction and a diffraction effect at the corners with sides parallel to the linear arrays. Acoustic speckle patterns were apparent. Holography of a different type was used by A. Allippi who showed how absorbtion coefficient measurements for ultrasonic waves could be
LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY Ultrasonics and Signal Processing A research fellow is required to work on the application of ultrasonics and signal processing to the detection of cracks in rails. Candidates should have had several years relevant experience either in industry or at auniversity. Salary in the region of f&500 depending on experience and qualifications. Applications shouId be addressed to Professor J.W.R. Griffiths,Department of Electronic and Electrical Engineering, Loughborough, Leicestershire
ULTRASONICS April 19’70
127
derived from the spectrum of the holographically reconstructed light diffracted from an ultrasonic pulse train and from the light reflected in the opposite direction and partially absorbed. His method does not require any translational movement of the light beam through the ultrasonic field. The vibration of the tympanic membranes of cats were studied using a time averaging holographic technique by J. Tonndorf and S. Klnuma who said that the development of a silent shutter and finding a suitable substance for coating the membranes had been the greatest problem in their work. Ethyl bronze powder was finally chosen and they were then able to show that the eardrum vibrates at l/3 of the amplitude of other parts of the membrane where it joins the malleus. An inexpensive system for stroboscopic holographic vibration analysis was described by P. Fryer who gave emphasis to the treatment given to the object and photographic plates made necessary in order to avoid light wastage. He described a stroboscopic system using a synchronous motor and a strobe disc. A technique for mapping an ultrasonic sound field that may ultimately be used to take ultrasonic holograms was described by B. D. Cook and R. Werchan. They used liquid crystals sprayed onto a thin plastic sheet. On absorbing a small fraction of the ultrasonic field, colours are seen corresponding to various temperatures between solid and liquid state. The process is at the moment not relatively sensitive and resolution is limited by the thermal properties of the substrate used. Another related technique was discussed by Landry
et al who showed their celebrated slide of a tropical fish reconstructed from light Bragg-diffracted from ultrasound that had passed through the fish. The light beam that they used was cylindrically convergent and the vertical and horizontal resolutions of the system were different. Theoretical expressions for these were given. A theoretical treatment which may be useful for surface deformation ultrasonic holography was described by T. J. Bander and B. P. Hilderbrand. They gave a partial dlfferential equation describing the motion of a liquid surface impinged upon by two ultrasonic waves at equal but opposite angles. The solution was separated into a ripple displacement superimposed onto a uniform displacement both of which oscillate. They showed graphs of ripple height of oscillation versus pulse width for different frequencies. Several papers were presented covering aspects of sound propagation underwater. Shock waves created by exploding 31b charges of TNT at 60, 500 and 1000 feet under water were investigated by L. C. Maples and W. H. Thorpe. They picked up the shock waves with a hydrophone suspended at 4200ft below the ship and one bottom moored at 1400ft. The wave trains were sampled at 3OkHz rate and edited trains were desampled onto analogue tape for analysis with l/3 octave filters. A graph of the energy flux density decreasing with frequency was shown. The conference ended with several interesting papers on the intelligibility of speech and direction finding using sound underwater. P. Fryer
Basle, Switzerland, 7 February 1970
Swiss Society of Diagnostic Ultrasound The annual meeting was held at the GynaecologicalObstetrical Clinic of the University of Basle, and was admirably organized by Dr Hinselmann. Professor Kaeser, Director of the Clinic, stressed in his welcome address the importance of diagnostic ultrasonic methods in obstetrics and gynaecology. It was also pointed out that the increasing use of long-term Doppler monitoring makes it desirable to do more research concerning possible biological interactions of low energy ultrasound. A large part of the scientific session was devoted to physical bases of medical ultrasonics. Professor Filipczynski of Warschaw gave an excellent lecture on the measurement of small ultrasonic intensities. Both electrostatic and electrodynamic transducers were developed by the author for this purpose. The output of different sonographic equipment being used in Poland, averaged over the beam cross-section, was demonstrated to be from 1.4 to 6.5 x 102Wmm-2 (peak) and from lO-7OmW