Index of content:
Volume 25, Issue 1, January 1953
25(1953); http://dx.doi.org/10.1121/1.1907002View Description Hide Description
This review is concerned with (1) certain physiological and structural changes produced in tissues of the central nervous system caused by high level ultrasound and (2) investigations into the physical mechanisms underlying these changes.
The cell bodies of neurons in the central nervous system are particularly susceptible to change by ultrasound. The effect of irradiation is immediately evident as a loss of function which may be reversible or irreversible depending on the dosage. Irreversible changes in function are accompanied by changes in the structure of the cell. The susceptibility of neurons studied so far is graded according to size, the larger neurons exhibiting a greater susceptibility. The dose of ultrasound can be adjusted to cause irreversible changes in neurons without causing any observable damage to the vascular and supporting components of the tissue. This selective and specific effect of ultrasound is being used as a tool in neuroanatomical studies now in progress and has considerable potential value in neurosurgery. The ultrasound alters the state of the nerve cells and, therefore, affords a basis for studying intracellular structure and function.
The physical basis for the ultrasonically produced biological effects has been investigated part. The following aspects of temperature have been analyzed and rejected: (1) High average (space) level, (2) interface heating, (3) rapid time rate of change, (4) temperature changes resulting from cavitation, (5) heating at gas nuclei. The phenomenon of cavitation is also shown to play no essential direct role in producing the effects.
25(1953); http://dx.doi.org/10.1121/1.1907011View Description Hide Description
This paper is concerned with the technique of temperature measurement in living tissue during irradiation by high intensity ultrasound. The interpretation of data obtained by the use of thermocouples is presented. The specific biological object used in this study is the spinal cord of rat exposed by laminectomy. This particular preparation serves to illustrate the relative importance of the heat conduction process in contributing to the temperature change as a function of the proximity of the imbedded thermocouple to bone and the time elapsed after initiation of the exposure.
The ultrasonic frequency used in these studies was 980 kc. The sound intensities incident on the cord were between 60 and 80 watts/cm2.
The experimental results presented in the paper are used to obtain values for the acoustic absorption coefficient of the tissue of the spinal cord. The range of values obtained for the intensity absorption coefficient per centimeter from measurements made on six adult rats at various positions in the spinal cord is 0.19 to 0.23 if the heat capacity of the tissue at constant pressure is 1.00 calorie/cm3.
25(1953); http://dx.doi.org/10.1121/1.1906986View Description Hide Description
Experiments have been carried out to investigate ultrasound as a possible therapeutic agent. These show that selective heating occurs in nerves and bones treated in situ. Effects of direct heat on nerves are strikingly similar to the effects of ultrasound. Blocking of nerves by sound or heat with present techniques seems impractical because the margin of safety is too narrow.
25(1953); http://dx.doi.org/10.1121/1.1907000View Description Hide Description
A review is presented of numerous biophysical studies concerning the action of ultrasound on living matter, with particular reference to therapeutic applications. On the basis of the experiments described. involving various physical, physiochemical, and chemical effects, it is concluded that selective heating caused by the radiation plays the major role quantitatively under therapeutic conditions. However, ultrasound also produces a mechanical effect. The diffusion layer at an interface is decreased by stirring, and thus exchange of metabolites is augmented.
25(1953); http://dx.doi.org/10.1121/1.1907003View Description Hide Description
The results of an experimental investigation of both the linear and nonlinear acoustic impedance of circular orifices with water and 10‐centistoke Dow Corning silicone fluid are presented. Experimental determinations of the end corrections for acoustic resistance and acoustic inductance are in good agreement with the classical end correction as given by Rayleigh for acoustic inductance. A series of determinations of the criteria for the onset of nonlinearity acoustic resistance and acoustic inductance together with photographs of the fluid flowpatterns developed are presented. A characteristic of one of the fluid flowpatterns is correlated with the onset of nonlinearity.
25(1953); http://dx.doi.org/10.1121/1.1907004View Description Hide Description
Relations expressing the acoustic inertance and resistance of short open tubes in terms of are reviewed. The tube radius is a; is the complex wave number for viscositywaves. Analogous relations giving the acoustical impedance of a cylindrical cavity in terms of , where is the thermal wave number, are considered. Viscous and thermal losses have sometimes been incorrectly separated in the literature. Past measurements, showing disagreement between theory and experiment for small , are cited. New measurements of inertance and associated resistance, in the range of from 1 to 77, are reported. The method of standing wave analysis used. In some instances the inertance tube is incorporated in an adjustable‐cavity resonator. Inertance values after end corrections agree with theory within 2.5 percent. Corrected resistance values generally agree with theory within 3 percent for Greater discrepancies for larger values of are attributed to excessive losses in very short cavities. Consideration of present results together with published data for larger values of indicates close agreement of theory and experiment for a range of extending at least from unity to several hundred.
