Volume 26, Issue 2, March 1954
Index of content:
26(1954); http://dx.doi.org/10.1121/1.1907299View Description Hide Description
In the analysis of the absorptioncharacteristics of a porous layer with perforated facing, it is usually assumed that the facing contributes only a mass reactance to the total impedance. In the present paper it is shown that the facing also causes an additional acoustic resistance which is often larger than the acoustic resistance of the porous layer itself. This resistance obtained only when the porous material is in close contact with the facing and becomes practically zero when the air space between facing and material exceeds approximately one perforation diameter. The effect is due to the near field (higher mode) losses in the porous material around the perforations. The resulting effect on the absorptioncharacteristics considerable, as demonstrated in some examples, and may at least partly explain reported disagreement between measurements and previous calculations.
26(1954); http://dx.doi.org/10.1121/1.1907300View Description Hide Description
The information transmission associated with elementary auditory displays consisting of a large number of independent stimulus aspects, e.g., the frequency or the sound level of a tone, was examined. In general, multiple stimulus encoding is a satisfactory procedure for increasing the information transmission associated with elementary auditory displays. Further, extreme subdivision of each stimulus aspect fails to produce substantial improvement in the information transmission.
26(1954); http://dx.doi.org/10.1121/1.1907301View Description Hide Description
Calculations based on the usual one‐dimensional theory of piezoelectric crystal systems have been carried out for several particular variable resonant frequency crystal systems radiating into water. Curves are presented showing the (1) power output as a function of frequency for operation both on and off resonance, (2) resonant frequency shift as a function of backing length, and (3) relative band width as a function of frequency. The validity of certain approximations is checked so that the operating characteristics of systems made up of different materials can be quickly estimated. The curves indicate that a variable resonant frequency system of crystals all of equal free‐free resonant frequency operating with an allowable power variation for fixed applied field strength of 2:1 can have a continuous variation of resonant frequency of about 2.3:1. A system half the crystals of which have twice the free‐free resonant frequency of the others for the same allowable field strength variation can operate over a frequency range of about 5.3:1.
26(1954); http://dx.doi.org/10.1121/1.1907302View Description Hide Description
The resonant frequency for the lowest mode of a circular plate is calculated for the conditions of concentrated mass loading at the center, water loading on one side, simply supported and clamped at the edge. These results are compared with those of Lamb and others.
26(1954); http://dx.doi.org/10.1121/1.1907303View Description Hide Description
Most investigations of the sound field of a moving source deal only with point sources. We have examined the sound field of finite sources, moving with constant velocity. The simplest possible vibrators have been chosen, viz., a pulsating circular cylinder and a pulsating sphere. The solution is an infinite series in terms of Mathieu functions in the two‐dimensional case, and in terms of prolate ellipsoidal functions in the three‐dimensional case. Besides the rigorous solutions, asymptotic formulas valid for large distances away from the source are given.
26(1954); http://dx.doi.org/10.1121/1.1907304View Description Hide Description
A theory is presented for the self‐maintained transverse oscillations which arise in a low‐velocity jet as it plays upon an obstacle, thus generating edge tones. It is assumed that transverse forces act on each particle of the jet as it travels toward an obstacle, and that this force is due to “sources” of hydrodynamic origin localized near the obstacle. The law of motion is set up in the form of an integral equation involving unknown functions for the force field. It is shown that a number of important properties of jet‐edge systems, and of the edge tones they produce, can be deduced from the equation, even for highly simplifying trial assumptions about the unknown functions.
26(1954); http://dx.doi.org/10.1121/1.1907305View Description Hide Description
The limitations imposed upon the usual elastic and viscoelastic constants are reinvestigated. Defining viscoelastic properties of a higher order, it is found that methods which have been used for the first‐order quantities are insufficient to determine relationships between the higher‐order constants. Necessary but non‐sufficient criteria are arrived at by means of arguments involving energies in dissipative processes.
26(1954); http://dx.doi.org/10.1121/1.1907306View Description Hide Description
The coefficient of variation for a series of sound pulses and the correlation function for successively received sound pulses are evaluated for arbitrary pulse length, using the assumption that the correlation function of the refractive index inhomogeneities is expressible as the product of time‐ and space‐dependent functions. For the case of pulse length much less than the time constant of refractive indexcorrelation function, the coefficient of variation is found to be independent of the pulse length, and the pulse‐correlation function is found to be identical with the time‐dependent part of the refractive indexcorrelation function. Comparison is made with data obtained from Sheehy. An approximate calculation shows that the motion of source and receiver will distort the pulse‐correlation function so that little information can be obtained from the data concerning the time dependance of the refractive index.
