Volume 57, Issue 6, June 1975
Index of content:
57(1975); http://dx.doi.org/10.1121/1.380615View Description Hide Description
Fredrick V. Hunt’s accomplishments in promoting and developing contemporary acoustics research, as well as his contributions to the field of underwater sound, studies of phonograph‐record reproduction, and acoustic signal processing, are outlined.
Subject Classification: 10.60; 05.60.
57(1975); http://dx.doi.org/10.1121/1.380617View Description Hide Description
The influence of Ted Hunt on the career and interests of this author, while both were working at the Harvard Underwater Sound Laboratory, is detailed. Outlined are the research projects and breakthroughs which, through Ted’s inspiration and his ever‐present guidance while director of the laboratory, set the pattern for most of the antisubmarine research to date.
Subject Classification: 05.60; 10.85, 10.60.
57(1975); http://dx.doi.org/10.1121/1.380618View Description Hide Description
Hunt’s relationship with the Office of Naval Research and his performance as an acoustics researcher on other Navy‐related activities are discussed. Particular attention is given to ’’Task Order X,’’ a series of projects carried on under Navy sponsorship over a 24‐year period.
Subject Classification: 05.60; 10.60, 10.30.
57(1975); http://dx.doi.org/10.1121/1.380619View Description Hide Description
Professor F. V. Hunt documented what he thought, what he invented, what he learned, and what he accomplished. During a 40‐year period, he published over 50 papers and reports. He also authored numerous technical memoranda, one complete book and sections of others, and many key speeches. Many of these contributions were in his classical field of interest, acoustics. However, he made significant contributions in other fields, such as improved instrumentation and education. Through open and classified publications, Professor Hunt greatly influenced the development of U.S. Navy sonar systems. The breadth and impact of this record of accomplishment is examined. Special attention is given to describing the evolution and status of two unfinished books. One of these manuscripts was developed initially to be a major textbook on physical acoustics. The other manuscript began its life as a historical introduction to the text on physical acoustics, became at times a scholarly hobby for Professor Hunt, and grew into a mature draft for a separate book on O r i g i n s i n A c o u s t i c s.
Subject Classification: 05.60; 10.60.
57(1975); http://dx.doi.org/10.1121/1.380620View Description Hide Description
Frederick Vinton Hunt profoundly affected the course of architectural acoustics. As the d e f a c t o successor to W. C. Sabine, he began his acoustical career by utilizing the Sabine reverberation chamber at Harvard University in the early 1930s to introduce accuracy into reverberation‐time measurements. Stimulated by P. M. Morse’s Chap. VIII in V i b r a t i o n a n d S o u n d (1936), Professor Hunt explored the sound field in a rectangular, marble‐walled box and published in 1939 a new means for measuringsound‐absorption coefficients of acoustical materials as a function of angle of wave incidence and, in addition, the details of a high‐impedance, research sound source. As a continuing step he, with his students, expanded the application of the normal‐mode‐of‐vibration theory of acoustics to full‐scale rectangular rooms in which acoustical materials were present on one or more walls. Although he subsequently developed research interests in other fields, his papers in architectural acoustics touched off scores of published works by his students and their colleagues. Examples of his contributions that can be found today include techniques for reverberation measurement, the meaning of sounddiffusion in spaces, the design of auditoriums for speech and music, and the principles of audio reinforcement and reproduction. This paper summarizes the present‐day status of concert hall design, with emphasis on those elements of architecture that must be controlled if satisfactory results are to be obtained.
Subject Classification: 10.60; 55.40.
57(1975); http://dx.doi.org/10.1121/1.380621View Description Hide Description
The usefulness of direct measurements of vector intensity for the calibration of acoustic transducers has been studied by laboratory experiments in anechoic and reverberant chambers and by analysis. Determination of total power output is feasible by intensity measurements in the nearfield and in reverberant spaces, situations where pressure measurements would be inadequate without elaborate calculation. However, the directional distribution of intensity differs from the farfield, free‐space directivity both near a source and far from a source but in a reverberant space. Analysis shows no theoretical way to correct for these differences. In some circumstances, even though intensity measurements do not yield the exact directivity pattern, the differences may be acceptable.
