- Email: [email protected]
Birmingham, UK, 5–7 April 1971 Spring meeting of the British Acoustical Society
showing two small objects, a pistol and a rifle, resting on a smooth sandy bottom. This represents a very severe test of such equipment: nevertheless, recognition was possible in a number of cases where the direction of illumination was favourable, In general, however, the acoustic image did not agree with its optical counterpart and recognition was further hampered when the object was resting upon a rough bottom. Notwithstanding this, the authors claim that their equipment will be very valuable for the detection of the larger class of object of interest to the police. Perhaps the most interesting development was that described by Dr R. W. G. Haslett and M. Heaton (Kelvin-Hughes Ltd). This makes use of the Lloyd mirror effect in which the direct signal and the one reaching the sea bed via a single surface reflection form an interference fringe pattern superimposed upon the plan view of the bottom. M. Heaton showed how it was possible, by identifying the order of a particular fringe, to deduce the depth of the bottom along that fringe line with an accuracy of +0.3m provided that the wave height on the surface is less than Cl.3m. This technique adds the benefit of rapid contouring to the side-scan surveying process. Unfortunately, this technique breaks down in certain locations where bottom features such as short period
‘sand waves break up the fringe patterns, and the surveyor must then return to conventional echo-sounding for his depth information. The final paper was scheduled to be given by J.D. Mills (Underwater Instrumentation Ltd, and E. G. G. Inc) on ‘Recent American developments’. Unfortunately J. D. Mills was unable to be present and K. Klein (Klein Associates) presented a light-hearted paper in its place, in which he gave an account of his recent exploits in Loch Ness where he spent the Summer of 1970 using equipment developed by his company in the search for the ‘monster’. The most important point to emerge from the papers and from subsequent discussion was that, although many advances have been made in providing corrected trueito-scale records, it is essential, if the present rate of progress is to be maintained, that some means of calibration be incorporated into the sidescan equipment to enable acoustic target strengths to be assigned to the features displayed. Only with this information will it be possible to quantify the data obtained by a side-scan sonar for comparison with data obtained at different times and different frequencies with other equipments. B. K. Gazey
Birmingham, UK, 5-7 April 1971
Spring
Meeting
of the British Acoustical
Society
UNDERWATER ACOUSTICS Non-linear
acoustics
meeting
Although one of the more interesting features of underwater acoustics during the last decade has been the development of non-linear acoustics, this topic was not specifically among those discussed in the formal programme. The reason is that the Society considered that it was sufficiently important to warrant a separate two-day meeting on its own. This immediately preceded the Spring Meeting, and was also held in the Electronic and Electrical Engineering Department of the University of Birmingham, where most of the work in the UK on the engineering aspects of non-linear acoustics has been carried out. A conclusion which can be drawn from this meeting and the conference is that the science of underwater acoustics has reached a stage where the physical principles are understood and the available methods of theoretical analysis axe generally adequate. The limit to what can be achieved is often set now by cost and complexity of apparatus rather than by physical barriers, particularly as far as non-military applications are concerned. A continuing need, too, is for reliable environmental information; for example, that relating to target strengths and to scattering from the sea-bed, on which the design of practical sonar systems should be based. Underwater
acoustics
session
The subjects of the 19 papers presented were fairly evenly divided between propagation of sound in the sea, transducers and arrays, and sonar and telemetry 174
ULTFf,ASONICS July 1971
systems. The papers on propagation tended to confirm the feeling that the powerful mathematical techniques which have been developed to handle the refraction and scattering of waves are now sufficiently advanced for the purpose. But the usefulness of any mathematical formula, even when backed by a large computer, is ultimately limited by the extent and accuracy of the information which is available for feeding into it. This highlights again the demand for more and more experimental data about what really happens, rather than a study of hypothetical models. An interesting physical explanation was given,for the fact that some communication is possible across the boundary of a so-called ‘shadow zone’; this now appears to be a result of the scattering of energy at the surface of the ocean. The exact explanation of the unexpectedly high observed acoustic attenuation coefficient in the sea at frequencies less than 1Hz is still not certain but one speaker suggested that it might possibly be due to finite-amplitude effects caused by the use of highenergy explosive sources for measurement purposes. There was little new to report as far as transducers were concerned; here, the emphasis was on the practical development of known techniques. These included the application of electro-mechanical filter theory to designing an efficient transmitting transducer with a bandwidth which could be made to exceed half the midband frequency, the designing of light-weight ‘bender’ type transducers and of sandwich-type transducers for the range of sizes
