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Research and education in physical acoustics at the University of Mississippi, USA
Applied Acoustics 41 (1994) 285-293 ,,
World Acoustics
Research and Education in Physical Acoustics at the University of Mississippi, USA Henry E. Bass Jamie L. Whitten National Center for Physical Acoustics, The Universityof Mississippi, Mississippi 38677, USA (Received and accepted 11 February 1993)
ABSTRACT The University of Mississippi provides graduate education for a significant fraction of US students pursuing careers in physical acoustics. The education program emphasizes traditional physics coursework with research opportunities #1 acoustics. Research is conducted in a new, dedicated state-~fthe-art research facility with access to modern data acquisition and analysis systems. Research projects include agroacoustics, outdoor sound propagation, thermoacoustics, acoustical O' active surfaces, active noise control, nondestructit, e testing and machhle perception and non-linear acoustics.
1 INTRODUCTION A formal research program in physical acoustics at the University of Mississippi began in 1972. Faculty in the Department of Physics and Astronomy formed the Physical Acoustics Research Group at the University of Mississippi (PARGUM). Early research efforts were concerned with acoustic studies of molecular relaxation processes, the nonlinear properties of solids, and propagation of sound outdoors. During the following 15 years, P A R G U M grew in terms of graduate students, faculty, and breadth of research activities. In 1986, the US Congress appropriated 285 Applied Acoustics 0003-682X/94/$07.00 © 1994ElsevierScienceLimited,England.Printed in Great Britain
286
H. E. Bass
funds to establish on the University of Mississippi campus the National Center for Physical Acoustics (NCPA). Funds were provided for a 78 000 ft z (1 ft 2 ~0"1 m 2) research facility and state of the art equipment. The new facility was completed in 1989. The name was changed to the Jamie L. Whitten National Center for Physical Acoustics upon dedication in 1990. In 1992, the old P A R G U M was merged with N C P A to combine education and research activities. N C P A has basic and applied research programs in agroacoustics, ocean acoustics, o u t d o o r sound propagation, thermoacoustics, non-linear acoustics, non-destructive testing, acoustically active surfaces, and active noise control. These research programs involve close collaboration with various Agricultural Research Service Laboratories (primarily the National Sedimentation Laboratory), the National Aeronautics and Space Administration, the Naval Postgraduate School, Los Alamos National Laboratory and various Army and Navy laboratories. NCPA maintains close ties to the National Research Council in Canada and the Open University in the UK. The various research programs emphasize the physics of acoustics. Graduate students studying at NCPA are (with a few exceptions) physics majors. They take the standard curriculum for physics majors at the MS or PhD levels and receive advanced degrees in physics upon graduation. Students specializing in acoustics typically take two courses and one laboratory in addition to their physics core. Some students take more specialized courses in physical acoustics at the Naval Postgraduate School in Monterey, CA, USA. In addition to the large number of physics majors, there are also students majoring in Electrical Engineering, Mechanical Engineering, Telecommunications, and Biology. 2 R E S E A R C H ACTIVITIES The primary research activities as mentioned above involve acoustics as a tool to study physical phenomena, the study of acoustic waves interacting with gases, liquids, or solids, and application of acoustics to specific problems of interest to N C P A personnel and a sponsor. Although Congress and the State of Mississippi provide base support for NCPA, most research programs must compete for funding on a national basis.
2.1 Agroacoustics A unique area of research at N C P A is the application of acoustics to agricultural problems. This work grew out of the study of outdoor sound propagation. The ground surface has a major effect on propagation near grazing incidence so it seemed reasonable that this same interaction could be
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used to investigate soil properties. Using sound level measurements above the ground and a probe microphone pushed into the surface to various depths, information about soil flow resistance and tortuosity can be determined. ~ This entry in agroacoustics led to a number of other interesting projects. Catfish grown in commercial ponds have become a major crop in Mississippi. These ponds are muddy, so it is difficult to directly observe how many fish are in the pond, feeding patterns, etc. A side scanning sonar has been developed which allows the farmer to inventory his crop and to observe fish behavior. A downlooking high-frequency sonar is now being developed to sample fish size. Another important application of acoustics is the detection and monitoring of insect infestations in stored commodities (e.g. grain) and fruit. The sound made by small insects in these commodities is of a low level but can quite often be detected and identified. A parallel program involves the use of acoustic signals to modify insect behavior. Most effort has been devoted to crickets. 2 Female crickets converge on a sound source that emulates a male cricket call. These same insects avoid regions where a bat signal is emitted. Ultrasonic pulse-echo systems have proven to be useful for monitoring water level in small drainage ditches and experimental flumes. In some cases, the transport of soil near the bottom of the channel is of primary concern. The proper choice of frequency can provide excellent depth resolution at modest costs.
2.2 Outdoor sound propagation The last decade has seen dramatic progress in our ability to predict sound transmission loss through a quiet atmosphere with an index of refraction which varies with height. Numerical techniques such as the Fast Field Program 3 and Parabolic Equation Solver4 applied to propagation over a complex impedance boundar3/agree with each other and with analytical 5'6 solutions where available. The remaining effects which limit prediction accuracy are turbulence 7'8 and topography. Current research programs address both these issues. Atmospheric absorption limits long range propagation to low frequencies. At low frequencies, turbulent structures many meters in scale have the greatest effect on sound. 9 This large-scale turbulence falls in the source region of the turbulence spectrum so it cannot be treated using the statistical approaches which work well in the Komolgorov region of the spectrum. The treatment of sound propagation through large-scale turbulence will probably require a time domain description using the mathematics of chaos.