rnrnez (time averaged); the overall sensitivity of diagnostic equipment, expressed in dB, is the next important parameter both for equipment standardization and clinical application. The author showed that it is necessary to measure the output of commercial diagnostic equipment, and then to limit the peak and average ultrasonic intensities, although there is no clear evidence that the intensity levels being used at present could be hazardous. Dr Niklas of Cologne then read a paper on the sound field of ultrasonic probes. After having treated the physics of the near and far fields with a demonstration of most instructive Schlieren-pictures, he mentioned that from the form and behaviour of the field some rules for the selection of the optimal probe diameter can be found. The most important
128
ULTRASONICS April 1970
are: to find small reflectors (small compared with the probe diameter); the near zone length should be equal to the distance of this reflector; and to find large, plain, or nearly plain reflectors in the neighbourhood of disturbing small reflectors; the near zone length should be shorter or longer than the distance to the large reflector so that this reflector lies in the widely opened part of the sound field. After a communication by Dr Blauenstein of Basle concerning some types of axial sonographical artefacts, Dr Hinselmann read a most interesting paper on obstetrical ultrasonics. Within a half a year this author did 1500 lnvestigations on approximately 500 patients. The application of both A- and B-scan techniques allowed him not only to follow the growth of biparietal diameter, but also to clearly demonstrate in early pregnancy the posterior wall and the low segment of the uterus. Thus, misinterpretations, including those of localization of the placenta, could largely be avoided. In his own contribution, the writer was dealing with the use of acoustical windows of the skull for sonography. Craniolomies can be utilized for intraoperative sonography in tumor surgery. Cranioplastics of material with low sound attenuation allow one to demonstrate details of intracranial structures. Through a frontal bone defect the author clearly demonstrated the foramen of Monroe. Examples confirming the value of the orbit as an acoustical window for echoencephalography were given. Further discussion of the proposed foundation of a European Federation of Medical Ultrasound Societies was conducted during the business meeting which followed. H. R. Mtiller
International
Electronics Conference
The Conference, sponsored by the Canadian Region of the Institute of Electrical and Electronics Engineers, is a regular event taking place every second year. The Conference is combined with an Exposition of electronic equipment and is the largest conference in Canada dealing with electronics, with the total registration of well over 10,000. The technical program comprises over 100 papers treating a wide variety of electronics subjects. At the 1967 conference, a session on Ultrasonics was introduced with five papers: ‘UltrasonicsGeneral Review’, T. S. Hutchison, RMC, ‘Recent developments in piezoelectric transducer materials,’ S. R. Burlage, Clevite Corp, ‘Instrumentation for Ultrasonic studies in solids.’ 0. M. M. Mitchell. Bell Telenhone Laboratories. ?Laser modulation by acoustic waves,’ A. J. De Maria, United Aircraft Research Laboratories and Ultrasonics and the physics of normal and superconducting metals,’ J. M. Perz, University of Toronto. A session of five review papers dealing with ultrasonics was included in the 1989 program. Two papers were on high intensity sound and ultrasound. The first paper, by A. J. Last, Ontario Research Foundation, dealt with a variety of high intensity sources operating in gases and in liquids, some of them developed by the author and his group. Large whistles up to 1OOkwof acoustic energy, pulsating combustion sources and electro-acoustic horns were described in detail. Of sources operating in liquids, focused beam generators deserve special attention. They make it possible to generate very high intensities at some distance from the radiating surface. Professor R. C. McMaster, who is a well known authority on applications of high intensity ultrasound, gave an amazing array of examples in various fields of engineering and industry: civil, ceramics and metallurgical .engineering in particular. In metal working operations the ultrasonically assisted metal deformation is of great interest and is under intensive investigation. Incidentally, at the first International Symposium on high intensity ultrasonics to be held in Graz, Austria, the emphasis will be on forming and processing of metals and plastics. Professor McMaster is also known to everybody using ultrasonics as a nondestructive technique, as the editor of the classic two volume set of books on Nondestructive Testing, American Society for Nondestructive Testing (1959) and we hear the second edition of this work is in preparation.
