Volume 33, Issue 11, November 1961
Index of content:
- PROGRAM OF THE SIXTY‐SECOND MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A: Underwater Sound—Transducers and Scatterers
- Contributed Papers
33(1961); http://dx.doi.org/10.1121/1.1936586View Description Hide Description
Reflections from the sea surface affect the acoustic power radiated by sound sources located at shallow depths. Furthermore, these surface reflections produce propagation anomalies characterized by a pattern of interference fringes and, at larger range, by abnormally high spreading losses known as the acoustic Lloyd mirroreffect. Both phenomena are well known for simple, nondirectional sources. They are studied here for directive sources whose directivity patterns have a horizontal or tilted main lobe. Directive sources differ from simple sources in that the surface‐reflected and direct rays are of different intensity. This causes a marked blurring of the interference fringe pattern, but has a negligible effect on the propagation anomaly at the long ranges where the Lloyd mirroreffect holds. The effect on power output of phase cancellations and reinforcements by surface‐reflected sound waves is generally less marked than for a simple source. (This work was sponsored by the Office of Naval Research.)
33(1961); http://dx.doi.org/10.1121/1.1936587View Description Hide Description
It is well known that a large plane reflector directly above and normal to the axis of a cylindrical transducer can have a rather drastic effect on the beam patterns of such a transducer. The degree of such an effect is, of course, dependent on size and material of the reflector, distance above the active face of the transducer, and the size of the active face of the transducer in wavelengths. Experimental data will be presented indicating some of the precautions one must take to guard against such effects or to use these effects to one's advantage if desired. Information will be presented, for both plane and conical reflectors, as to the beam depression, beam width, effective hydrophone sensitivity, and minor‐lobe structure as a function of reflector dimensions and for various distances of the reflectors above the active face. (This work is supported by the Bureau of Ships.)
Experimental Determination of the Depth of Focus of Transducers Using Elliptical and Modified Parabolic Reflector Systems33(1961); http://dx.doi.org/10.1121/1.1936588View Description Hide Description
When operating in the near zone of a large aperture transducer, one must use a focusing technique to realize the far‐field resolving power of the aperture. For many applications the depth of focus is an important consideration. Experimental data will be presented showing the beam width, minor‐lobe suppression, and sensitivity as a function of range near the far focus of an elliptic reflector. Data for similar conditions will also be presented for parabolic reflectors with the focal element shifted to obtain optimum directivity conditions. (This work is supported by the Bureau of Ships.)
Computation of the Directivity Pattern and Source Level of a Transducer from Near‐Field Measurements33(1961); http://dx.doi.org/10.1121/1.1936589View Description Hide Description
Kirchhoff's formula is used to compute the directivity pattern and the source level of a large cylindrical transducer from measurements made near the transducer. Directivity patterns are computed for a plane normal to the axis of the cylinder and a plane which contains the axis of the cylinder. The normal pressure gradient required in the formula is obtained by a simple approximation, and the computations are carried out by means of a numerical integration over the surface of a circular cylinder of finite length. Data have been taken by making various numbers of passes around the transducer at different positions along the axis with a probe hydrophone, or by making one pass with a line hydrophone. There is good agreement between measured and computed patterns and source levels. (This work is supported by the Bureau of Ships.)
33(1961); http://dx.doi.org/10.1121/1.1936590View Description Hide Description
This paper describes the construction of and behavior of a large spherical transducer formed as a mosaic of ceramic elements with no inactive material Formulas are shown that describe the several elements which form the building block of the assembly. The developed techniques indicate spheres up to 30 in. in diameter can be easily constructed and a modification of the basic element will permit much larger spheres to be built. Complete evaluation of a 10‐in.‐diam sphere is shown with efficiencies approaching 100%. Correlation between its evaluation and the theoretical report of S. Hanish at NRL is shown. Behavior in water indicates a good acoustic loading and wide‐band operation.
33(1961); http://dx.doi.org/10.1121/1.1936591View Description Hide Description
An interesting variation of the classical problems involved in synthesizing desired sonartransducerbeampatterns is to examine the effects caused by missing acoustic radiating elements. Beampatterns were calculated for a multi‐element circular array using discrete frequencies and a composite beampattern was calculated for a decade bandwidth. The same beampatterns were then calculated omitting single elements and groups of elements for analog and polarity sampling beamforming networks. The results are discussed in terms of rotating and fixed beamsonar systems.
33(1961); http://dx.doi.org/10.1121/1.1936592View Description Hide Description
An audiofrequency underwater transducer utilizing a folded exponential horn has been designed, developed, and tested. The horn is driven by a lead zirconate titanatepiezoelectric bimorph driver, producing a sound output of 74 db re 1 dyne/cm2 at 1 yd. The radiated soundpattern is essentially omnidirectional, with a Q of about 4. The horn length and mouth diameter are about one‐third of the wavelength of the center frequency. The transducer can operate at great depths since the horn and driver are free flooding. A physical description of the driver and horn is given. This is followed by a comparison of calculated impedance and frequency response curves with experimental results. (Represents work performed under subcontract to SIE Division, Dresser Electronics, for the Bureau of Ships.)
