Volume 36, Issue 12, December 1964

Influence of Radiation on the Cavitation Threshold of Degassed Water
View Description Hide DescriptionA study was made of cavitation induced in water at the center of a 12‐liter spherical resonator at a frequency of 25 kc/sec. For waterdegassed so that the equilibrium air pressure above it was less than 4 mm Hg, the threshold was found to be 9±3 atm, provided that the sound field was not radically different from an ideal radial mode. It was also possible to obtain thresholds consistently higher and up to 80 atm in a mode in which the central maximum was enhanced. A Picker‐Dresser neutron generator producing 14‐MeV neutrons at a rate of 10^{8}/sec was used to irradiate the resonator, and the threshold was always lowered by 10%–50%, whichever mode was used, there being a time lapse of about for the full effect of the irradiation to take place. A 1.0 cm‐thick lead screen placed over the resonator produced a rise in threshold of 10%–50% as often as it apparently produced no effect. It was concluded that neutron irradiation affects the cavitation threshold of degassedwater and that cosmicradiation probably does also. The time lags encountered are evidence of a cumulative effect, whose nature is briefly discussed.

Variability in Acoustic Output of Deep Explosive Sources
View Description Hide DescriptionSeveral salvos of nine explosive sources were lowered from a drifting ship to a depth of 600 m in water 4420 m deep and the charges were fired seriatim at 1‐ to 1.5 min intervals by electrical detonation. The signals generated were received by a hydrophone suspended at 300 m below the surface from an anchored ship and were recorded in 8 half‐octave frequency bands, between center frequencies of 63 and 715 cps, on photographically recording oscillographs. The variance in peak pressure of the signals in each of these frequency bands is calculated for the N arrivals of the same designation in the salvo. This variance in peak pressure is taken as the upper limit to the variance of the acoustic output in the chosen frequency band for the N sources of that salvo. The average values of the variance in peak pressure for all the one‐bottom‐bounce arrivals of the four best salvos are below 10% for the 63‐, 90‐, 125‐, and 180‐cps half‐octave bands and are 12%, 13%, and 19%, respectively, for the 250‐, 500 , and 715‐cps bands. Reflection from the free surface of the ocean does not appear to have a measurable effect on the variance of peak pressure in the frequency bands 63 180 cps, but such an effect does appear to exist in the higher frequency bands. Some characteristics of the bottom reflection process that are revealed in the experiment are noted and discussed.

Analysis of Spreading Loss for Refracted/Reflected Rays in Constant‐Velocity‐Gradient Media
View Description Hide DescriptionThe analytical properties of existing spreading‐loss formulas are not well‐known in the case where a ray experiences multiple boundary reflections. This paper investigates spreading loss in detail for refracted/bottom‐reflected rays when the velocity gradient is a negative constant and the bottom is horizontal. Expressions for the spreading loss are derived, special cases are examined, and upper bounds are obtained. It is shown that there are some points on every ray at which the spreading loss is less than inverse‐square loss and some points on some rays at which the loss exceeds inverse‐square loss. The loss behavior is examined as a ray path is traversed, and is found to be markedly dependent upon initial ray inclination, the ray lobe being considered, and velocity structure. The problem of multiple arrivals at a fixed receiver is also studied. Relationships are given between the number of arrivals and their intensity, travel time, initial inclination, and number of reflections. Two examples are presented. The results in this paper apply not only to refracted/bottom‐reflected transmissions, but also to surface‐channel and deep‐sound‐channel transmissions.

Correlation of Amplitude and Phase Fluctuations for Wave Propagation in a Medium with Random, Spheroidal Inhomogeneities
View Description Hide DescriptionA theoretical study is presented of the mutual correlation between amplitude and phase fluctuations when an acoustic (or electromagnetic)wave propagates in a random medium whose correlation function for refractive index is spheroidal‐Gaussian. Amplitude‐phase correlations are considered for both plane and spherical waves transmitted through a statistically isotropic medium in which the deviations of refractive index about a mean are small. Large‐scale inhomogeneities of arbitrary spheroidal shape are treated, and transmitter‐to‐receiver ranges are taken as large in comparison with the refractive‐index correlation length in the transmitter‐receiver plane. Comparison is made between the cases of spherical and nonspherical inhomogeneities, for both plane and spherical acoustic waves. The low‐ and high‐frequency extremes of the theory are given special note.

Coupler‐Reciprocity System for Hydrophone Calibration at High Pressures
View Description Hide DescriptionA reciprocity system has been developed for the absolute acoustic calibration of small hydrophones at hydrostaticpressures up to 15 000 psi and frequencies to 400 cps. The method involves electrical measurements on three transducers placed in a closed, stiff‐walled “coupler” vessel and a measurement of the static acoustic compliance of the coupler.

