Volume 26, Issue 3, May 1954
Index of content:
26(1954); http://dx.doi.org/10.1121/1.1907331View Description Hide Description
Several theoretical studies of the acoustic properties of homogeneous isotropic porous material have been presented in the past, and the results have been shown to be in good agreement with experiments. However, no corresponding satisfactory theoretical analysis of the commonly used perforated porous tile seems to have been given. In this paper this problem is treated in some detail, and from the resulting formulas and equivalent circuits the acoustic properties of the tile can be calculated in terms of the geometry and the properties of the porous material. As an example, the acoustic impedance and absorption coefficient have been calculated as a function of perforation depth for a typical tile, and the results are shown to be in good agreement with measurements.
Determination of Absolute Sound Levels and Acoustic Absorption Coefficients by Thermocouple Probes—Theory26(1954); http://dx.doi.org/10.1121/1.1907332View Description Hide Description
A detailed theoretical analysis of the operation of thermocouple probes used to determine absolute sound levels or acoustic absorption coefficients is presented.
The probe consists of a thermocouple imbedded in a sound absorbing medium which closely matches in density and soundvelocity the medium in which the sound level is to be determined. In use the transducer which generates the acoustic field is excited to generate sound pulses with a rectangular envelope. The initial time rate of change of the temperature at the thermocouple junction is determined. In addition to the measurement of the temperature change, the calculation of the absolute sound intensity requires only a knowledge of the absorption coefficient of the imbedding material and its heat capacity per unit volume at the temperature at which the measurements are made.
The theoretical discussion includes an analysis of the relation between the temperature rise at the junction resulting from absorption in the body of the imbedding medium and the sound level. In addition, the effects of (a) viscous forces arising from relative motion between the thermocouple and the imbedding fluid and (b) heat conduction between the thermocouple and the fluid, in contributing to the temperature change at the thermocouple junction are analyzed.
Based on the analysis, a set of design formulas is obtained which are summarized and illustrated for the convenience of other investigators who may wish to design and use such probes.
Determination of Absolute Sound Levels and Acoustic Absorption Coefficients by Thermocouple Probes—Experiment26(1954); http://dx.doi.org/10.1121/1.1907333View Description Hide Description
A stable, readily constructable thermocouple probe has been developed for determining absolute sound levels in ultrasonic fields in liquid media. This paper includes criteria for design of such probes and a discussion of experimental measurements made with such a device.
It is a consequence of the method that if the sound intensity at an appropriate size thermocouple junction imbedded in an absorbing medium is known the acoustic absorption coefficient of the material can be determined. The method thus makes possible the determination of absorption coefficients of minute quantities of material.
The probe consists of a thermocouple imbedded in a sound absorbing medium which closely matches in density and soundvelocity the medium in which the sound level is to be determined. In use the transducer which generates the acoustic field is exctied to generate sound pulses with a rectangular envelope. The initial time rate of change of the temperature at the thermocouple junction is determined. In addition to the measurement of the temperature change, the calculation of the absolute sound intensity requires only a knowledge of the absorption coefficient of the imbedding material and its heat capacity per unit volume at the temperature at which the measurements are made.
The experimental results include a comparison of the sound level determined by a thermocouple probe and a determination by radiation pressure methods. The values obtained by the two methods agree within the uncertainty of the experimental measurements.
26(1954); http://dx.doi.org/10.1121/1.1907334View Description Hide Description
Acoustically treated, doorless telephone booths can increase the effectiveness of telephone communication in noisy environments even though practical size and open area requirements limit the amount of noise reduction which can be achieved. The principal factors determining the noise reduction of an acoustically treated booth (where the noise reduction is considered as the decibel difference in octave band noise levels measured outside and inside the booth) have been investigated by use of full size and model booths.
The principal parameters affecting the reduction of an acoustically treated telephone booth were found to be (1) the sound absorption coefficients of the wall surfaces as function of frequency; (2) the ratio of open area to the wall area of the phone booth; (3) the transmission loss of the walls of the phone booth; and (4) the size and shape of the booth.
The investigation resulted in a new triangular design of booth which is fully as effective acoustically as the rectangular booths, yet has added practical advantages.
26(1954); http://dx.doi.org/10.1121/1.1907335View Description Hide Description
A theoretical analysis of the propagation of pressure waves through liquid filled flexible tubes is presented. Expressions are derived which show the dependence of phase velocity and damping factor on the viscosity of the liquid, and on internal damping in the tube wall. The analysis is restricted to tubes with thin walls and to waves whose amplitude is infinitesimal and whose wavelength is large compared to the radius of the tube.