288
H. E. Bass
Hills, vegetation, and rough ground can also affect sound propagation. In principle, one can include hills by transforming the terrain into variations in the index of refraction. In practice such a treatment will be limited by reflections when the hill becomes steep. Roughness introduced by vegetation or uneven ground could also prove to be important at high frequencies. Specifically, the scattering of high-frequency sound by dense grasses is presently being studied. A low-frequency pneumatically driven acoustic horn has been developed and tested. 1° The acoustic output is 140_+ 10dB re. 20/tPa, 1 m for tones between 12 and 500Hz. The loudspeaker performance is controlled and monitored from a remote computer via a radio link. The system consists of a diesel engine, compressor, heat exchangers, pneumatic valve, and 10 Hz horn which are mounted on a large trailer. The system is portable in that it can be moved on roads and highways and is easily set up and taken down in tens of minutes.
2.3 Thermoacoustics When sound propagates through a region where there is a temperature (or density) gradient, additional terms in the wave equation lead to interesting effects. In the vicinity of a surface, thermal conduction allows heat to flow into or out of sound wave. L~.~2 This gives rise to sound amplification in some cases (thermoacoustic prime mover ~3) and an enhanced temperature gradient in others (thermoacoustic refrigeration). Thermoacoustic prime movers are sound sources driven by a temperature gradient. The absence of moving parts can make such sources attractive for some applications. Thermoacoustic refrigerators can use inert gas mixtures as the working fluid hence no environmentally damaging chemicals are required. Thermoacoustic refrigerators are now being scaled up in size to investigate commercial feasibility.
2.4 Acoustically active surfaces In recent years polymers and composite materials have been made that can be formed into flexible sheets that have piezoelectric properties.14"~ s With such materials it is possible to make an acoustically active coating that can be driven electrically to generate, absorb or reflect sounds. If a second layer of such materials is added that can be used to sense the sound and the signal generated is passed through a filter and applied to a driving layer, it is possible to make a 'smart' acoustically active surface that responds in a prescribed way to sound falling on it. The active surface group is experimenting with such smart surfaces and using state-of-the-art digital
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signal processors to control sound reflection, transmission and radiation by them in both air and water. 16- 19
2.5 Active noise control If sound can be radiated into a noisy environment in such a way that its phase is inverted the radiated sound can be made to cancel the noise. 2° The radiated sound is sometimes referred to as 'antisound' and the cancellation process as active noise control. The active noise control group at N C P A is using state-of-the-art digital signal processors to adaptively filter the signal from a sensing microphone and apply it to an antinoise transducer inside a headset so as to reduce the noise entering the ear. 21 -25 Two recent projects involve (1) construction of an active noise-reducing headset to be used in a diving helmet, and (2) construction of an active noise-reducing stethoscope to be used by medical personnel in medivac helicopters.
2.6 Non-destructive testing and machine perception Because of their high strength-to-weight ratio, composites are being increasingly used to replace metals. Standard ultrasonic non-destructive testing techniques in the time domain cannot generally be used for composites, particularly fiber-reinforced composites, because structural defects do not generally occur as an isolated feature but are diffused over a larger region. Increasing use is therefore being made of frequency-domain methods. At N C P A , frequency-domain methods are applied in an immersion tank to back-scattered data at normal incidence. A technique has been developed where bodies of arbitrary shape are scanned under feedback control from a focused ultrasonic probe. The scanning device is a robot arm, which maintains the moving probe at a fixed perpendicular distance to the surface of the body. 26 This technique can be used both in air and in water. In air it can be used for machine perception on a production line.
2.7 Non-linear acoustics The propagation of sound waves through gases, liquids, and solids often cannot be described adequately by a linear wave equation. Non-linear effects are important even at relatively small amplitudes because media are inherently non-linear. 2v In gases and liquids the non-linear effects are traceable to both the equation of motion and the equation of state, and are described by the non-linearity parameter B, the ratio of coefficients of the non-linear term to the linear term in the non-linear wave equation. 28 - 30 In solids the description also can be made by introducing third order elastic constants. 31
290
H. E. Bass
Non-linear acoustical and acousto-optical effects in fluids are studied at N C P A . Third-order elastic constants of many solids, both crystalline 32- 35 and a m o r p h o u s 36 have been evaluated. The effect of piezoelectricity 37 and the non-linear behavior of High T c superconductors 38 are being examined. The behavior of third-order elastic constants in crystals and at phase transitions can provide fundamental information about this important aspect of solid-state physics. N on-linearity of inhomogeneous materials and composites, or geological samples, and of metals are leading to new characterization materials. This promises a new technique for fundamental investigations in physics, for non-destructive testing and for materials evaluation.