25(1953); http://dx.doi.org/10.1121/1.1907005View Description Hide Description
The sound fieldgenerated by a vibrating cylinder of infinite length, whose dynamic configuration is periodic in φ and z, is expressed in terms of acoustic impedance ratios. It is noted that symmetrical modes of vibration are suppressed at certain frequencies because the corresponding reactive impedance is infinite, and that all z dependent modes become nonradiating below certain “cut‐off” frequencies, the corresponding impedance being purely reactive. Graphs are presented for the impedance ratios corresponding to certain modes.
For modes independent of z, the sound field is in the form of concentric cylindrical waves. For z dependent modes, as the plane wave wavelength increases from zero to a certain critical cut‐off value, the sound field changes from a set of concentric cylindrical waves to two sets of conical waves of decreasing vertex angle; at and beyond the cut‐off point, the conical waves have degenerated into a set of plane standing waves normal to the z axis. Simultaneously, the sound field has ceased being periodic in the radial direction, and the phase velocity in that direction has become infinite.
Practical applications of these phenomena are suggested.
25(1953); http://dx.doi.org/10.1121/1.1907006View Description Hide Description
The problem of the spherical acoustic resonator containing a slightly viscous medium is considered. Derivation of the equations of sound waves in a viscous medium is carried out, and it is shown that separation of the particle velocity into a divergenceless vector and a curl‐less vector gives rise to both a vector Helmholtz equation and a scalar Helmholtz equation. By using known solutions to these equations in spherical coordinates and imposing suitable boundary conditions, the frequencies of the free vibrations are calculated as well as the attenuation constants associated with each frequency. The effect of viscosity is, thus, seen to decrease slightly the natural frequencies and to cause a time rate of decay of the free vibrations. The attenuation constant for the purely radial modes of vibration is much smaller than that for modes having tangential velocity components.
25(1953); http://dx.doi.org/10.1121/1.1907007View Description Hide Description
The first part of this investigation deals with the case of a simple‐harmonic wave which becomes eventually distorted into a relatively stable shape resembling the shape of a saw‐tooth wave with rounded‐off corners. A simplified‐picture theory is developed in which the wave is idealized as consisting of a non‐dissipative linear portion of length λ1 and a dissipative simple‐harmonic portion of length λ2. The amplitude A of the distorted wave and its rate of absorption α A are computed (1) from the dissipative wavelength 2λ2 and (2) from the “build‐up distance” L = Nλ at which stability of shape is achieved, both λ2 and N being regarded as experimental data. Tests of the theory are made by using shadowgrams of waves generated in air by a piezoelectricquartz at the frequency of 0.405 megacycle (see reference 2). The second part of the paper presents an approximation treatment of the classical wave equation in which either of two different transformations of variables lead to a quasi‐linear differential equation of the type . In the first transformation, more suitable for periodic waves,x is used for the distance from the source and y for the time, while the reverse is done in the second transformation, more suitable for shock waves. An exact solution is given for the case of the simple‐harmonic source already discussed in the first part.
25(1953); http://dx.doi.org/10.1121/1.1907008View Description Hide Description
The present paper starts with a brief discussion of the two components of acoustical radiationpressure in contrast with the single component obtained for electromagnetic radiation in vacuum. The two components of acoustical radiationpressure are correlated in a simple manner with the two distinct interaction possibilities of the obstacle and the medium, i.e., interaction with the wave motion only or interaction with the wave motion as well as with the medium itself. This picture appears to be consistent with the earlier conclusions of Brillouin and Richter.
Afterwards the result is generalized for dispersive media. It is shown that the component which is usually stated to be independent of the equation of state in the existing nondispersive theories should contain the time parameters of the equation of state in the form of a multiplying factor group, velocity over phase velocity.
The present paper stresses the basic concepts rather than mathematical detail and concludes with an extensive bibliography and commentary.
25(1953); http://dx.doi.org/10.1121/1.1907009View Description Hide Description
Starting from the Navier‐Stokes equations, general equation governing the generation of vorticityR is obtained: , wherein R is the time‐independent vorticity in the Eulerian frame and 〈ρ0 uu〉 is the average value of Reynolds' stress dyadic. The solution to this vorticity equation, when properly transformed to the particle coordinates, is shown to be divergence free.
A specialization of the vorticity equation to the case of solenoidal first‐order motion is shown to lead to the generating term employed by Rayleigh and by Schlichting; a specialization to the case of irrotational first‐order motion is shown to lead to the generating term employed by Eckart. The sum of the two specialized driving terms does not equal the general term, indicating that the contributions to vorticity from rotational and compression effects are not independent of one another.
The theory is applied to find the streaming generated by a well‐defined beam of sound giving results agreeing with Eckart and with Markham. However, when it is applied to a two‐dimensional standing wave problem, the configuration of the streaming velocity in the boundary layer is found to differ from the results obtained by Rayleigh.
Finally, the effect on streaming of a time‐dependent viscosity coefficient is examined.