Subject Classification: 10.60; 85.22.
57(1975); http://dx.doi.org/10.1121/1.380603View Description Hide Description
This is a historical look at the contributions to the field of acoustical signal processing which originated under the nurture of F. V. Hunt at the Harvard Acoustics Research Laboratory. The early research is highlighted by the work of Faran and Hills on the use of correlators for space–time processing. This early work was followed by research in the use of multiple element arrays and by the development of real‐time processors for correlation and beamforming applications. The impact of these contributions can be seen in the expanding field of signal‐processing research today. Upon retirement, Hunt transferred the pursuit of his last research interest, ’’Sir Transit Sonitus’’—a ship transit detector, to the Marine Physical Laboratory, where a real‐ocean investigation of the technique is currently underway.
Subject Classification: 10.60; 60.10.
57(1975); http://dx.doi.org/10.1121/1.380604View Description Hide Description
Theoretical predictions and reported experimental evidence of sound behavior in an enclosure are discussed and are shown in certain cases (e.g., normal‐mode theory) to be inconsistent. A concrete model of a rectangular room was constructed so that the number of boundaries could be varied. Steady‐state and transient responses are presented tor two‐, three‐, and four‐boundary configurations, and these are compared with digital computations from a time‐domain image simulation. The experimental results were found to be consistent with those of other researchers and the computermodel was able to account for all the principal phenomena, including those at wavelengths comparable with the boundary dimensions where normal‐mode concepts are usually substantiated. It was found that tone‐burst responses in a bounded configuration whose reflection density increases with time can exhibit amplitude, phase, and ’’frequency’’ modulation throughout the frequency range. In reconciling these results with the normal‐mode theory, the normal mode must be regarded as a mathematical function valid only in the context of an infinite series or integral in which, if principal features of the behavior are to be reproduced, the exact phase relationships are of vital importance. Finally, the tone‐burst results are discussed in the light of recent work on the hearing mechanism.
Subject Classification: 55.20, 55.55, 55.40.
57(1975); http://dx.doi.org/10.1121/1.380605View Description Hide Description
A series of experiments were conducted at a number of concert halls in order to evaluate their acoustic ’’quanlity.’’ The quality was determined from a comparison of subjective assessments of a hall with a set of specified parameters describing physical characteristics of the sound field. Notable differences in the sound field in the hall and detected by artificial heads were correlated to position, reverberation time, brilliance, and so on. Using a factor analysis a finite number of independent factors emerged. It is concluded that the quality of a hall is better described by a set of such factors rather than by a single ’’reduced’’ number, as was done in the past.
Subject Classification: 55.45.
57(1975); http://dx.doi.org/10.1121/1.380606View Description Hide Description
The method of maximizing the directional gain of a receiving array (heretofore useful only at a single frequency) is extended to the case of operation at a finite bandwidth. It is also shown how to design for maximum effective gain in the presence of noise which might arise within the individual transducers or their preamplifiers. Some necessary noise‐field correlations are computed, and numerical examples are included to show the effects of bandwidth and self noise on the overall gain for reception which can be achieved. The directional gain of a broadside linear array for operation at a finite bandwidth is always less than that for operation at a single frequency, but is always greater, and often considerably greater, than the gain realized by use of the single‐frequency design at the finite bandwidth. Unlike the single‐frequency design, whose directional gain falls very rapidly as the operating bandwidth is increased from zero, the wide‐band designs operate with good gain over a wide range of bandwidths.
Subject Classification: 30.82; 85.40; 60.30.
57(1975); http://dx.doi.org/10.1121/1.380607View Description Hide Description
Some authors writing about brassmusical instruments have used the term ’’effective length,’’ usually meaning the length of a cylindrical tube having the same resonance frequencies as a given horn, but possibly with different end conditions. In this paper, alternative definitions of effective length are considered, and one definition is chosen and generalized to all frequencies, not just discrete resonance frequencies. Within the framework of lossless plane‐wave horn theory, a nonlinear first‐order differential equation is derived that yields effective length as a function of frequency and horn contour. Effective length has been calculated for some horn contours resembling French horns and trumpets. The solutions are qualitatively consistent with the experience of instrument makers and players, and with the effective lengths of actual instruments, determined from measured resonance frequencies.