26(1954); http://dx.doi.org/10.1121/1.1907336View Description Hide Description
The speed of a plane compression wave in a gross mixture (a suspension or a porous medium) which obeys a simple density mixture law is derived. The results agree with a relation proposed by Wood and others. The derivation of these investigators requires an addition to a density mixture law an assumption concerning the compressibility of the mixture in terms of the constituent compressibilities. It is shown that the latter assumption is a direct consequence of the density mixture law, and is hence not required. It is indicated that the derivation is applicable to waves of arbitrary amplitude.
26(1954); http://dx.doi.org/10.1121/1.1907337View Description Hide Description
A radially symmetric progressive acoustic beam has been set up in a closed, gas‐filled tube to cause streams driven by the Eckart, volume, source of verticity. A careful determination of the acoustic pressure at all points of a cross section of the beam has been used successfully to predict the stream velocities at this cross section, for argon. The work with argon and the results of further experiments with dry nitrogen and moist air indicate that: (1) the Navier‐Stokes equation, the equation of continuity, and the acoustic “equation of state” , provide a valid basis for second‐order studies of streaming; (2) thermal conductioneffects may be introduced into the Eckart streaming equations in the same manner as they appear in the attenuation formulas; (3) the macroscopic quantity, bulk viscosity, is made up of contributions from the intramolecular processes of vibrational and rotational relaxation.
26(1954); http://dx.doi.org/10.1121/1.1907338View Description Hide Description
The method described in Part III has been applied to another normal mode of a particular disk by combining the motions corresponding to the one imaginary and two real values of α. The residual stresses at the cylindrical surface of the disk are small, and the displacement distribution of the plane surfaces agrees reasonably well with that obtained experimentally.
26(1954); http://dx.doi.org/10.1121/1.1907339View Description Hide Description
26(1954); http://dx.doi.org/10.1121/1.1907340View Description Hide Description
Previous analysis of the equivalent circuit for the ultrasonic delay line was applied to quartz crystal transducers and a vitreous silica delay medium. The analysis has been extended to evaluate the use of other materials such as magnesium, aluminum, and steel, and to include ceramictransducers. Calculations show the decrease in loss to be expected as the impedance of the delay medium is decreased and conversely the increase in band width to be expected as the impedance of the delay medium is increased. The increase in band width resulting from symmetrical loading of the transducers is also evaluated. Modifications to the equivalent circuits for the case of ceramictransducers are considered. Theoretically losses can be reduced to 10 db or less using the ceramictransducers with a vitreous silica medium. The band width is slightly less than for quartz crystal transducers. However, by utilizing higher impedance materials, band widths approaching 100 percent should be achieved.
26(1954); http://dx.doi.org/10.1121/1.1907341View Description Hide Description
The acoustic noise spectrum associated with the collapse of transient cavities produced by a stirring rod in water was measured in the frequency range 1–3500 kc. While the spectrum, in general, goes as f −2 above 1 kc, flat regions are found between 20 kc and 100 kc, and again above 500 kc to at least 3500 kc. If the flat regions are due to discrete random impulses, two time constants, and , may be obtained by using the criterion 2πfτ≃1, where f is the −3 db point beyond the flat region.
With intense cavitation, the acoustic level fails off and becomes erratic in the 600‐kc region. Transmission measurements made through the cavitating volume show strong sound absorption, which appears to be caused by a cloud of small, stable bubbles.
26(1954); http://dx.doi.org/10.1121/1.1907342View Description Hide Description
The velocity of ultrasonic waves in sulfur hexafluoride (a symmetrical molecule with fifteen vibrational degrees of freedom) has been measured at frequencies of 251.22 kc and 1001.8 kc in the range from 0.38 to 15.89 Mc sec−1 atmos−1 at 36.10°C. The observed disperison in ultrasonic velocity is ascribed to the slow rate of energy exchange between translational and vibrational states. The dispersion region involves the disappearance of the whole of molecular vibrational energy, and corresponds to a singlerelaxation time. The relaxation time as derived from dispersion measurements is found to be 5.89×10−7 sec at one atmosphere corresponding to 4.68×103 a collisions required for energy exchange. The relaxation time for a sulfur hexafluoride mixture with 0.066 mole fraction of helium is found to be 5.46×10−7 sec.
26(1954); http://dx.doi.org/10.1121/1.1907343View Description Hide Description
The density and viscosity of the refractive media of the eye have been measured. The velocity of propagation for 5 mc sound waves is found to be 1497 m/sec for the aqueous humor, 1516 for the vitreous humor, and 1616 for the lens. The absorption coefficient was determined at 30 mc for the liquid part of the eye and at 3 mc for the lens.
