Volume 28, Issue 2, March 1956
Index of content:
28(1956); http://dx.doi.org/10.1121/1.1908237View Description Hide Description
The results of an experimental study of a relatively unfamiliar mode of propagation are presented: the zero‐order mode in a tube with massive wall reactance. The observed and theoretical disperison curves, which show good agreement, display a high‐pass cut‐off frequency. There is a dead zone between this frequency and the ring resonant frequency of the tube.
The phase velocitiesc2 are measured indirectly from the directivity patterns of a receiver in the form of a long rubber hose coupled to a microphone; these patterns are characterized by two symmetrical lobes whose angular separation is 2 cos−1 c/c 2, c being the sound velocity.
28(1956); http://dx.doi.org/10.1121/1.1908239View Description Hide Description
A theory is developed for the propagation of stress waves in a porous elastic solid containing compressible viscous fluid. The emphasis of the present treatment is on materials where fluid and solid are of comparable densities as for instance in the case of water‐saturated rock. The paper denoted here as Part I is restricted to the lower frequency range where the assumption of Poiseuille flow is valid. The extension to the higher frequencies will be treated in Part II. It is found that the material may be described by four nondimensional parameters and a characteristic frequency. There are two dilatational waves and one rotational wave. The physical interpretation of the result is clarified by treating first the case where the fluid is frictionless. The case of a material containing viscous fluid is then developed and discussed numerically. Phase velocity dispersion curves and attenuation coefficients for the three types of waves are plotted as a function of the frequency for various combinations of the characteristic parameters.
Theory of Propagation of Elastic Waves in a Fluid‐Saturated Porous Solid. II. Higher Frequency Range28(1956); http://dx.doi.org/10.1121/1.1908241View Description Hide Description
The theory of propagation of stress waves in a porous elastic solid developed in Part I for the low‐frequency range is extended to higher frequencies. The breakdown of Poiseuille flow beyond the critical frequency is discussed for pores of flat and circular shapes. As in Part I the emphasis of the treatment is on cases where fluid and solids are of comparable densities. Dispersion curves for phase and group velocities along with attenuation factors are plotted versus frequency for the rotational and the two dilational waves and for six numerical combinations of the characteristic parameters of the porous systems. Asymptotic behavior at high frequency is also discussed.
Production of Lesions in the Central Nervous System with Focused Ultrasound: A Study of Dosage Factors28(1956); http://dx.doi.org/10.1121/1.1908244View Description Hide Description
Applications of ultrasonics in neurosurgery involve the production of lesions of a specified size at a specified location, and in a reproducible fashion. Control of the dosage required for a given lesion depends on the geometry of the focal region, the exact determination of focal intensity and the relation between irradiation parameters and biological effect. From an analysis of these factors an optimum frequency for each focal depth is determined and an empirical dosage relationship based on ultrasonically produced paralysis in mice is stated. The mechanism of cell destruction by ultrasound is shown to be a temperature‐dependent mechanical effect originating at weak points in the tissue. The difficulties in extrapolating results obtained with experimental animals to applications in human neurosurgery are pointed out.
28(1956); http://dx.doi.org/10.1121/1.1908246View Description Hide Description
It is shown that the detailed group‐theoretical analysis of molecular vibrations can be extended to continuous bodies of various shapes by regarding them as the limits of discrete systems with the number of particles becoming indefinitely large. In this paper an elementary review of the method is given and its application is illustrated with 2‐dimensional systems and thin systems having pyramidal symmetry (i.e., tents and shells). Both stationary and rotating systems containing any number of particles arranged with the symmetry of a regular polygon or polygonal pyramid are considered. For macroscopic systems, unlike molecules, different possible constraints must be taken into account in enumerating the genuine vibrations of various types. Otherwise, one proceeds as in the molecular case. By going to the limit of large numbers of particles, one obtains the fractional occurrence numbers of vibrations of various symmetry types for stationary or spinning membranes, plates, tents, or shells of all regular polygonal shapes.
28(1956); http://dx.doi.org/10.1121/1.1908247View Description Hide Description
An integral solution to the differential equations of elastodynamics is given. The solution for the displacement at any point in an isotropic homogeneous, elastic medium is obtained in terms of the initial distribution of body forces within a given volume and the initial distribution of displacements and stresses over the bounding surface of the volume. These quantities appear in the solutions as retarded displacements, stresses, and body forces, with two types of retardation depending upon the velocities of longitudinal and transverse waves.
From the integral solution the diffraction of elastic waves through large apertures bounded by opaque walls in solidmaterials is formulated. The incident wave motions may be either of longitudinal or transverse polarizations. The expected shear‐compression interaction is obtained in the course of the computation. By analogy with optical procedure, Fraunhofer diffraction of elastic waves may be investigated as a special case.