Subject Classification: 85.60; 75.40.
57(1975); http://dx.doi.org/10.1121/1.380608View Description Hide Description
Analysis of submerged vibrating structures and of the associated sound fields requires the simultaneous solution of the vibrational and acoustical problems, because surfacepressures exerted by a dense medium are typically comparable to inertia and damping forces in metal structures. The scattered field generated by a submerged structure may be similarly altered by the elastic response to the incident wave. This feedback phenomenon, whereby forces exciting the structure must be combined with radiation loading dependent on the structural response, is unimportant in the atmosphere. Consequently, as underwater sound approached airborne sound in practical importance during World War I, acousticians were forced to develop new analytical and experimental techniques for studying structure–water interactions. The development of this hybrid branch of acoustics and structural mechanics is reviewed with regard to radiation and scattering. Hunt, even though never personally active in this field, was interested in its applications. He performed his characteristic function of an unselfish teacher by encouraging doctoral candidates and postdoctoral fellows in the Harvard Acoustics Research Laboratory to venture into this novel area of acoustics.
Subject Classification: 30.30, 30.40, 30.50; 10.85.
57(1975); http://dx.doi.org/10.1121/1.380609View Description Hide Description
If necessity is the mother of invention, F. V. Hunt surely was its father. Confronted with the need to play some delicate recordings of the Harvard Tercentenary in 1936, Ted invented the first phonograph pickup capable of operation with 5‐g bearing forces—a 10:1 advance in the state of the art—thereby ushering in the era of high‐fidelity sound reproduction. This was followed in rapid succession by a definitive study of tracing distortion with Pierce and Lewis, including an uncanny prediction of the feasible parameters of a long‐playing record. In 1941 and 1945 he received patents on inventions related to pickup and stylus structures, and in 1962 he again reduced the pickup bearing weights by an order of magnitude, producing successful 1/10th‐g models which are yet to be commercialized. But Ted’s favorite saying was that he would receive most accolades for the work done through and by his students. Two among them, Frank G. Miller (1950) and James V. White (1970), made significant contributions to the understanding of a stylus–groove contact, thus laying a new theoretical basis for continued leadership of the disc record as a means of high‐fidelity sound reproduction in the home.
Subject Classification: 10.60; 85.66, 85.68.
57(1975); http://dx.doi.org/10.1121/1.380610View Description Hide Description
This paper describes a theory for predicting the vibrational motion of a phonograph stylus sliding in a modulated record groove. Previous theories of this interaction suffer from two main defects: (1) they are based on the assumption that the groove deformation is perfectly elastic, whereas groove impedance data have shown that this assumption is inappropriate at both high and very low tracking forces; (2) they are based on the assumption that the groove wall shape is paraboloidal in the stylus‐contact region. This second assumption is inappropriate when the recorded wavelength is comparable to the width of the contact region and leads to a complete breakdown in these theories at very short wavelengths, where scanning loss is important. This paper describes a generalized formulation of the stylus–groove interaction in which the nonlinear deformationproperties of the groove are specified in terms of several functions that can be determined experimentally by measuring the mechanical impedance of an unmodulated groove and by measuring scanning loss. The resulting theory is consistent with existing data on groove impedance and scanning loss. Predictions of nonlinear playback distortion based on this theory compare favorably with experimental results reported by Woodward and Werner on difference‐tone distortion. More comparisons with experiment are needed to determine the range of validity of the theory.
Subject Classification: 85.66, 85.68.
57(1975); http://dx.doi.org/10.1121/1.380611View Description Hide Description
Professor Hunt once asked, ’’Can you ’blow’ a whistle under water?’’ This question was perhaps the first of a series of events that has led to the emergence of a new power‐conversion technology for the efficient conversion of hydraulic power to acoustic or vibratory power. This hydroacoustic power‐conversion process embodies flow‐switching techniques that are analogous to switching methods that are employed in electronic dc to ac power‐conversion practice. The hydroacoustic technology has been developed for the generation of high‐power underwater sound for communication and echo location and for such industrial uses as high‐frequency rotary percussion rock drilling and pile driving. The origins of the power‐conversion technology are reviewed, analytical models used for predicting equipment performance are described, and illustrations of specific applications anticpated by Hunt and the author are set forth.