3 GRADUATE STUDY Specific courses for students specializing in acoustics are (i) Acoustics, a one semester course at the level of Kinsler, Frey, Coppens and Sanders, (ii) a one semester laboratory course in Acoustics, and (iii) a one semester advanced course at the level of Pierce's text. All other courses are those required by the department in which the student chooses to major. In physics, a total of 36 credit hours of course work beyond the BS degree is typical. Research towards preparation of a P h D dissertation require from 2 to 5 years. The average time between a BS and a PhD is about 6 years. The Master's degree requires 24 h of course work plus a thesis or 30 h of course work. Students typically require 2 years to complete a Master's degree.
4 FACILITIES AND EQUIPMENT The 7 8 0 0 0 f t 2 National Center for Physical Acoustics includes a 7 8 m 3 anechoic chamber, a modern machine shop with computer-controlled machine tools, a 17-m-long wave tank, a 56 ft long low noise flow tunnel for aeroacoustic measurements, a clean room, and laboratories which are vibration and EM isolated. Special purpose Gaussian beam, solid dielectric, and direct to digital transducers are routinely fabricated in the machine shops. N C P A features state-of-the-art data acquisition and analysis equipment allowing for measurements at frequencies between 1 Hz and 100MHz. Capabilities include laser Doppler vibrometry and measurements at very low temperatures (below 77 K). All laboratories and offices are connected through one of two Ethernet systems in the Center. One Ethernet system utilizes a S U N minicomputer as the host. A variety of Vax Stations, S U N
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Workstations, NEXTs, H P Workstations and PCs are connected to this Ethernet system. A second Ethernet system is serviced by a cluster of 46 D E C Risc based processors to provide data throughput for analysis in excess of 18 B words/s. Both Ethernet systems are connected through fiber optics cable to the parallel Cray X M P s on campus.
REFERENCES 1. Sabatier, J. M., Hess, H., Arnott, W. P., Attenborough, K., Romkens, M. J. M. & Grissinger, E. H., In situ measurements of soil physical properties by acoustical techniques. Soil Sci. Soc. Am., 54(3) (1990) 658-72. 2. Forrest, T. G. & Raspet, R., Models of passive attraction and active female choice in acoustic communication. American Naturalist (1993). 3. Lee, S. W., Richards, W. F., Bong, N. & Raspet, R., Impedance formulation of the fast field program for acoustic wave propagation in the atmosphere. J. Acoust. Soc. Am., 79(3) (1986) 628-34. 4. White, M. J. & Gilbert, K. E., Application of the parabolic equation to the outdoor propagation of sound. Appl. Acoust., 27 (1989) 227-38. 5. Raspet, R., Baird, G. E. & Wu, W., The relationship between upward refraction above a complex impedance plane and the spherical wave evaluation for a homogeneous atmosphere. J. Acoust. Soc. Am., 89 (1991) 107 14. 6. Raspet, R., Baird, G. E. & Wu, W., Normal mode solution for low frequency sound propagation in a downward refracting atmosphere above a complex impedance plane. J. Acoust. Soc. Am., 91(3) (1992) 1341-52. 7. McBride, W. E., Bass, H. E., Raspet, R. & Gilbert, K. E., Scattering of sound by atmospheric turbulence: prediction in a refractive shadow zone. J. Acoust. Soc. Am., 91(3) (1992) 1336-40. 8. Gilbert, K. E., Raspet, R. & Di, X., Calculation of turbulence effects in an upward refracting atmosphere. J. Acoust. Soc. Am., 87(2) (1990) 2428-37. 9. Noble, J. M., Bass, H. E. & Raspet, R., The effects of large scale atmospheric inhomogeneities on acoustic propagation. J. Acoust. Soc. Am., 92(2) (1992) 1040-6. 10. Sabatier, J. M., Bass, H. E. & Bolen, L. N., Design and performance of a highpower, low-frequency sound source. Presented at the 124th Meeting of the Acoustical Society of America (1992). 11. Arnott, W. P., Bass, H. E. & Raspet, R., General formulation ofthermoacoustics for stack having arbitrarily shaped pore cross-sections. J. Acoust. Soc. Am., 90(6) (1991) 3228-37. 12. Arnott, W. P., Bass, H. E. & Raspet, R., Specific acoustic impedance measurements of air-filled thermoacoustic prime mover. J. Acoust. Soc. Am., 92(6) (1992) 3432-4. 13. Atchley, A., Bass, H. E., Hofler, T. J. & Lin, H.-T., Study of a thermoacoustic prime mover below onset of self-oscillation. J. Acoust. Soc. Am., 91 (1992) 734-43. 14. Furukawa, T., lshida, K. & Fukada, E., Piezoelectric properties in the composite systems of polymers and PZT ceramics. J. Appl. Phys., 50 (1979) 4904-12. 15. Gururaja, T. R., Shulze, W. E., Cross, L. E. & Newnham, R. E., Piezoelectric
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16. 17.
18. 19.
20. 21. 22. 23.
24.
25. 26.
27. 28. 29.
30. 31.