25(1953); http://dx.doi.org/10.1121/1.1907010View Description Hide Description
Theories for calculating steady streaming associated with sound fields are reviewed, comparing the methods and approximations of various authors. Two illustrative problems are worked out, both for rectilinear flow due to irrotational sound fields. The first deals with a single attenuated plane wave traveling down a tube, as in Cady's quartz wind experiments. In the second, a pair of crossed plane waves is treated, giving rise to a quite different kind of streaming. In obtaining solutions, attention is given to boundary conditions; here, gradients of She excess static pressure, another second‐order quantity, come into consideration. Significantly, streaming speeds depend critically upon α, the attenuation constant, where α may be due to any common cause, such as heat conduction, scattering, thermal relaxation, etc. From these results it appears that streaming measurements cannot be used to distinguish between absorption mechanisms. Numerical values are given for a few cases; high flow speeds may be expected in a bubbly medium.
25(1953); http://dx.doi.org/10.1121/1.1907012View Description Hide Description
The effect of a finite circular baffle board on the acoustic radiation was computed by the use of the oblate spheriodal wave function developed by Kotani and by Boukamp. The directivity, power radiation, and radiation impedance of the vibrating disk with a concentric circular baffle board are shown together with a design diagram for a circular baffle board.
25(1953); http://dx.doi.org/10.1121/1.1907013View Description Hide Description
A method for the precise measurement of the velocity of sound is described in which the driving‐point impedance of a loudspeaker connected to a closed tube of variable length is the frequency‐controlling element of a bridge‐stabilized oscillator. The operation of the system and the accuracy of measurement are analyzed. Results of measurements in air at 1 kc/sec, room temperature, and atmospheric pressure yield, when corrected to standard conditions, a velocity of 331.45±0.05 m/sec.
25(1953); http://dx.doi.org/10.1121/1.1907014View Description Hide Description
As an aid to acoustical designers, this paper presents some experimentally determined values of the attenuation coefficient and the speed of propagation of sound in Fiberglas. The propagation parameters α and β in the equation for a plane wave are determined by comparing the amplitudes and phases of the sound pressures existing simultaneously at separate points in the Fiberglas. The principle of measurement is similar to that used by Beranek and Scott involving a longitudinal movable probe in a long narrow metal duct filled with rockwool. However, in the redesigned apparatus, edge effects were reduced by using a tube of larger cross section; wall vibrations were damped by laminated rubber and Masonite walls; and the shunting effect of the longitudinal probe hole was eliminated by the use of transversely inserted microphones.
Experimental data were obtained within one db for frequencies between 50 and 1000 cycles for two samples of Fiberglas with nominal weights of 9 lb/cu ft PF (hard) and TWF (soft). The direction of propagation of the sound was normal to the surface of the blanket. For both samples the attenuation increases almost as the square root of the frequency, but the speed of propagation is found to increase almost as the and root of the frequency for the hard and the soft samples, respectively.
25(1953); http://dx.doi.org/10.1121/1.1907015View Description Hide Description
25(1953); http://dx.doi.org/10.1121/1.1907016View Description Hide Description
25(1953); http://dx.doi.org/10.1121/1.1907017View Description Hide Description
A fixed path interferometer, using variable temperature for the measurement of the absorption in gases in the low f/p region is described. Its use is illustrated by measurements on argon, neon, air, nitrogen, and nitrogen‐hydrogen mixtures at 250 kc. The values found for argon and neon were above classical by 40 and 65 percent, respectively. The discrepancies are ascribed to Rayleigh cross modes in the ultrasonic waves excited by the quartz crystal transducer rather than to any departure from classical theory. Cross modes and their influence on ultrasonicmeasurements are discussed. Data on the performance of two crystals at 250 and 500 kc are given.
25(1953); http://dx.doi.org/10.1121/1.1906981View Description Hide Description
By an extension of the techniques of Lindsay, the reflection from a semi‐infinite attenuating medium is derived for the case of oblique angles of incidence. The results are applied to the design of a water tank each wall of which is assumed to be covered by a sheet of absorbing material. The optimum physical properties of the absorber required to make the tank anechoic in the mesacycle region are then determined theoretically. Experimental measurements at 3.35 megacycles/sec indicate that the properties of tan rho‐crubber are very close to optimum. With a thick sheet backed by aluminum the reflection coefficient is about −30 db at normal incidence and rises to −20 db at an incident angle of 60°. Beyond 60° the rubber is of little value in reducing the reflectivity. The results obtained are in good agreement with those expected from the theoretical treatment when modified for reflections from the back surface and the phase difficulties previously described by the author.
25(1953); http://dx.doi.org/10.1121/1.1906982View Description Hide Description
The consonant environments of vowels were varied by forming nonmeaningful stimulus syllables consisting of 72 combinations of six vowels and 12 consonants. The syllables were spoken by subjects, and the duration, fundamental frequency, and relative power of the vowels were measured. All three factors varied significantly in response to changes of the consonant environment. The variations were systematically related to the attributes of the consonants, the most powerful attribute being the presence or absence of vocal fold vibration, followed by manner of articulation and place of articulation, in that order.