Ultrasonic inspection methods were the subject of the paper Material Evaluation by Ultrasonics by G. D. Martens, Automation Industries Inc, Sperry Division. After dealing with inspection concepts, including the delta configuration, the advances in the equipment were described. Gating, ultrasonic multiplexing, multi-channel instrumentation and C-Scan recording were explained. Ultrasonic Holography was the subject of the paper by E. Marom, Bendix Research Laboratories. Dr Marom, who is well known for his contribution in acoustic# holography, first outlined the theory of light and acoustical holography and then discussed various detection configurations used in ultrasonic holography in particular. Acoustic holograms have been generated by point-by-point scanning of a detector in both air and water. Fast hologram generators can be obtained by either electronic or light scanning, utilizing a TV monitor to display the holograms. The possibility of real-time spatial modulation of a laser beam was discusses.. Finally, an underwater imaging system based on holographic principles was described. Numerous holograms mainly from the authors laboratory were shown. The last paper dealt with Physical Measurements with Lasers-Ultrasonics by A. A. Gundjian and S. Sizgoric, McGill University, Montreal. The paper gave a review of the different experimental methods utilizing the interaction of laser radiation with ultrasonic structure in order to obtain information about the acoustic and electro-mechanical properties of materials. Methods for determining the velocity and attenuation of ultrasonic and hypersonic waves in transparent materials were described. Laser beam can be used also in the study of the propagation characteristics of ultrasonic surface waves. Further, measurements of small mechanical deformations can be made either by modulation of the laser frequency by perturbation of the resonant cavity length, or by the use of an external Michelson-type interferometer where mechanical deformations modulate in amplitude the resulting, superposted laser beam. Two page abstracts of papers are available in the PreConference Digest (IEEE Cat 89C40-Reg 7). W. J. Bratina
San Diego, California, 4-7 November 1969
78th Meeting of the Acoustical Society of America
An attendance .__ of 1012 gathered at San Diego to hear a record number of 427 papers including special sessions on interaction of light with sound and urban noise. Probably the session of greatest interest to readers of Ultrasonics was one containing a group of five papers on ultrasonic cavitation. CAVITATION Robert E. Apfel of the Acoustics Research Laboratory, Harvard discussed the role of vapour-cavity dynamics in cavitation threshold determinations (Paper 6Ml). Following his work on nucleation of cavities from cracks on motes (solid impurities) Apfel has now shown by numerical solutions of a nondimensional equation that a small spherical cavity will only grow in a few cycles to many times its initial radius if a certain threshold value of acoustic pressure amplitude is exceeded. Viscosity as well as surface tension is shown to play an important role. 126
ULTRASONICS April 1970
H. Scott Fogler of the University of Michigan (Paper 6M2) presented some studies of the stability and collapse of spherical cavities in visco-elastic fluids. Taking a linear rheological model for the liquid, a non-linear integro-differential equation of motion for the cavity was obtained. Analytical solutions were derived for limiting values of the parameters, and numerical results for other values. One of the more interesting results of this work is the finding that elasticity in the liquid can significantly retard the collapse of a void and produce prolonged oscillatory motion whenever the relaxation time is moderately large in comparison with the Rayleigh collapse time. F. A. Angona of Mobil Research and Development Corporation described measurements (Paper 6M3) of the threshold of cavitation in water at ambient pressures from 1 to 15atm. A 3OkHz beam was focused by an aluminum parabolic reflector to a small volume in the interior of the pressure chamber, thus minimizing wall effects and erosion damage. The