33(1961); http://dx.doi.org/10.1121/1.1936593View Description Hide Description
To meet the requirement for the simple high‐powered sound sources necessary in studies of underwater acoustic propagation and geophysical exploration various substitutes for solid explosives such as TNT have, from time to time, been investigated. This experimental study of the acoustic characteristics of explosions of hydrogen‐oxygen mixtures represents a systematic attempt to achieve a simple acoustic pulse free from the complicating bubble oscillations that have often discouraged previous investigators. Experimental data are shown relating the intensity and character of the acoustic signal produced to the proportions and amount of gas mixture; to point of ignition; to hydrostaticpressure; to length‐to‐diameter ratios of chambers and to various chamber appendages and terminations. It is shown that for certain conditions it is possible, almost completely, to suppress or dissipate the bubble pulse. Some information is given about the experimental techniques used and the difficulties which may be associated with the realization of a practical device.
33(1961); http://dx.doi.org/10.1121/1.1936594View Description Hide Description
It is often desirable for an acoustic target to have a large target strength independent of target aspect. A sphere is considered to have a low target strength that is independent of target aspect, whereas a cylinder is considered to have a high target strength for a particular target aspect but low target strength for other aspects. The measured target strength in water of a finite 90‐deg biconical surface as a function of target aspect is compared to similar data for a finite cylinder and sphere with the same relative dimensions. The biconical surface targets are two metal cones, having a common axis, and joined at their vertices with the vertex angles equal to 90 deg. The finite 90‐deg biconical surface is shown to have a lower maximum target strength than the cylinder but has a target strength that is less dependent upon target aspect than that of the cylinder. Target strength data for four right‐circular conical surfacesmeasured in water at two frequencies in the plane normal to the conical axis are compared to the empirical formula , where R is the radius of the base of the finite 90‐deg biconical surface and λ is the wavelength of the incident acoustic wave. (This work is supported by the Bureau of Ships.)
33(1961); http://dx.doi.org/10.1121/1.1936595View Description Hide Description
The echo structure of backward scattered acoustic energy from hollow air‐filled metal cylinders in water was investigated experimentally using pulses which were small compared to the physical dimensions of the cylinders. This is a continuation of the work reported by Barnard and McKinney [J. Acoust. Soc. Am. 33, 226–238 (1961)]. Pulses of 25 and 50μ sec duration at the frequencies 125, 290, and 470 kc were employed. True backscattering was accomplished through the use of a single transducer for transmitting and receiving. For a single transmitted pulse multiple periodic echo pulses were received. The time between the echo pulses and their rate of decay were measured. A periodic circular path about the cylinders was assumed and velocities computed and compared to the longitudinal velocity in infinite plates. Two sets of cylinders, one with constant outside diameter and four wall thicknesses and another having constant wall thickness and four different outside diameters were chosen to represent both thin and thick shells over a wide range of ka values. (This work is supported by the Office of Naval Research.)
- Session B: Musical and Electroacoustics
33(1961); http://dx.doi.org/10.1121/1.1936596View Description Hide Description
Previous work has shown that the clarinet reed is primarily stiffness controlled, its motion reproducing the pressure variations in the mouthpiece fairly closely [J. Acoust. Soc. Am. 33 806 (1961)]. Subsequent investigations have provided more information on the properties and behavior of the reed. The Young's modulus for the cane from which reeds are made varies by more than a factor of two among samples, even for strips of the same cane. The density also varies, but over a smaller range. These variations make finding a good clarinet reed an unpredictable matter. The reed, when wet (as under playing conditions), has a smaller modulus and larger density than when dry. The deviations from the simple stiffness behavior mentioned above are important to the performance of the instrument. The damping provided by the player's lip is important in reducing the tendency to produce high‐frequency vibrations. The mass of the reed is an important factor determining the production of higher harmonics and is thus of considerable importance to the quality of the tone produced. (This work is supported by a grant from the National Science Foundation.)
33(1961); http://dx.doi.org/10.1121/1.1936597View Description Hide Description
The attenuation‐frequency characteristic of typical pipe‐organ swell shutters was measured, using pipe tones as the source of signal. For shutters fully closed, relative to fully open, the average response shows a uniform negative slope. Several “swell shutter volume control” circuits have been synthesized to achieve this response.
33(1961); http://dx.doi.org/10.1121/1.1936598View Description Hide Description
A supplementary soundsystem was one of the technical innovations of the 1961 summer season of grand opera at the Cincinnati Zoo Pavilion, a covered but unenclosed area seating 2500 people. Neither the 85‐piece orchestra nor the Metropolitan and European soloists and the chorus required or were provided sound re‐enforcement. The versatile, flexible sound‐amplification and reproduction system was intended and used instead for a variety of supplementary purposes as follows: (a) auditory relocation of individual instruments or groups from the orchestra pit (actual) to backstage or on‐stage (apparent) positions, to fulfill dramatic requirements; (b) solo voice modification and motion, when “supernatural” timbre or spatial effects were appropriate; (c) soundeffects which were difficult for the orchestra percussionists to produce as realistically or dramatically as desired; (d) electronic musical instruments. Microphones in the orchestra pit and backstage, stereo tape playback, and a Baldwin electronic organ were the program sources. Soundeffects filters, artificial reverberation, and a Choratone (for organ) were available for modification effects. In addition to patch panel interconnection, a keyboard of variable resistance switches provided rapid, smooth transitions among the seven amplification channels leading to different loudspeaker locations (e.g., backstage, on stage, proscenium, ceiling, and rear).