Subject Classification: 10.60; 30.70; 28.65; 85.40.
57(1975); http://dx.doi.org/10.1121/1.380612View Description Hide Description
In addition to serving as an ideal discipline for introducing the concepts of quantum field theory and general relativity,nonlinear acoustics provides a new means for absorbing and generatingsound. The many applications of this exciting subject are based in part on the fact that, in common with vector (electromagnetic and shear) and tensor (gravitational) waves, the scalar sound wave transports energy, angular momentum, linear momentum, and, incidentally, information. The methods of nonlinear acoustics can account for such disparate phenomena as the parametric array and the radar time delay in a gravitational field; the absorption of sound in superfluid helium and shock‐wave attenuation in a gas; radiation pressure and sonic boom; the scattering of sound by sound; and the generation of gravitational waves. Based on the ability of nonlinear acoustics to subsume within its methodology, and hence to unify, different physical processes, as well as its demonstrated ability to contribute solutions to practical problems, it semms reasonable to expect that this subject will become a part of the curriculum in more colleges and universities than is the case at present.
Subject Classification: 10.60; 25.10.
57(1975); http://dx.doi.org/10.1121/1.380613View Description Hide Description
The author has shown previously [J. Acoust. Soc. Am. 49, 119(A) (1971)] that the nonlinear properties of the medium can induce coupling among the modes in a multiresonant system and that the threshold and steady‐state response of subharmonic components can be predicted. In the present work, coupling induced by nonlinear properties is further illustrated by observing the subharmonic generation phenomenon in an acoustic interferometer. The experimental investigation was conducted with a 1.5‐MHz driving signal and used filtered and degassed water as the medium. Data concerned with the actual modes of the interferometer, the loss factor associated with the subharmonic modes, and the threshold of subharmonic generation, with length of the interferometer and the driving frequency varying, are consistent with the analytic results, provided that cavitation is carefully avoided.
Subject Classification: 25.15, 25.20; 30.75.
57(1975); http://dx.doi.org/10.1121/1.380614View Description Hide Description
This paper concerns the translational forces exerted on pulsating air bubbles in a stationary sound field. These forces, normally called Bjerknes forces, are derived by simple arguments and classified as to their origin. Measurements have been made of the relative velocity of appoach of two bubbles undergoing a mutual Bjerknes force. The measurements were made in a rigid glass container oscillated in a vertical direction at 60 Hz by a shaker table. The ambient pressure above the liquid was reduced in order to obtain large pulsations, and the attracting bubbles were photographed with a movie camera. Oberved and calculated values for the velocity of approach are in agreement provided a drag law assuming interfacial slippage is used.
Subject Classification: 25.60.
57(1975); http://dx.doi.org/10.1121/1.380622View Description Hide Description
A novel technique for measuring the tensile strength of liquids was recently reported [R. E. Apfel, J. Acoust. Soc. Am. 49, 145 (1971); Nature (Phys. Sci.) 233, 119 (11 Oct. 1971)]. In this technique, a droplet of the sample rises slowly in a column of another liquid, the ’’host,’’ in which it is nearly insoluble. When the droplet reaches a certain position in the host, an acoustic standing‐wave field at about 50 kHz is established in the column, serving two purposes: (1) to balance the buoyancy force on the droplet and thus levitate it and (2) to rend the sample apart by raising the acoustic pressure to a great enough amplitude. If the droplet is simultaneously superheated, the cavitation event takes the form of an irreversible explosion of the sample into its vapor. Taking advantage of the levitation scheme, we have filmed this explosion with both diffuse and shadow lighting at about 3500 frames/sec; this shows resonant oscillations and what appears to be jet formation during the collapse of the vapor bubble.
Subject Classification: 35.68; 25.60.