transducer was driven by a 1Okw amplifier. The system was calibrated with a hydrophone under non-cavitating conditions. The distilled water sample was first degassed by energizing at low level under vacuum and then pressurised with a water pump. The cavitation was observed to occur on a target surface of aluminum or rock wafers. The threshold was measured with decreasing air saturation (loo’& and 353 saturation and degassed). The first two values agreed only qualitatively with the Hsieh-Plesset rectified diffusion theory.1 The threshold for degassed water agreed with Akulichev’s theory2 for degassed water with nucleation on a solid nucleus of 2 x 10-a mm. Even with degassed water at the highest static pressure applied, in that case lOatm, threshold was lower than ambient pressure. A. Masse, M. L. Chu, Jr and R. D. Finch of the University of Houstonc (Paper 6M4) showed movies of cavitation in superfluid helium. The cavitation was generated by a PZT4 cylindrical transducer at 52kHz. The bubbles were found to occur in the form of streaks or streamers which persisted from frame to frame. The streamers were frequently paired, apparently having a common point of origin. The authors were unable to tell if the mechanism of streamer formation in normal liquids was applicable or whether the bubbles originated on vortex filaments. In a further paper from the University of Houston, J.R. Shadley and R. D. Finch (Paper 6M5) discussed nucleation of cavitation in liquid helium by alpha-particle irradiation. They reported that the presence of a 1mCi Pozlo source, an alpha emitter, caused a lowering of the cavitation noise threshold in helium II by as much as 50”& the sound field being generated by a cylindrical PZT4 transducer resonant at 57kHz. The lowest temperature investigated was 1.80% Thresholds corresponding to various specific sound levels increments were determined from graphic records. There was no discernable effect on the threshold in helium I, so that the nucleation mechanism cannot be the formation of micro-bubbles at hot spots as in Seitz’s theory.3 Neither the presence of a 13 mCi tritiated titanium beta source nor electric fields affected the nucleation, so that ions, per se, are also ruled out as a nucleating mechanism. However, several similarities to behaviour of the threshold in the presence of a rotating shaft and bucket were observed. Evidence was obtained indicating that the cavitation noise was localized in the vicinity of the alpha source. It was concluded that the nucleation occurs on vortex filaments created by the alpha particles. R. D. Finch
REFERENCES 1
Hsieh, D-Y and Plesset, M. S. Journal of the Acoustical Society of America, Vol 33 (1961), p 206.
2
Akulichev, V. A., Soviet Physics Acoustics, Vol 11 (1965), p 15.
3
Seitz, F., Physics of Fluids, Vol 1 (1958), p 2.
OTHER SESSIONS There were also papers involving ultrasonics in some of the other sessions. The latest advances in the techniques for observing Brillouin scattering were described. A. Monkewicz reported on a sensitive apparatus for observing Brillouin scattering in gases. A Bendix photomultiplier giving 20 counts per minute dark current, a Fabry-Perot interferometer with a bandwidth of ~OMHZ,and a single mode helium neon laser were used. This apparatus was used to study (by varying the pressure and hence the mean free path) the transition from the kinetic to the hydrodynamic point of view. A clever technique for utilising a multimode laser was described by R. Mohr et al employing a Fabry-Perot etalon whose length was the same as the cheap multimode laser used. Thi% made all the modes degenerate at the detector and gave an effective line width of 2MHz. They used this equipment to observe the Brillouin scattering from SF, near its critical point for angles of 166” and 45”.