‘sand waves break up the fringe patterns, and the surveyor must then return to conventional echo-sounding for his depth information. The final paper was scheduled to be given by J.D. Mills (Underwater Instrumentation Ltd, and E. G. G. Inc) on ‘Recent American developments’. Unfortunately J. D. Mills was unable to be present and K. Klein (Klein Associates) presented a light-hearted paper in its place, in which he gave an account of his recent exploits in Loch Ness where he spent the Summer of 1970 using equipment developed by his company in the search for the ‘monster’. The most important point to emerge from the papers and from subsequent discussion was that, although many advances have been made in providing corrected trueito-scale records, it is essential, if the present rate of progress is to be maintained, that some means of calibration be incorporated into the sidescan equipment to enable acoustic target strengths to be assigned to the features displayed. Only with this information will it be possible to quantify the data obtained by a side-scan sonar for comparison with data obtained at different times and different frequencies with other equipments. B. K. Gazey
Birmingham, UK, 5-7 April 1971
Spring
Meeting
of the British Acoustical
Society
UNDERWATER ACOUSTICS Non-linear
acoustics
meeting
Although one of the more interesting features of underwater acoustics during the last decade has been the development of non-linear acoustics, this topic was not specifically among those discussed in the formal programme. The reason is that the Society considered that it was sufficiently important to warrant a separate two-day meeting on its own. This immediately preceded the Spring Meeting, and was also held in the Electronic and Electrical Engineering Department of the University of Birmingham, where most of the work in the UK on the engineering aspects of non-linear acoustics has been carried out. A conclusion which can be drawn from this meeting and the conference is that the science of underwater acoustics has reached a stage where the physical principles are understood and the available methods of theoretical analysis axe generally adequate. The limit to what can be achieved is often set now by cost and complexity of apparatus rather than by physical barriers, particularly as far as non-military applications are concerned. A continuing need, too, is for reliable environmental information; for example, that relating to target strengths and to scattering from the sea-bed, on which the design of practical sonar systems should be based. Underwater
acoustics
session
The subjects of the 19 papers presented were fairly evenly divided between propagation of sound in the sea, transducers and arrays, and sonar and telemetry 174
ULTFf,ASONICS July 1971
systems. The papers on propagation tended to confirm the feeling that the powerful mathematical techniques which have been developed to handle the refraction and scattering of waves are now sufficiently advanced for the purpose. But the usefulness of any mathematical formula, even when backed by a large computer, is ultimately limited by the extent and accuracy of the information which is available for feeding into it. This highlights again the demand for more and more experimental data about what really happens, rather than a study of hypothetical models. An interesting physical explanation was given,for the fact that some communication is possible across the boundary of a so-called ‘shadow zone’; this now appears to be a result of the scattering of energy at the surface of the ocean. The exact explanation of the unexpectedly high observed acoustic attenuation coefficient in the sea at frequencies less than 1Hz is still not certain but one speaker suggested that it might possibly be due to finite-amplitude effects caused by the use of highenergy explosive sources for measurement purposes. There was little new to report as far as transducers were concerned; here, the emphasis was on the practical development of known techniques. These included the application of electro-mechanical filter theory to designing an efficient transmitting transducer with a bandwidth which could be made to exceed half the midband frequency, the designing of light-weight ‘bender’ type transducers and of sandwich-type transducers for the range of sizes