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composite materials for ultrasonic applications. Part I: Resonant modes of vibration of PZT rod-polymer composites. Part II: Evaluation of ultrasonic medical applications. I E E E Trans. Son. UItrason., SU-32 (1985) 481-98, 499 513. kafleur, L. D., Shields, F. D. & Hendrix, J. E., Acoustically active surfaces. J. Aeoust. Soe. Am., 90 (1991) 123(~7. Bao, X.-Q. Varadan, V. K., Varadan, V. V. & Howarth, T. R., Model of a bilaminar actuator for active acoustic control systems. J. Aeoust. Soe. Am., 87 (1990) 2350 2. Howarth, T. R., Varadan, V. K., Bao, X. & Varadan, V. V., Piezoelectric composite coating for active underwater sound reduction. J. Acoust. Soe. Am., 91 (1992) 823 31. Ruppel, T. H. & Shields, F. D., Cancellation of air-borne acoustic plane waves obliquely incident upon a planar phased-array of active surface elements. J. Aeoust. Soe. Am. (1993) (in press). For extensive bibliography see Active Noise and Vibration Control R~li, renee Bibliography (3rd edn) plus supplement 1991, D. Guicking Drittes Physickalisches lnstitut, University of Gottingen, Germany. Addendum to Volume I of the Second Annual TMS320 Educators Conference, Houston, Texas, USA (1992). Hendrix, J. E., Harley, T. R. & Shields, F. D., A predictive filter for active noise control in small spaces. Presented at the 120th Meeting of the Acoustical Society of America (1990). Harley, T. R., Hendrix, J. E. & Shields, F, D., Computational optimization ot" predictive filter for noise cancellation. Presented at the 120th Meeting of the Acoustical Society of America (1990). Harley, T. R., Hendrix, J. E. & Shields, F. D., DSP control algorithms for canceling broadband random noise with a single microphone and speaker. Presented at the 122nd Meeting of the Acoustical Society of America (1991). Hendrix, J. E., Harley, T. R. & Shields, F. D., An adaptive predictive filter for controlling broadband noise or vibration with tightly coupled transducers. Presented at the 122th Meeting of the Acoustical Socidty of America (1991). Hickling, R., Jao, S. J. & Kolodziejczak, J. J., N D E using modal analysis and automated ultrasonic testing. In Proceedings International Symposium on Vihroaeoustie Characterization of Materials and Structures, 1992, pp. 79 84. Breazeale, M. A., What to do when your world turns nonlinear. In Physieal Acoustics: Fundamentals and Applications, ed. O. Leroy & M. A. Breazeale. Plenum Press, New York, USA, 1991, pp. 21 30. Breazeale, M. A., Whither nonlinear acoustics? In Review Progress in Quantitative Nondestructive Evaluation (Vol. 9), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1990, pp. 1653 60. Breazeale, M. A., Anharmonicity, piezoelectricity and solid state nonlinearity. In Review of Progress in Quantitative Nondestructive (Vol. 10), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1991, pp. 1797 1803. Breazeale, M. A., Nonlinear acoustics and how she grew. In Review o f Progress in Quantitative Nondestruetive Evaluation (Vol. 11B), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1992, pp. 2015-23. Breazeale, M. A., Philip, J., Zarembowitch, A., Fischer, M. & Gesland, Y.,
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33.
34. 35. 36. 37.
38.
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Acoustical measurement of solid state nonlinearity: application in CsCdF 3 and KZnF 3. J. Sound Vibration, 88 (1983) 133~40. Cao, W., Barsh, G. R., Jiant, W. & Breazeale, M. A., Temperature dependence of third order elastic constants of potassium manganese fluoride. Phys. Rev., B38 (1988) 10244-54. Electric potential in piezoelectric medium and its influence on measurement of ultrasonic nonlinearity parameter', Wenhua Jiang, Gonghuan Du and M. A. Breazeale, in Proceedings of the 14th International Congress on Acoustics, Vol. 1A, Li Peizi, edc, Academica Sinica, Paper A1-5 (1992). Jiang, W., Du, G. & Breazeale, M. A., Electrical potential in piezoelectric medium and its influence on measurement of ultrasonic nonlinearity parameter. In Proc. 14th International Congress on Acoustics (Vol. 1A), ed. Li Peizi. Academic Sinica, 1992, paper AI-5. Jiang, W. & Breazeale, M. A., Temperature variation of elastic nonlinearity of NaC1. J. Appl. Phys., 68 (1990) 5472-7. Joharapurkar, C., Gerlich, D. & Breazeale, M. A., Temperature dependence of elastic nonlinearities in single crystal gallium arsenide. J. Appl. Phys., 72 (1992) 2202-8. Cantrell, Jr, J. H. & Breazeale, M. A., An ultrasonic investigation of the nonlinearity of fused silica for different hydroxyl ion contents and homogeneities between 300 and 3 K. Phys. Rev., BI7 (1978) 4864 70. Na, J. N. & Breazeale, M. A., Second harmonic generation of ultrasound in piezoelectric materials. In Frontiers of Nonlinear Acoustics: Proceeding of 12th ISNA, ed. M. F. Hamilton & D. T. Blackstock. Elsevier Science Pub. Ltd, London, UK, 1990, pp. 571-6. Breazeale, M. A. & Jiang, W. H., Ultrasonic nonlinearities of high temperature superconductor YBazCu30 v a from 300 to 77K, IEEE 1990 Ultrasonics Engineering Proceedings, ed. B. R. McAvoy. Inst. of Electrical and Electronic Engineers Pub., New York, USA, 1990, pp. 1297 300.