26(1954); http://dx.doi.org/10.1121/1.1907307View Description Hide Description
The reflection of a plane wave at a rough interface separating two fluid media is examined in the approximation of “small roughness,” such that second‐order terms in the magnitude of the roughness may be neglected. Exact solutions are obtained when the roughness is harmonic, while asymptotic results are obtained for arbitrary distributions. The analysis deals principally with an incident wave that is harmonic in time, but the problem of the reflection of a pulse from a perfectly reflecting, sinusoidal boundary is solved. It is found that such a boundary acts as a band‐pass filter of the nonspecular components of the reflected wave. Outisde of this pass band the reflection is not only specular but distortionless. Rather less generality is possible when the boundary is not perfectly reflecting, but the pass band is found to be independent of the properties of the reflecting medium.
26(1954); http://dx.doi.org/10.1121/1.1907308View Description Hide Description
The propagation of dominant mode plane sound waves within a cylindrical tube having arbitrary cross section and extending indefinitely from an open end is studied theoretically. A quantity of special interest is the end correction, which characterizes the reflection coefficient at wave lengths large compared to the transverse tube dimensions. The reflection coefficient and end correction fit naturally into a boundary value formulation of the wave propagation problem, whose solution hinges on that of one or more integral equations. Since the integral equations do not in general admit of exact solution, practical techniques for approximately determining the physical quantities are important. Among these are the stationary or variational principles, which can be cast into a variety of independent forms, corresponding to the different nature of boundary distributions. Another technique is based on initial modification of an integral equation so as to utilize the possibility of explicit and rigorous solution. Approximate forms of the reflection coefficient and end correction are obtained in this manner, with the distinction of yielding exact results when the tube cross section is circular. A brief comparison of the different procedures is included.
26(1954); http://dx.doi.org/10.1121/1.1907309View Description Hide Description
Oral speech intelligibility tests were conducted with, and without, supplementary visual observation of the speaker's facial and lip movements. The difference between these two conditions was examined as a function of the speech‐to‐noise ratio and of the size of the vocabulary under test. The visual contribution to oral speech intelligibility (relative to its possible contribution) is, to a first approximation, independent of the speech‐to‐noise ratio under test. However, since there is a much greater opportunity for the visual contribution at low speech‐to‐noise ratios, its absolute contribution can be exploited most profitably under these conditions.
26(1954); http://dx.doi.org/10.1121/1.1907310View Description Hide Description
Simple loudness judgment tests were performed by a jury of 15 observers on the recorded noises of approximately 100 highway trucks. The loudness of the noises calculated by a modification of the equivalent tone method proposed by Beranek and co‐workers ranges from about 20 to 200 sones. Observers were instructed to rank the noises in six loudness classes and were allowed to listen to a sample of class 1 and class 6 after each 20 recordings. The calculated loudnesses were then divided into six classes with class limits set by a 40 percent loudness increase per class. Although the subjective tests were conducted with no elaborate controls, a correlation coefficient of 0.94 was found between the average judgment of the group and the calculated loudness class. The correlation coefficient between average judgment and total sound pressure level was 0.78, whereas the correlation coefficient between the average judgment and the level on the A‐network of the sound level meter was 0.83. Experimental techniques and the capabilities and limitations of the loudness‐calculation methods are discussed.
26(1954); http://dx.doi.org/10.1121/1.1907311View Description Hide Description
The new USRL tank for making acoustic measurements under hydrostaticpressures up to 1000 psig and controlled temperatures from near freezing to near boiling is described. The internal treatment of the tank with wedges of Insulkrete, a sound absorbing material composed of pine sawdust and Portland cement, is pictured. Oscillograms of the pulsed signal show the reduction in reverberation level effected by Insulkrete. The conversion by Insulkrete of high‐frequency sound energy into heat is illustrated by schlieren photographs.
26(1954); http://dx.doi.org/10.1121/1.1907312View Description Hide Description
This paper presents the results of measurements of the sound absorption in aqueous solutions of several salts. While magnesium sulfate is the salt of primary interest in this case, some results for sea water at different temperatures, solutions of magnesium acetate, zinc sulfate, and beryllium sulfate are given. The frequency range of the measurements is approximately 50 to 500 kc. Although the resonator method used is not very precise, it is the only known laboratory method which is able to give reliable absorptionmeasurements in water and water solutions in this low‐frequency range where the sound absorption in these liquids is extremely small.