26(1954); http://dx.doi.org/10.1121/1.1907344View Description Hide Description
The subjective relation “octave of” is demonstrated to be a valid interval for determining scales of musical pitch for pure tones presented successively. Octave judgments of trained musicians have a standard deviation averaging about 0.6 percent. Inter‐observer variability is 2 to 5 times as great, increasing with frequency. Judgments vary significantly from day to day, but as the direction of shift at different frequencies for a single observer is random, the shifts cannot be attributed to changes in the octave criterion. Instead, this variability, and also (1) differences between right‐ and left‐ear judgments of a given observer, (2) the change in difference between subjective and physical octaves as a function of frequency, and (3) the high inter‐observer variability, all confirm the basic instability of pitch‐frequency relations implied by the facts of binaural diplacusis.
Individual and group scales of musical pitch are deduced. In these scales, the average rate of change of musical pitch with respect to frequency level is less than unity by a small but significant amount. Although this discrepancy not explained, tests show that it is not an obvious artifact of method. Simultaneous presentation raises variability, but affects means only slightly. The relation betwen the peculiarities of individual scales and binaural diplacusis discussed.
26(1954); http://dx.doi.org/10.1121/1.1907345View Description Hide Description
A two‐component tone was presented under a number of frequency and intensity conditions. The loudness of each component and the total loudness of the combination were traced as the intensity of the higher frequency and were brought from masked threshold to well above masked threshold. Loudnesses were determined by equating the tone or tones under consideration to a comparison tone.
When the higher frequency of the pair was at or below 1000 cps, it showed a loudnessloss, relative to its loudness when presented alone, at medium intensities above its masked threshold; but as its intensity was increased, it tended to regain its normal loudness. When the higher frequency of a pair was above 1000 cps, it exhibited a greater than normal loudness at medium intensities above its masked threshold which again approached normal with increased intensity. The lower frequency of a pair seemed to be little affected by increases the intensity of the higher frequency within the limits of intensity used. Total loudness tended to be the algebraic sum of the components' loudnesses. The results indicate that the effect of a tone upon the loudness of another is not so simple a phenomenon as a tone's effect upon the threshold of another.
26(1954); http://dx.doi.org/10.1121/1.1907346View Description Hide Description
There is described an enclosure for a standard loudspeaker mechanism which provides a compact sound source or artificial voice. With a size slightly larger than a human head, the frequency range is 100 to 7000 cps. A simple and reproducible arrangement of damping felt gives unusual flatness.
This artificial voice can either be used in free space or with a small chamber arranged as an acoustic labyrinth, for testingmicrophones and hearing aids. In the chamber, calibratingmicrophone is mounted opposite the test microphone for continuous monitoring. Simple rc compensating networks are utilized to equalize the sound source for either free‐space or labyrinth measurements.
26(1954); http://dx.doi.org/10.1121/1.1907347View Description Hide Description
Twenty operators were given a task which required answering one of two simultaneous voice messages. The task was performed under a variety of conditions produced by combinations of four experimental “aid” variables: horizontal spatial separation of the sound sources, aural shaping filters which made the tone quality different in each channel, visual cues which indicated the channel about to call the operator, and facilities to “pull down” a desired message from the initial source into a headphone or a loudspeaker near the operator's ear. It was found that the use of horizontal separation and/or the filtered messages greatly improved the operator's performance. Visual cues had no apparent effect on ability to answer the message except when used with pull down facilities. The pull down facilities aided the operator to some extent. Some speculations are advanced about the nature of recognizing and attending to a message in the presence of another message.
26(1954); http://dx.doi.org/10.1121/1.1907348View Description Hide Description
Air control tower operators listened to test transmissions, consisting of speech intelligibility tests couched in airplane‐tower phraseology, over four channels. Part of the time the messages of two channels overlapped each other and the degree of overlap was systematically varied. The task was to respond to all messages.
The results showed a decrement in performance related directly to the amount of overlap, the leading message being understood better (unless it was considerably lower in intensity). For those parts of the message which had low informational content, the messages of the overlapping channel increased the total assimilated information per unit time above the total assimilated in the absence of the competing channel messages.
26(1954); http://dx.doi.org/10.1121/1.1907349View Description Hide Description
The effect of several factors upon voice identification was examined. These factors were: the size of the class of possible voices, the duration of the speech signal, the frequency range of the speech signal, voicing vs nonvoicing speech characteristics, and the simultaneous presentation of several voices. One of the most effective factors for speaker identification was the duration of the speech signal. Duration, as such, appears to be important, however, only insofar as it admits a smaller or larger statistical sampling of the speaker's speech repertoire.
26(1954); http://dx.doi.org/10.1121/1.1907350View Description Hide Description
The conventional sound spectrograph affords the analysis of short signal samples and considerable flexibility in the construction of amplitude sections. Other major interests in sound spectrography are in the instantaneous portrayal of signals, and in the permanent record display of continuous linear and logarithmic analyses. This paper suggests instrument methodology for carrying out these operations. Values which are considered practical are suggested for the parameters of certain of the systems which are discussed.