World Acoustics
Research and Education in Physical Acoustics at the University of Mississippi, USA Henry E. Bass Jamie L. Whitten National Center for Physical Acoustics, The Universityof Mississippi, Mississippi 38677, USA (Received and accepted 11 February 1993)
ABSTRACT The University of Mississippi provides graduate education for a significant fraction of US students pursuing careers in physical acoustics. The education program emphasizes traditional physics coursework with research opportunities #1 acoustics. Research is conducted in a new, dedicated state-~fthe-art research facility with access to modern data acquisition and analysis systems. Research projects include agroacoustics, outdoor sound propagation, thermoacoustics, acoustical O' active surfaces, active noise control, nondestructit, e testing and machhle perception and non-linear acoustics.
1 INTRODUCTION A formal research program in physical acoustics at the University of Mississippi began in 1972. Faculty in the Department of Physics and Astronomy formed the Physical Acoustics Research Group at the University of Mississippi (PARGUM). Early research efforts were concerned with acoustic studies of molecular relaxation processes, the nonlinear properties of solids, and propagation of sound outdoors. During the following 15 years, P A R G U M grew in terms of graduate students, faculty, and breadth of research activities. In 1986, the US Congress appropriated 285 Applied Acoustics 0003-682X/94/$07.00 © 1994ElsevierScienceLimited,England.Printed in Great Britain
286
H. E. Bass
funds to establish on the University of Mississippi campus the National Center for Physical Acoustics (NCPA). Funds were provided for a 78 000 ft z (1 ft 2 ~0"1 m 2) research facility and state of the art equipment. The new facility was completed in 1989. The name was changed to the Jamie L. Whitten National Center for Physical Acoustics upon dedication in 1990. In 1992, the old P A R G U M was merged with N C P A to combine education and research activities. N C P A has basic and applied research programs in agroacoustics, ocean acoustics, o u t d o o r sound propagation, thermoacoustics, non-linear acoustics, non-destructive testing, acoustically active surfaces, and active noise control. These research programs involve close collaboration with various Agricultural Research Service Laboratories (primarily the National Sedimentation Laboratory), the National Aeronautics and Space Administration, the Naval Postgraduate School, Los Alamos National Laboratory and various Army and Navy laboratories. NCPA maintains close ties to the National Research Council in Canada and the Open University in the UK. The various research programs emphasize the physics of acoustics. Graduate students studying at NCPA are (with a few exceptions) physics majors. They take the standard curriculum for physics majors at the MS or PhD levels and receive advanced degrees in physics upon graduation. Students specializing in acoustics typically take two courses and one laboratory in addition to their physics core. Some students take more specialized courses in physical acoustics at the Naval Postgraduate School in Monterey, CA, USA. In addition to the large number of physics majors, there are also students majoring in Electrical Engineering, Mechanical Engineering, Telecommunications, and Biology. 2 R E S E A R C H ACTIVITIES The primary research activities as mentioned above involve acoustics as a tool to study physical phenomena, the study of acoustic waves interacting with gases, liquids, or solids, and application of acoustics to specific problems of interest to N C P A personnel and a sponsor. Although Congress and the State of Mississippi provide base support for NCPA, most research programs must compete for funding on a national basis.
2.1 Agroacoustics A unique area of research at N C P A is the application of acoustics to agricultural problems. This work grew out of the study of outdoor sound propagation. The ground surface has a major effect on propagation near grazing incidence so it seemed reasonable that this same interaction could be
Worm acoustics
287
used to investigate soil properties. Using sound level measurements above the ground and a probe microphone pushed into the surface to various depths, information about soil flow resistance and tortuosity can be determined. ~ This entry in agroacoustics led to a number of other interesting projects. Catfish grown in commercial ponds have become a major crop in Mississippi. These ponds are muddy, so it is difficult to directly observe how many fish are in the pond, feeding patterns, etc. A side scanning sonar has been developed which allows the farmer to inventory his crop and to observe fish behavior. A downlooking high-frequency sonar is now being developed to sample fish size. Another important application of acoustics is the detection and monitoring of insect infestations in stored commodities (e.g. grain) and fruit. The sound made by small insects in these commodities is of a low level but can quite often be detected and identified. A parallel program involves the use of acoustic signals to modify insect behavior. Most effort has been devoted to crickets. 2 Female crickets converge on a sound source that emulates a male cricket call. These same insects avoid regions where a bat signal is emitted. Ultrasonic pulse-echo systems have proven to be useful for monitoring water level in small drainage ditches and experimental flumes. In some cases, the transport of soil near the bottom of the channel is of primary concern. The proper choice of frequency can provide excellent depth resolution at modest costs.
2.2 Outdoor sound propagation The last decade has seen dramatic progress in our ability to predict sound transmission loss through a quiet atmosphere with an index of refraction which varies with height. Numerical techniques such as the Fast Field Program 3 and Parabolic Equation Solver4 applied to propagation over a complex impedance boundar3/agree with each other and with analytical 5'6 solutions where available. The remaining effects which limit prediction accuracy are turbulence 7'8 and topography. Current research programs address both these issues. Atmospheric absorption limits long range propagation to low frequencies. At low frequencies, turbulent structures many meters in scale have the greatest effect on sound. 9 This large-scale turbulence falls in the source region of the turbulence spectrum so it cannot be treated using the statistical approaches which work well in the Komolgorov region of the spectrum. The treatment of sound propagation through large-scale turbulence will probably require a time domain description using the mathematics of chaos.