33(1961); http://dx.doi.org/10.1121/1.1936599View Description Hide Description
“Optimum generator utilization” is a method of distributing the outputs of a number (G) of continually oscillating generators to a number (S) of stops, with G<S, in such a way that the most likely stop combinations will derive from the greatest number of generators.
33(1961); http://dx.doi.org/10.1121/1.1936600View Description Hide Description
The Panoramic Tone system is a new means for enhancing electrically produced or reproduced sound.Organ music has traditionally been heard in environments with long reverberation times. Therefore, it is desirable from a musical standpoint for this effect to be present in the sound produced by electronic organs played in homes and small churches. The heart of the new system is a long wire equipped with an electromechanical transmitter and receiver at its ends. Multiple reflections and termination losses control the properties of the tonal enhancement. The wire is coiled into a helical shape only for convenience in packaging. The sound transmitted along the wire has been translated to the ultrasonic frequency range. This greatly attenuates any disturbance from noises and shock at audible frequencies. It also gives a more uniform trend for the response‐frequency characteristic and the reverberation time‐frequency characteristic than other methods have provided.
33(1961); http://dx.doi.org/10.1121/1.1936601View Description Hide Description
Measurements were made on the low frequency tones radiated from a very large pipe organ. In the purer stops, such as the contra bourdon, the first partial predominates, even in the C0‐C1 octave (16–32 cps). This indicates that an electroacoustic tone radiating system for electronic organs should be capable of radiating fundamental tones as low as 16 cps. Such a system is demonstrated and described.
33(1961); http://dx.doi.org/10.1121/1.1936602View Description Hide Description
This paper discusses loudspeakercharacteristics which must be published by the manufacturer in order for the acoustical engineer to intelligently design a given loudspeaker system as part of the room acoustics design. It considers those characteristics of cone‐type direct radiator loudspeakers, column loudspeakers, and horn‐loaded loudspeakers most important to typical distributed and central loudspeaker systems. Reasons are given for preferring continuous pure‐tone measurements to measurements using bands of noise, square waves, etc., and for preferring anechoic or “free‐field” measurements to so‐called “typical‐room” measurements. Errors in sound‐system design that can result from a lack of information or from misinformation are discussed.
33(1961); http://dx.doi.org/10.1121/1.1936603View Description Hide Description
It is impertinent—but fair—to ask why we should care whether the frequency response of a loudspeaker is flat or not when the point‐to‐point steady‐state transfer function of an ordinary listening room is sure to introduce a much greater degree of frequency‐response irregularity in the over‐all chain of transduction leading from electrical signal to perception. Of course, we do care; so we are led to the hypothesis that a time‐invariant pattern of frequency response (such as that of the loudspeaker)can be discriminated in the presence of a variable pattern of frequency‐response irregularity (such as the room response when the listener keeps moving slightly) when, but only when, the listening interval is of sufficient duration to allow the variable part of the total response pattern to be averaged out. Qualitative evidence seems to support the hypothesis, but quantitative evaluation of the needed integration times is still wanting. If the effect is real, however, it may have an important bearing on the interpretation of the conventional A‐B tests used for the assessment of sound‐reproducing systems.
33(1961); http://dx.doi.org/10.1121/1.1936604View Description Hide Description
A high‐quality condenserearphone will be described in this paper. It consists of a solid backplate and a metallized Mylar foil which is stretched across the backplate. Its frequency response is within ±1.5 db between 25 and 14 000 cps and within ±3 db between 20 and 16 500 cps. In this frequency range the harmonic distortion is less than 1%, and the intermodulation distortion is less than 3% at 100 db SPL. The earphone shows an impulse response which is an almost true image of the applied voltage. The high quality of this earphone makes it useful for many purposes particularly in psycho‐acoustic experiments.
- Session C: Semiconductor Transducers
- Invited Symposium Papers
33(1961); http://dx.doi.org/10.1121/1.1936605View Description Hide Description
Semiconductor materials cut in particular directions have a very large change of resistance with strain. This is a consequence of the multivalley nature of the shape of the energy surfaces. Lightly doped samples have a gauge factor that varies inversely proportional to the temperature but heavily doped samples have a much smaller change in gauge factor than do lightly doped samples. Heavy doping also improves the linearity of the specimen. Silicon has the widest temperature range and the greatest sensitivity of any known material. Another pressure‐sensitive device is the heavily dopedp‐n junction known as the Esaki or tunnel diode. A negative resistance occurs over parts of the voltage‐current characteristic. The properties are pressure‐sensitive because of the thickness change and because of the energy gap change under pressure. The latter usually dominates and causes a reversal of effect between silicon and germanium. The present paper discusses the general properties, while other papers in the series discuss applications.