Acoustic Radiation from a Finite Cylinder
View Description Hide DescriptionA semianalytic method is presented that can be used to calculate the acoustic radiation in a domain exterior to a source that is vibrating harmonically in time. This method follows the traditional technique of an expansion of the boundary conditions in an infinite series of eigenfunctions and of a least‐squares approximation of this expansion by a finite series of the same functions. For simply connected bodies, such as a finite cylinder, that are not constant coordinate surfaces and for which a complete set of geometrically appropriate eigenfunctions is unavailable, the spherical eigenfunctions are employed as the complete set of eigenfunctions. Farfield patterns produced by radially pulsating cylinders of length equal to twice the diameter have been calculated for ka = 1,2, and 5 and are compared to the calculated farfield patterns of the physically similar situations of a finite‐length line source and a radially pulsating band of finite length on an otherwise inert, infinite cylinder. In addition, the radiation resistance and directivity index are calculated and compared for the finite and infinite cylinder sources.

Influence of Sound upon Local Heat Transfer from a Cylinder
View Description Hide DescriptionMeasurements far local heat transfer on a horizontal heated circular cylinder in a horizontal transverse standing sound field are reported. Observations were made by Schmidt's method, which makes use of optical properties of thermal boundary layers and requires no correction for thermal conduction or radiation. Significant local effects are found at intensities well below the “critical” sound‐pressure level. The “critical” level determined from average heat‐transfer measurements is found to have no intrinsic fluid‐mechanical significance.

Diffraction of Sound by an Array of Rectangular Reflective Panels
View Description Hide DescriptionAn investigation in an anechoic room of the diffraction of sound by rectangular panels, similar to panels that have been used for acoustical purposes in concert halls and other auditoriums, confirms certain expectations based on well‐known properties of both optical and acoustical diffraction: namely, (1) that, for wavelengths that are large as compared to the dimensions of the panels and the spacing between them, the incident sound on the panels is essentially all transmitted, and therefore for frequencies below a certain critical value, arrays of such panels are essentially nonreflective; (2) that, for wavelengths that are small, the panels reflect most of the sound incident upon them; and (3) that, for wavelengths that are of the same order of magnitude as the dimensions of the panels and spacings, there are complex diffractioneffects.

Detection of Markovian Sequences of Signals
View Description Hide DescriptionThe influence of constrained stimulus sequences on detection was studied in a two‐alternative temporal forced‐choice task with feedback. Three observers listened to a weak pure tone embedded in noise whose probabilities of occurrence and repetition in an interval were governed by a first‐order Markov process. Each observer listened to examples of each of nine different Markov chains. Results were (a) a single function relating detections to false alarms fitted individual sets of data well, in agreement with the theory of signal detectability, except (b) that detection was higher for more extreme repetition probabilities; (c) responses depended strongly on the previous stimulus with (d) the dependence being peculiar to a given chain; (e) detection probabilities increased during runs of signals in the same interval, yet (f) probability of detection on the first trial of a run in a given interval did not depend on the length of the preceding run in the other interval.

Ohm's Acoustical Law and Short‐Term Auditory Memory
View Description Hide DescriptionThe ability of listeners to “hear out” individual components of a tone combination was examined under a variety of experimental procedures. Contrary to the expectation of Ohm's acoustical law, listeners were relatively unable to accurately “extract” components from nonharmonically related tone combinations.

Circularity in Judgments of Relative Pitch
View Description Hide DescriptionA special set of computer‐generated complex tones is shown to lead to a complete breakdown of transitivity in judgments of relative pitch. Indeed, the tones can be represented as equally spaced points around a circle in such a way that the clockwise neighbor of each tone is judged higher in pitch while the counterclockwise neighbor is judged lower in pitch. Diametrically opposed tones—though clearly different in pitch—are quite ambiguous as to the direction of the difference. The results demonstrate the operation of a “proximity principle” for the continuum of frequency and suggest that perceived pitch cannot be adequately represented by a purely rectilinear scale.

Effect of Noise on Listening Levels in Conference Telephony
View Description Hide DescriptionThe present study outlines the effects of background noise on requested loudspeaker levels of incoming speech signals at the conference table for four basic listening assessments. Two general classes of loudspeaker systems were employed proximity units in the “wings” of individual high‐back chairs and a loudspeaker display above the long axis of the conference table. In addition, a brief outline of the effect of noise on vocal output has been included. The results show that for either loudspeaker system the preferred listening level under relatively quiet conditions (NC‐23) was 63 dB (A+B)/2 as measured at the ears of the observers. This is approximately the level of so called conversational speech at a meter. For a 37‐dB increase in the noise level (to NC‐60), the participants requested a 15.5 dB increase in the preferred listening level. This compares with a 13.5‐dB increase in the vocal output of the participants for the same 37‐dB increase in noise level. The range between the strain level (threshold of message intelligibility) and the maximum acceptable listening level for conference purposes was about 50 dB under quiet conditions but only about 20 dB at NC‐60. This rather rapid convergence stresses the importance of operating at a relatively low noise level in order to insure good intelligibility of all incoming voice signals.