26(1954); http://dx.doi.org/10.1121/1.1907313View Description Hide Description
Precision sound velocity measurements are reported in sucrose solutions in the concentration range up to 3 molal, and in solutions of NaCl and up to 2 molal. Measurements were made at three temperatures—24.76°C, 40.37°C, and 59.60°C—using 15‐Mc sound. For sucrose and NaCl the velocityvs molality relation is linear out to about 1 molal, where a slight decrease in slope sets in. For the velocityvs molality relation has a negative slope, and a rise from linearity is already evident at 0.5 molal.
A simple theory of the concentration dependence of the sound velocity in solutions is developed and correlated with experimental data.
26(1954); http://dx.doi.org/10.1121/1.1907314View Description Hide Description
When underwater sound strikes the bottom of the sea in shallow or deep water, a portion of its intensity is scattered back toward the source of sound. A study of the backscattering from the bottom as a function of pulse length, frequency, and tilt‐angle at several locations in a harbor along the East Coast, was made by using a tiltable transducer mounted on a barge. The results are expressed in terms of quantity called the “scattering strength” of a unit area of bottom.
The results suggest that the backscattering of sound from a natural bottom is due more to its roughness rather than to the fact that it consists of sedimentary particles. Even at normal incidence the bottom of a sea or bay appears sufficiently rough so that little sound is returned by specular reflection. Some evidence in support of these conclusions is given.
26(1954); http://dx.doi.org/10.1121/1.1907315View Description Hide Description
Experimental investigations indicate that ultrasound may prove useful for therapy, for diagnosis, and for biologic measurement.
Ultrasound, although it may be dangerous in unskilled hands, is an excellent agent for the structural heating of living tissues. The term “structural heating” is used to denote heating at abrupt interfaces.The living organism presents numerous abrupt interfaces.
Ultrasound promises to become a reliable diagnostic tool. Either the transmitted or the reflected ultrasonic energy can be used in this application. Tumors, gallstones, kidney stones, stones in the salivary glands, foreign bodies, and abnormal collections of fluid have been located accurately.
Preliminary observations on the use of ultrasound for certain biologic measurements are encouraging. Instrumentation for biologic measurement includes ultrasonic flowmeters, devices for measuring thickness of some tissue structures, and equipment for measuring the viscosity of blood.
26(1954); http://dx.doi.org/10.1121/1.1907316View Description Hide Description
Ultrasonic processing possibilities are limited mainly by the economic factors of high first cost of equipment and high‐power cost per unit of processed material. In addition transducers suitable for large volume applications have only recently been developed. Processing possibilities which have led to industrial applications or which seem quite promising are reviewed. Among them are ultrasonic maturing of liquids, extraction, dispersing of dyes and pigments, emulsification, floatation ore dressing, degassing of water, oil and glass, soldering of aluminum,drilling, and cleaning of metal parts.
26(1954); http://dx.doi.org/10.1121/1.1907317View Description Hide Description
Ultrasonics is one of the newest tools to be applied to metal cleaning. Through the years abrasives, soaps, compounded alkaline materials, and solvents have been used to remove soils from metals. Trichlorethylene Solvent degreasing has had a wide reception in industry since its introduction in 1930. The Detrex Soniclean process combines sound energy and trichlorethylene solvent degreasing for metal cleaning. This is accomplished by immersing especially treated bariumtitanatetransducers in chlorinated solvents producing cleaning results previously unobtainable. Crossrod, conveyorized equipment incorporating constant distillation and filtration of the solvent in the sonics chamber and providing a final vapor rinse and drying is now producing as many as 8000 parts per hour in industrial plants.
26(1954); http://dx.doi.org/10.1121/1.1907318View Description Hide Description
Dispersion of the velocity of sound was measured by simultaneously transmitting continuous trains of sound waves of the two frequencies through water. One transducer was used to generate the two signals and a second transducer, which could be moved in the direction of the sound beam, received the signals. Dispersion was determined by measuring the change in the relative phase of the two received signals as the length of path between the transducers was varied. These measurements indicated that the velocity of sound at 500 kc was equal to the velocity at 1500 kc to within one part in 290 000.