In the example of diffraction by a slit, an incident compression wave is diffracted into (1) a compression wave with the “ordinary” spatial distribution similar to that obtained in the corresponding optical problem and (2) a weaker shear wave with an “extraordinary” distribution. In the case of incident shear waves, there is obtained a diffracted shear wave with an ordinary distribution, and a possible weak compression wave with the extraordinary distribution. The presence of the diffracted compression wave depends upon the polarization of the incident shear wave.
Opacity in the Kirchhoff sense is a property of materials which are perfectly rigid.
Diffraction Effects in the Ultrasonic Field of a Piston Source and Their Importance in the Accurate Measurement of Attenuation28(1956); http://dx.doi.org/10.1121/1.1908249View Description Hide Description
A study is made of the ultrasonic field produced by a circular quartz crystal transducer and the integrated response of a quartz crystal receiver with the same dimensions as the transducer. The transducer and receiver are taken to be coaxial, and it is assumed that the transducer behaves as a piston source while the integrated response is proportional to the average pressure over the receiver area. Computations are made for cases of interest in the megacycle frequency range (ka=50 to 1000; a=piston radius; λ=wavelength; k=2π/λ). The results contain features of use in identifying and correcting for diffraction errors. These features which apparently have been missed in previous investigations are compared with available experimental data. Finally correction formulas to account for diffractioneffects in the accurate measurement of attenuation are discussed. It is shown that the order of magnitude of the diffractionattenuation is given by one decibel per a 2/λ.
28(1956); http://dx.doi.org/10.1121/1.1908252View Description Hide Description
The interaction between an acoustic field and a thermal field is worked out for the case of a sound wave propagating down a channel in which the convection of heat occurs in a potential flow. The analysis restricted to inviscid fluids shows that the magnitude of the sound wave is reinforced as it propagates through the thermal field. The extent of this reinforcement is given in the form of an amplification factor which depends on the parameter of the flow and on the frequency of the sound wave.
28(1956); http://dx.doi.org/10.1121/1.1908253View Description Hide Description
Fifty‐three references from the open literature dealing with organ‐pipe combustion oscillations are reviewed; these oscillations were generated in various types of combustion systems by the flame, or the combustion process. The many systems discussed range from simple singing‐flame burners to industrial furnaces and high‐duty rockets. The combustion systems are classified, for purposes of discussion, according to the basic arrangement of the components of the unit. These classes are burners with singingdiffusionflames, flash tubes, gauze‐tone burners, rocket‐shaped burners, burners utilizing secondary air, and ram‐jet type burners. The acoustical behavior of each system is described, and possible driving mechanisms for producing the observed oscillations are indicated. Some general remarks are made, based on the survey, concerning the mathematical formulation of mechanisms to explain the oscillations in particular cases.
28(1956); http://dx.doi.org/10.1121/1.1908255View Description Hide Description
Data are presented on acoustic oscillations produced by a burner using a hexagonal bank of hypodermic tubes as deep ports. Most of the tests were made with ethane as the fuel, but some tests were run with methane and propane, for comparison. Both the diameter and length of the combustion chamber were varied.
It was found that the combustion chamber could be considered as a driver which forced the slugs of gas in the ports to oscillate. Burning of the incremental pulses of combustible mixture periodically issuing from the ports furnished energy to drive the oscillations when the pulses burned in phase with the oscillating component of the pressure in the chamber. The phasing depended on a time‐lag factor which was a function of the velocity of the gases through a space, similar to dark space, between the burner ports and the mean burning points, and the width of the space itself.
The oscillations not only ceased when there was a failure to satisfy the timing criterion, but also ceased upon approach to either rich or lean blowoff limits of the conventional type. This premature cessation apparently results from the flame burning from fewer ports as the limits are approached, less driving energy being available.
28(1956); http://dx.doi.org/10.1121/1.1908257View Description Hide Description
Systemic effects in hairless mice were studied following local abdominal and scrotal skin exposure to intense air‐borne sound. Comparisons were made between effects of moderately high (150 db, 18 kc) and high (160–168 db, 20 kc) levels of sound. Examination of control mice revealed that 10‐min daily immobilization for one to three months was sufficiently stressful to cause an hypertrophy of the adrenal cortex and involution of the thymus. Evidence was also obtained which indicated there was an increased adrenal response in immobilized mice which were exposed to moderately high levels of sound for periods of one to three months. It was concluded from these studies that immobilization and sound vibrations on the skin surface can both be considered stressful. The absence of gonadal damage and slight changes in the hemopoietic system indicated these are mild rather than severe stress stimuli. In other experiments where more intense sound was used, the local and systemic response was similar to that known to occur following ordinary skin burns. In these cases heat and not sound was the stressor agent involved.