Consistency of Identification of Consonants Heard in Noise
View Description Hide DescriptionFive measures of response consistency were obtained for three different sets of four consonants over a wide range of speech to noise ratios. The measures are related to uncertainty and variance statistics. Consistency measures were compared for two conditions of presentation: an identical set of utterances in which a given syllable and its noise background were both recorded and played back over the course of 12 replications of the experiment; and an equivalent set of utterances in which 12 separate readings of each utterance were recorded against independent noise backgrounds. Each of the measures demonstrated higher response consistency for identical than for equivalent sets of utterances.

Propagation of Acoustic Transients in Absorbing and Relaxing Liquids
View Description Hide DescriptionA study has been made of the propagation of pressure impulses approximately 0.1 μsec wide in liquids exhibiting viscousabsorption, i.e., constant α/ω^{2}, and relaxation effects. Computed variations of impulse shape as a function of path length are obtained by solving numerically the appropriate linearized wave equations. These are compared with experimental data obtained in such liquids as carbon tetrachloride, carbon disulfide, and acetic and propionic acids. Significant differences are noted in the variation of shape and the veocity of propagation of the impulse maximum in the two types of media.

Combined Effect of Shear Viscosity, Thermal Conduction, and Thermal Relaxation on Acoustic Propagation in Linear‐Molecule Ideal Gases
View Description Hide DescriptionAn acoustic‐propagation equation is derived for the case of shear viscosity,thermal conduction, and rotational relaxation in a linear‐molecule ideal gas that is fully excited rotationally. By assumption, the rotational relaxation is characterized by a single relaxation time but no assumption is made as to the magnitude of the absorption. The propagation equation is solved approximately, by truncated series, in the general case and in some special cases. Significant interaction appears only when the ratio of the fundamental rotational‐relaxation time to the fundamental translational‐relaxation time is of the order of unity. The special case of no thermal conduction is treated.

Ultrasonic Absorption and Dispersion in Monatomic Gases
View Description Hide DescriptionThe propagation of ultrasound of frequency ω through a classical, dilute, monatomic gas is studied in terms of the Boltzmann collision equation. The collision term is replaced by the relaxation model

Absorption of Sound in Air below 1000 cps
View Description Hide DescriptionFor many acoustic problems, it is of importance to have accurate data of absorption of sound in air as a function of humidity. Such information has not been available below 2000 cps. This paper presents data at lower frequencies in the range from about 125 to 1000 cps. The intensity attenuation coefficient m is given as a function of relative humidity at 20°, 0°, and −20°C for normal atmospheric pressure. Similar data are given at pressures of 400 and 200 mm Hg for a temperature of 20°C. The experimental results are compared with the theoretical values predicted by Kneser.

Attenuation of Acoustic Waves in Dilute Emulsions and Suspensions
View Description Hide DescriptionA study is made of the attenuation of acoustic waves by a suspension of fluid droplets in a fluid medium. Special attention is given to the case of small droplets for which the effect of the surface tension is not negligible. Both the droplets and the surrounding fluid medium are considered to be viscous and thermal conducting. The droplets are allowed to execute large translational motion and to undergo a small deformation from a spherical shape. It is shown that the result of Epstein and Carhart on the attenuation of sound waves in air with the suspension of water droplets is applicable even when the displacement of the droplets is large as compared to its radius. The effect of surface tension is to increase sound attenuation in a two‐phase medium by increasing the thermal dissipation. This effect is important in the suspension of gaseous bubbles in a liquid for small droplets and is negligible in the case of a gaseous medium containing liquid droplets. The explicit forms for the attenuation, the droplet displacement, the drag force on the droplet, and the heat‐transfer rate between phases are given for the case that is applicable to a gas containing the liquid and solid droplets. The expression for the attenuation that is applicable to the suspension of gaseous bubbles in a liquid is also given and is found to be completely dominated by the thermal dissipation.

Spherical Waves in Viscoelastic Media
View Description Hide DescriptionClosed‐form solutions to the propagation of spherical waves from an internal steady oscillating pressure source as well as to the transient response of a viscoelastic medium to a pressure impulse are presented. The assumption that the loss factors, quantities related to the dissipative property of a viscoelastic medium, are independent of frequency is used here for investigating the wave‐propagation phenomenon due to the action of an impulse. Numerical examples for the displacement and stress functions have been used to illustrate the damping effect of a viscoelastic medium by comparing with its special case—the elastic medium. It is found that this effect shows its significance with increased traveling distance. Also, in the steady‐oscillation problems it is of importance when the vibration frequency is high, while in the transient case it causes the decrease in the peak amplitude and also a time lag in the arrival of the wavefront.

Acoustic Field of a Circular Plane Piston
View Description Hide DescriptionKing's integral for the acoustic field of a circular plane piston in an infinite rigid baffle is recast into a form that has the product of two Bessel‐function operators (algebraic‐differential) acting upon the wavefunctione^{ikz} /(−ikz). Various exact or approximate formulations, previously known, can readily be derived. One of the two operators “displaces” a wavefunction given on the s axis of cylindrical coordinates to its correct value at (r,s). The other acts upon a spherical wave at (r,s), emanating from the origin of cylindrical coordinates, and transforms it to the piston expression.