Sound velocities deduced from these observations were compared with existing ultrasonic data and shown to be consistent with a single temperature dependent relaxation frequency. Several authors extended the theoretical treatment of sound radiation from plates with H. Davies describing the radiation from a simply supported square plate, into a fluid filled halfspace. The analysis was based on the in-vacua modes of the plate, and the modal coupling coefficients were evaluated below the acoustic critical frequency (where only the corners of the plate radiate). He said that coupling is due to both fluid inertia and radiation damping effects and depends on the widths of the modal resonance peaks and the frequency spacing of the resonances. He finished by showing graphs of the computed spectral density of the radiated acoustic power. D. Feit and H. Saureman were less general in their treatment and gave an expression for the sound radiated from a beam stiffened thin plate, to a point in a liquid. This expression was then used as a means of finding the inter-frame resonances and graphs were shown of the radiated sound pressure with the plates stiffened with one, two and three stiffening beams. The propagation characteristics of Rayleigh waves on LiNb, were described by Lean et al. They used a probing light beam to study the surface deformation which in this case was the only factor contributing to the deformation of the reflected beam. The transmitted beam is diffracted by index of refraction variation also and these are related to the particle displacements of the propagating Rayleigh wave. A graph of attenuation increasing with frequency was shown. Rayleigh waves were also the topic of a paper by D. E. Bray and R. D. Finch who gave the arrival times for Rayleigh, asymmetrical Lamb and symmetrical Lamb waves for different positions of the 4mm diameter receiver around the rim of a l/4 scale model of a railway wheel. The asymmetrical Lamb waves arrived later than predicted by the theory. They pointed out that the pulses propagating around the interior of the rim were similar to modes observed in a rectangular bar. Sound radiation from a collimated source of large amplitude was described by R. H. Mellen and D. G. Browning who pointed out that there were three regions of sound from such a source: The near-field plane-wave region, the intermediate region where secondary-wave divergence balances its growth and the farfield where the collimated volume tends to act as an end fire radiator for the secondary radiation. Sound vibration visualisation techniques were much in evidence at the conference, and Willard Wells described an acoustic holographic system using N linearly arranged receivers listening simultaneously to N transmitters arranged linearly at right angles to them sending in rapid sequence the virtual hologram obtained contains three aberrations: an image field rotation proportional to pulse rate, a distortion with uncorrelated optical reconstruction and a diffraction effect at the corners with sides parallel to the linear arrays. Acoustic speckle patterns were apparent. Holography of a different type was used by A. Allippi who showed how absorbtion coefficient measurements for ultrasonic waves could be
LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY Ultrasonics and Signal Processing A research fellow is required to work on the application of ultrasonics and signal processing to the detection of cracks in rails. Candidates should have had several years relevant experience either in industry or at auniversity. Salary in the region of f&500 depending on experience and qualifications. Applications shouId be addressed to Professor J.W.R. Griffiths,Department of Electronic and Electrical Engineering, Loughborough, Leicestershire
ULTRASONICS April 19’70
127
derived from the spectrum of the holographically reconstructed light diffracted from an ultrasonic pulse train and from the light reflected in the opposite direction and partially absorbed. His method does not require any translational movement of the light beam through the ultrasonic field. The vibration of the tympanic membranes of cats were studied using a time averaging holographic technique by J. Tonndorf and S. Klnuma who said that the development of a silent shutter and finding a suitable substance for coating the membranes had been the greatest problem in their work. Ethyl bronze powder was finally chosen and they were then able to show that the eardrum vibrates at l/3 of the amplitude of other parts of the membrane where it joins the malleus. An inexpensive system for stroboscopic holographic vibration analysis was described by P. Fryer who gave emphasis to the treatment given to the object and photographic plates made necessary in order to avoid light wastage. He described a stroboscopic system using a synchronous motor and a strobe disc. A technique for mapping an ultrasonic sound field that may ultimately be used to take ultrasonic holograms was described by B. D. Cook and R. Werchan. They used liquid crystals sprayed onto a thin plastic sheet. On absorbing a small fraction of the ultrasonic field, colours are seen corresponding to various temperatures between solid and liquid state. The process is at the moment not relatively sensitive and resolution is limited by the thermal properties of the substrate used. Another related technique was discussed by Landry