288
H. E. Bass
Hills, vegetation, and rough ground can also affect sound propagation. In principle, one can include hills by transforming the terrain into variations in the index of refraction. In practice such a treatment will be limited by reflections when the hill becomes steep. Roughness introduced by vegetation or uneven ground could also prove to be important at high frequencies. Specifically, the scattering of high-frequency sound by dense grasses is presently being studied. A low-frequency pneumatically driven acoustic horn has been developed and tested. 1° The acoustic output is 140_+ 10dB re. 20/tPa, 1 m for tones between 12 and 500Hz. The loudspeaker performance is controlled and monitored from a remote computer via a radio link. The system consists of a diesel engine, compressor, heat exchangers, pneumatic valve, and 10 Hz horn which are mounted on a large trailer. The system is portable in that it can be moved on roads and highways and is easily set up and taken down in tens of minutes.
2.3 Thermoacoustics When sound propagates through a region where there is a temperature (or density) gradient, additional terms in the wave equation lead to interesting effects. In the vicinity of a surface, thermal conduction allows heat to flow into or out of sound wave. L~.~2 This gives rise to sound amplification in some cases (thermoacoustic prime mover ~3) and an enhanced temperature gradient in others (thermoacoustic refrigeration). Thermoacoustic prime movers are sound sources driven by a temperature gradient. The absence of moving parts can make such sources attractive for some applications. Thermoacoustic refrigerators can use inert gas mixtures as the working fluid hence no environmentally damaging chemicals are required. Thermoacoustic refrigerators are now being scaled up in size to investigate commercial feasibility.
2.4 Acoustically active surfaces In recent years polymers and composite materials have been made that can be formed into flexible sheets that have piezoelectric properties.14"~ s With such materials it is possible to make an acoustically active coating that can be driven electrically to generate, absorb or reflect sounds. If a second layer of such materials is added that can be used to sense the sound and the signal generated is passed through a filter and applied to a driving layer, it is possible to make a 'smart' acoustically active surface that responds in a prescribed way to sound falling on it. The active surface group is experimenting with such smart surfaces and using state-of-the-art digital
World acoustics
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signal processors to control sound reflection, transmission and radiation by them in both air and water. 16- 19
2.5 Active noise control If sound can be radiated into a noisy environment in such a way that its phase is inverted the radiated sound can be made to cancel the noise. 2° The radiated sound is sometimes referred to as 'antisound' and the cancellation process as active noise control. The active noise control group at N C P A is using state-of-the-art digital signal processors to adaptively filter the signal from a sensing microphone and apply it to an antinoise transducer inside a headset so as to reduce the noise entering the ear. 21 -25 Two recent projects involve (1) construction of an active noise-reducing headset to be used in a diving helmet, and (2) construction of an active noise-reducing stethoscope to be used by medical personnel in medivac helicopters.
2.6 Non-destructive testing and machine perception Because of their high strength-to-weight ratio, composites are being increasingly used to replace metals. Standard ultrasonic non-destructive testing techniques in the time domain cannot generally be used for composites, particularly fiber-reinforced composites, because structural defects do not generally occur as an isolated feature but are diffused over a larger region. Increasing use is therefore being made of frequency-domain methods. At N C P A , frequency-domain methods are applied in an immersion tank to back-scattered data at normal incidence. A technique has been developed where bodies of arbitrary shape are scanned under feedback control from a focused ultrasonic probe. The scanning device is a robot arm, which maintains the moving probe at a fixed perpendicular distance to the surface of the body. 26 This technique can be used both in air and in water. In air it can be used for machine perception on a production line.
2.7 Non-linear acoustics The propagation of sound waves through gases, liquids, and solids often cannot be described adequately by a linear wave equation. Non-linear effects are important even at relatively small amplitudes because media are inherently non-linear. 2v In gases and liquids the non-linear effects are traceable to both the equation of motion and the equation of state, and are described by the non-linearity parameter B, the ratio of coefficients of the non-linear term to the linear term in the non-linear wave equation. 28 - 30 In solids the description also can be made by introducing third order elastic constants. 31
290
H. E. Bass
Non-linear acoustical and acousto-optical effects in fluids are studied at N C P A . Third-order elastic constants of many solids, both crystalline 32- 35 and a m o r p h o u s 36 have been evaluated. The effect of piezoelectricity 37 and the non-linear behavior of High T c superconductors 38 are being examined. The behavior of third-order elastic constants in crystals and at phase transitions can provide fundamental information about this important aspect of solid-state physics. N on-linearity of inhomogeneous materials and composites, or geological samples, and of metals are leading to new characterization materials. This promises a new technique for fundamental investigations in physics, for non-destructive testing and for materials evaluation.