28(1956); http://dx.doi.org/10.1121/1.1908259View Description Hide Description
The experimentally determined characteristics of a continuously variable resonant frequency crystal transducer are presented for the system operating under radiating conditions. These characteristics are presented for a high‐power variable resonant frequency transducer of large radiating area, which utilizes the second harmonic and operates over the 2:1 frequency range from 22 kc to 44 kc with no more than a 2:1 variation in the pressure amplitude at a fixed point on the axis of the beam. Curves of pressure amplitude, input conductance, band width, half‐power beam width, and efficiency, all as a function of the resonant frequency, are shown. The measured characteristics are compared with calculations based on a one‐dimensional theory.
28(1956); http://dx.doi.org/10.1121/1.1908261View Description Hide Description
Measurements on a type B fog horn indicate that the nominal frequency of 180 cps is far from any resonance peak in the resistance ratio of the horn. Efficiencies of much less than one percent are therefore commonly encountered. Actual diaphone frequencies in the field range between 180 and 245 cps. The frequency stability with pressure is poor, and large deviations from symmetrical piston movement occur. It can generally be stated that a fog horn that “grunts” is not operating efficiently.
28(1956); http://dx.doi.org/10.1121/1.1908263View Description Hide Description
The magnetostrictive properties of Alfenol, a cold‐rolled Al‐Fe alloy, were investigated to determine its suitability for use in electromechanical transducers. The effects of various processing techniques are included. The material, in the form of laminated toroids, was evaluated by the motional impedance method. Measured values of the electromechanical coupling coefficient, the mechanical Q in air, the reversible permeability, the dynamic magnetostrictive constant, and other parameters are given. The measurements indicate that the performance of 12‐ or 13‐Alfenol is comparable to that of nickel, at least for low‐power applications. The electromechanical coupling coefficient (approximately 0.29) is comparable to that of nickel; the electrical resistivity is at least ten times greater than that of nickel.
28(1956); http://dx.doi.org/10.1121/1.1908265View Description Hide Description
As a result of the nonlinear properties of magnetostrictive transducers their impedance and efficiency is a function of the amplitude of the driving field. Earlier studies of high power operation of magnetostrictive transducers at the Harvard Underwater Sound Laboratory showed the need for further study of this subject. This earlier study has been extended by the use of a high power amplifier recently installed at the ORL Black Moshannon Calibration Station.
Impedance and efficiency data obtained on a nickel ring stack transducer operated with and without polarization are presented for a driving field range of 0.1 to 100 oersteds. The efficiency of the unpolarized transducer is shown to be comparable to that of the polarized transducer at high driving powers.
Input current wave form was also observed in this study, and data are presented to show the dependence of wave form on current level. With the unpolarized transducer it is shown that the wave form is a function of nonlinear coupling as well as nonlinear magnetic properties.
Both the experimental procedure used in this study, and the importance of the results to transducer operation at high power levels are discussed.
28(1956); http://dx.doi.org/10.1121/1.1908267View Description Hide Description
The behavior of the Rayleigh disk whose size cannot be neglected in comparison with the wavelength is investigated. Results obtained are as follows. The sensitivity of the disk set to an angle 45° increases with the frequency of sound and reaches a maximum at the frequency corresponding to , (a is the radius of the disk, λ the wavelength of sound). This maximum value is about 50% larger than that calculated from the König's expression. When k becomes larger than 1.6, however, the sensitivity begins to fall off and its sign, at last, changes to negative when k≅4.5. In the high‐frequency region, the negative torque acts on the disk if the angle between its axis and the wave normal is small, but the positive torque if the angle is large. The disk displays the greatest sensitivity at an angle different from the ordinary angle 45°.
28(1956); http://dx.doi.org/10.1121/1.1908269View Description Hide Description
Naive listeners were administered PB word articulation tests under four conditions of successive repetition and three conditions of signal‐to‐noise ratio. At all three signal‐to‐noise ratios, successive repetition of test words yields slightly higher articulation scores. The major portion of improvement in scores occurs with the second presentation; third and fourth presentations have negligible effect.
28(1956); http://dx.doi.org/10.1121/1.1908271View Description Hide Description
Energy density spectra of gated segments of fricative consonants were measured. The spectral data were used as a basis for developing objective identification criteria which yielded fair results when tested. As a further check gated segments of fricatives were presented for identification to a group of listeners and their responses evaluated in terms of the objective identification criteria.
- LETTERS TO THE EDITOR
28(1956); http://dx.doi.org/10.1121/1.1908273View Description Hide Description
28(1956); http://dx.doi.org/10.1121/1.1908275View Description Hide Description