et al who showed their celebrated slide of a tropical fish reconstructed from light Bragg-diffracted from ultrasound that had passed through the fish. The light beam that they used was cylindrically convergent and the vertical and horizontal resolutions of the system were different. Theoretical expressions for these were given. A theoretical treatment which may be useful for surface deformation ultrasonic holography was described by T. J. Bander and B. P. Hilderbrand. They gave a partial dlfferential equation describing the motion of a liquid surface impinged upon by two ultrasonic waves at equal but opposite angles. The solution was separated into a ripple displacement superimposed onto a uniform displacement both of which oscillate. They showed graphs of ripple height of oscillation versus pulse width for different frequencies. Several papers were presented covering aspects of sound propagation underwater. Shock waves created by exploding 31b charges of TNT at 60, 500 and 1000 feet under water were investigated by L. C. Maples and W. H. Thorpe. They picked up the shock waves with a hydrophone suspended at 4200ft below the ship and one bottom moored at 1400ft. The wave trains were sampled at 3OkHz rate and edited trains were desampled onto analogue tape for analysis with l/3 octave filters. A graph of the energy flux density decreasing with frequency was shown. The conference ended with several interesting papers on the intelligibility of speech and direction finding using sound underwater. P. Fryer
Basle, Switzerland, 7 February 1970
Swiss Society of Diagnostic Ultrasound The annual meeting was held at the GynaecologicalObstetrical Clinic of the University of Basle, and was admirably organized by Dr Hinselmann. Professor Kaeser, Director of the Clinic, stressed in his welcome address the importance of diagnostic ultrasonic methods in obstetrics and gynaecology. It was also pointed out that the increasing use of long-term Doppler monitoring makes it desirable to do more research concerning possible biological interactions of low energy ultrasound. A large part of the scientific session was devoted to physical bases of medical ultrasonics. Professor Filipczynski of Warschaw gave an excellent lecture on the measurement of small ultrasonic intensities. Both electrostatic and electrodynamic transducers were developed by the author for this purpose. The output of different sonographic equipment being used in Poland, averaged over the beam cross-section, was demonstrated to be from 1.4 to 6.5 x 102Wmm-2 (peak) and from lO-7OmW rnrnez (time averaged); the overall sensitivity of diagnostic equipment, expressed in dB, is the next important parameter both for equipment standardization and clinical application. The author showed that it is necessary to measure the output of commercial diagnostic equipment, and then to limit the peak and average ultrasonic intensities, although there is no clear evidence that the intensity levels being used at present could be hazardous. Dr Niklas of Cologne then read a paper on the sound field of ultrasonic probes. After having treated the physics of the near and far fields with a demonstration of most instructive Schlieren-pictures, he mentioned that from the form and behaviour of the field some rules for the selection of the optimal probe diameter can be found. The most important
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are: to find small reflectors (small compared with the probe diameter); the near zone length should be equal to the distance of this reflector; and to find large, plain, or nearly plain reflectors in the neighbourhood of disturbing small reflectors; the near zone length should be shorter or longer than the distance to the large reflector so that this reflector lies in the widely opened part of the sound field. After a communication by Dr Blauenstein of Basle concerning some types of axial sonographical artefacts, Dr Hinselmann read a most interesting paper on obstetrical ultrasonics. Within a half a year this author did 1500 lnvestigations on approximately 500 patients. The application of both A- and B-scan techniques allowed him not only to follow the growth of biparietal diameter, but also to clearly demonstrate in early pregnancy the posterior wall and the low segment of the uterus. Thus, misinterpretations, including those of localization of the placenta, could largely be avoided. In his own contribution, the writer was dealing with the use of acoustical windows of the skull for sonography. Craniolomies can be utilized for intraoperative sonography in tumor surgery. Cranioplastics of material with low sound attenuation allow one to demonstrate details of intracranial structures. Through a frontal bone defect the author clearly demonstrated the foramen of Monroe. Examples confirming the value of the orbit as an acoustical window for echoencephalography were given. Further discussion of the proposed foundation of a European Federation of Medical Ultrasound Societies was conducted during the business meeting which followed. H. R. Mtiller