3 GRADUATE STUDY Specific courses for students specializing in acoustics are (i) Acoustics, a one semester course at the level of Kinsler, Frey, Coppens and Sanders, (ii) a one semester laboratory course in Acoustics, and (iii) a one semester advanced course at the level of Pierce's text. All other courses are those required by the department in which the student chooses to major. In physics, a total of 36 credit hours of course work beyond the BS degree is typical. Research towards preparation of a P h D dissertation require from 2 to 5 years. The average time between a BS and a PhD is about 6 years. The Master's degree requires 24 h of course work plus a thesis or 30 h of course work. Students typically require 2 years to complete a Master's degree.
4 FACILITIES AND EQUIPMENT The 7 8 0 0 0 f t 2 National Center for Physical Acoustics includes a 7 8 m 3 anechoic chamber, a modern machine shop with computer-controlled machine tools, a 17-m-long wave tank, a 56 ft long low noise flow tunnel for aeroacoustic measurements, a clean room, and laboratories which are vibration and EM isolated. Special purpose Gaussian beam, solid dielectric, and direct to digital transducers are routinely fabricated in the machine shops. N C P A features state-of-the-art data acquisition and analysis equipment allowing for measurements at frequencies between 1 Hz and 100MHz. Capabilities include laser Doppler vibrometry and measurements at very low temperatures (below 77 K). All laboratories and offices are connected through one of two Ethernet systems in the Center. One Ethernet system utilizes a S U N minicomputer as the host. A variety of Vax Stations, S U N
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Workstations, NEXTs, H P Workstations and PCs are connected to this Ethernet system. A second Ethernet system is serviced by a cluster of 46 D E C Risc based processors to provide data throughput for analysis in excess of 18 B words/s. Both Ethernet systems are connected through fiber optics cable to the parallel Cray X M P s on campus.
REFERENCES 1. Sabatier, J. M., Hess, H., Arnott, W. P., Attenborough, K., Romkens, M. J. M. & Grissinger, E. H., In situ measurements of soil physical properties by acoustical techniques. Soil Sci. Soc. Am., 54(3) (1990) 658-72. 2. Forrest, T. G. & Raspet, R., Models of passive attraction and active female choice in acoustic communication. American Naturalist (1993). 3. Lee, S. W., Richards, W. F., Bong, N. & Raspet, R., Impedance formulation of the fast field program for acoustic wave propagation in the atmosphere. J. Acoust. Soc. Am., 79(3) (1986) 628-34. 4. White, M. J. & Gilbert, K. E., Application of the parabolic equation to the outdoor propagation of sound. Appl. Acoust., 27 (1989) 227-38. 5. Raspet, R., Baird, G. E. & Wu, W., The relationship between upward refraction above a complex impedance plane and the spherical wave evaluation for a homogeneous atmosphere. J. Acoust. Soc. Am., 89 (1991) 107 14. 6. Raspet, R., Baird, G. E. & Wu, W., Normal mode solution for low frequency sound propagation in a downward refracting atmosphere above a complex impedance plane. J. Acoust. Soc. Am., 91(3) (1992) 1341-52. 7. McBride, W. E., Bass, H. E., Raspet, R. & Gilbert, K. E., Scattering of sound by atmospheric turbulence: prediction in a refractive shadow zone. J. Acoust. Soc. Am., 91(3) (1992) 1336-40. 8. Gilbert, K. E., Raspet, R. & Di, X., Calculation of turbulence effects in an upward refracting atmosphere. J. Acoust. Soc. Am., 87(2) (1990) 2428-37. 9. Noble, J. M., Bass, H. E. & Raspet, R., The effects of large scale atmospheric inhomogeneities on acoustic propagation. J. Acoust. Soc. Am., 92(2) (1992) 1040-6. 10. Sabatier, J. M., Bass, H. E. & Bolen, L. N., Design and performance of a highpower, low-frequency sound source. Presented at the 124th Meeting of the Acoustical Society of America (1992). 11. Arnott, W. P., Bass, H. E. & Raspet, R., General formulation ofthermoacoustics for stack having arbitrarily shaped pore cross-sections. J. Acoust. Soc. Am., 90(6) (1991) 3228-37. 12. Arnott, W. P., Bass, H. E. & Raspet, R., Specific acoustic impedance measurements of air-filled thermoacoustic prime mover. J. Acoust. Soc. Am., 92(6) (1992) 3432-4. 13. Atchley, A., Bass, H. E., Hofler, T. J. & Lin, H.-T., Study of a thermoacoustic prime mover below onset of self-oscillation. J. Acoust. Soc. Am., 91 (1992) 734-43. 14. Furukawa, T., lshida, K. & Fukada, E., Piezoelectric properties in the composite systems of polymers and PZT ceramics. J. Appl. Phys., 50 (1979) 4904-12. 15. Gururaja, T. R., Shulze, W. E., Cross, L. E. & Newnham, R. E., Piezoelectric
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composite materials for ultrasonic applications. Part I: Resonant modes of vibration of PZT rod-polymer composites. Part II: Evaluation of ultrasonic medical applications. I E E E Trans. Son. UItrason., SU-32 (1985) 481-98, 499 513. kafleur, L. D., Shields, F. D. & Hendrix, J. E., Acoustically active surfaces. J. Aeoust. Soe. Am., 90 (1991) 123(~7. Bao, X.-Q. Varadan, V. K., Varadan, V. V. & Howarth, T. R., Model of a bilaminar actuator for active acoustic control systems. J. Aeoust. Soe. Am., 87 (1990) 2350 2. Howarth, T. R., Varadan, V. K., Bao, X. & Varadan, V. V., Piezoelectric composite coating for active underwater sound reduction. J. Acoust. Soe. Am., 91 (1992) 823 31. Ruppel, T. H. & Shields, F. D., Cancellation of air-borne acoustic plane waves obliquely incident upon a planar phased-array of active surface elements. J. Aeoust. Soe. Am. (1993) (in press). For extensive bibliography see Active Noise and Vibration Control R~li, renee Bibliography (3rd edn) plus supplement 1991, D. Guicking Drittes Physickalisches lnstitut, University of Gottingen, Germany. Addendum to Volume I of the Second Annual TMS320 Educators Conference, Houston, Texas, USA (1992). Hendrix, J. E., Harley, T. R. & Shields, F. D., A predictive filter for active noise control in small spaces. Presented at the 120th Meeting of the Acoustical Society of America (1990). Harley, T. R., Hendrix, J. E. & Shields, F, D., Computational optimization ot" predictive filter for noise cancellation. Presented at the 120th Meeting of the Acoustical Society of America (1990). Harley, T. R., Hendrix, J. E. & Shields, F. D., DSP control algorithms for canceling broadband random noise with a single microphone and speaker. Presented at the 122nd Meeting of the Acoustical Society of America (1991). Hendrix, J. E., Harley, T. R. & Shields, F. D., An adaptive predictive filter for controlling broadband noise or vibration with tightly coupled transducers. Presented at the 122th Meeting of the Acoustical Socidty of America (1991). Hickling, R., Jao, S. J. & Kolodziejczak, J. J., N D E using modal analysis and automated ultrasonic testing. In Proceedings International Symposium on Vihroaeoustie Characterization of Materials and Structures, 1992, pp. 79 84. Breazeale, M. A., What to do when your world turns nonlinear. In Physieal Acoustics: Fundamentals and Applications, ed. O. Leroy & M. A. Breazeale. Plenum Press, New York, USA, 1991, pp. 21 30. Breazeale, M. A., Whither nonlinear acoustics? In Review Progress in Quantitative Nondestructive Evaluation (Vol. 9), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1990, pp. 1653 60. Breazeale, M. A., Anharmonicity, piezoelectricity and solid state nonlinearity. In Review of Progress in Quantitative Nondestructive (Vol. 10), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1991, pp. 1797 1803. Breazeale, M. A., Nonlinear acoustics and how she grew. In Review o f Progress in Quantitative Nondestruetive Evaluation (Vol. 11B), ed. D. O. Thompson & D. E. Chimenti. Plenum Press, New York, USA, 1992, pp. 2015-23. Breazeale, M. A., Philip, J., Zarembowitch, A., Fischer, M. & Gesland, Y.,
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Acoustical measurement of solid state nonlinearity: application in CsCdF 3 and KZnF 3. J. Sound Vibration, 88 (1983) 133~40. Cao, W., Barsh, G. R., Jiant, W. & Breazeale, M. A., Temperature dependence of third order elastic constants of potassium manganese fluoride. Phys. Rev., B38 (1988) 10244-54. Electric potential in piezoelectric medium and its influence on measurement of ultrasonic nonlinearity parameter', Wenhua Jiang, Gonghuan Du and M. A. Breazeale, in Proceedings of the 14th International Congress on Acoustics, Vol. 1A, Li Peizi, edc, Academica Sinica, Paper A1-5 (1992). Jiang, W., Du, G. & Breazeale, M. A., Electrical potential in piezoelectric medium and its influence on measurement of ultrasonic nonlinearity parameter. In Proc. 14th International Congress on Acoustics (Vol. 1A), ed. Li Peizi. Academic Sinica, 1992, paper AI-5. Jiang, W. & Breazeale, M. A., Temperature variation of elastic nonlinearity of NaC1. J. Appl. Phys., 68 (1990) 5472-7. Joharapurkar, C., Gerlich, D. & Breazeale, M. A., Temperature dependence of elastic nonlinearities in single crystal gallium arsenide. J. Appl. Phys., 72 (1992) 2202-8. Cantrell, Jr, J. H. & Breazeale, M. A., An ultrasonic investigation of the nonlinearity of fused silica for different hydroxyl ion contents and homogeneities between 300 and 3 K. Phys. Rev., BI7 (1978) 4864 70. Na, J. N. & Breazeale, M. A., Second harmonic generation of ultrasound in piezoelectric materials. In Frontiers of Nonlinear Acoustics: Proceeding of 12th ISNA, ed. M. F. Hamilton & D. T. Blackstock. Elsevier Science Pub. Ltd, London, UK, 1990, pp. 571-6. Breazeale, M. A. & Jiang, W. H., Ultrasonic nonlinearities of high temperature superconductor YBazCu30 v a from 300 to 77K, IEEE 1990 Ultrasonics Engineering Proceedings, ed. B. R. McAvoy. Inst. of Electrical and Electronic Engineers Pub., New York, USA, 1990, pp. 1297 300.