Volume 25, Issue 4, July 1953
Index of content:
- PROGRAM OF THE FORTY‐FIFTH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A. Sound Reproduction
25(1953); http://dx.doi.org/10.1121/1.1917647View Description Hide Description
Three distortion methods, the harmonic, the SMPE intermodulation, and the CCIF, a difference frequency, have been studied, mainly on a theoretical basis as a means of analyzing distortion encountered in phonograph reproduction. The harmonic method is somewhat difficult to apply and some of the harmonics may be beyond the range of the system. The IM has been used in the past with considerable success and appears to be a good method of analyzing tracing distortion. The difference frequency methods appears to be somewhat insensitive to tracing distortion but offers theoretical advantages in that the analysis may be made right up to the cut‐off frequency of the system; practically it is somewhat difficult to apply.
25(1953); http://dx.doi.org/10.1121/1.1917648View Description Hide Description
A combined sound level meter and analyzer has been developed for the U.S. Navy Bureau of Aeronautics, incorporating many novel features and meeting all pertinent ASA and military specifications. The main assembly, consisting of a combined sound level meter and octave band analyzer, is smaller and lighter than many ordinary sound level meters alone and is of a shape particularly adapted for placement in remote compartments of aircraft. This main assembly can be used as a complete sound level meter and analyzer by itself or it can be operated in conjunction with a remote control unit which can be mounted in the pilot's cockpit. All important functions of the sound level meter and analyzer can be controlled remotely from this unit, which occupies a volume of any 0.05 cubic foot. Although designed specifically for tests in single‐seater fighter aircraft, the new instrument's versatility and compactness make it applicable to a wide variety of uses. The development is unclassified, and the resulting equipment is available on special orders to commercial customers as well as the U.S. Government.
25(1953); http://dx.doi.org/10.1121/1.1917649View Description Hide Description
Residual mechanical stresses in press‐polished plastic film material used for embossed type sound recording are in many cases responsible for relatively high noise levels. Results of noise measurements of different plastic materials on a 400 grooves per inch recorder are presented. The relation between optically observed stresses and recorded noise level is given. Practical consequences are discussed.
25(1953); http://dx.doi.org/10.1121/1.1917650View Description Hide Description
A method is described whereby measurements of the acoustic impedance of small orifices and membranes are made by comparison to a known impedance. The equipment required consists of a high impedance and a low impedance source of sound, a sound level meter, phase angle indicator, and a few easily constructed impedance references. The method is analogous to the electrical one where a known impedance is compared to an unknown using a constant voltage or constant current source and measuring the current or voltage. Data on the acoustic impedance of various types of cloth and metallic screens and membranes are given. Experimental results of impedance measurements of two orifices for various separations as well as for different shapes of orifices is also shown.
25(1953); http://dx.doi.org/10.1121/1.1917651View Description Hide Description
Experimental data on the directional characteristics of a number of quasi‐cylindrical sector radiators vibrating radially are given. The variables considered are those of sector angle, height, and frequency. The exact pressure distributions over equiphase surfaces in the vicinity of several of the horns are shown. These data are useful in predicting the directional characteristics of other similar radiators having different height dimensions. The accuracy of this application is also shown by measurements made on a similar radiator.
25(1953); http://dx.doi.org/10.1121/1.1917652View Description Hide Description
The system displays the polar characteristic of a microphone upon an ordinary oscilloscope. The microphone rotor is coupled to a control unit outside the anechoic chamber by a pair of selsyns. Provision is made for manual positioning of the microphone from outside the chamber. The system has been used with low level, low impedance gradient microphones.
25(1953); http://dx.doi.org/10.1121/1.1917653View Description Hide Description
The reverberation room such as used for measurement of acoustic materials makes a very convenient laboratory for acoustic calibration of sound sources and microphones under random field conditions. The randomness of the sound field is maintained by having a large volume and by using rotating vanes, diffusing cylinders, and signals from a broad‐band noise or warble‐tone source. The total absorption of the sound in the room can be quickly measured for acoustic power determinations. Generally, only one reference microphone is needed, since the space variation of the sound field averaged with time does not exceed ±2 db above 100 cps and over most of the frequency region is less than ±0.5 db. Data will be presented on (1) the power and efficiency spectra of loud‐speakers, which check the free‐field integration within the error of free‐field measurements, (2) the random field calibration of microphones and sound level meters, (3) the measurement of the acoustic power spectra of noise sources such as fans, appliances, and automobile horns.
- Session B. Wave Propagation
25(1953); http://dx.doi.org/10.1121/1.1917654View Description Hide Description
A detailed review will be presented, summarizing available data on absorption of sound in homogeneous air and its main constituents as a function of frequency, pressure, temperature, and humidity. Disagreement exists between results of the various investigators, especially when different techniques are used. It is customary to compare experimentally observed absorption coefficients αexp with theoretical values αtheor, where the latter include the effects of viscosity,heat conduction,heat radiation,diffusion, and vibrational thermal relaxation. Observed values of r = αexp/αtheor vary from slightly less than unity up to about 4 for measurements in the laboratory and up to somewhat higher values for measurements out‐of‐doors. Even in carefully dried air, nitrogen, and oxygen, reported values of r vary over a wide range, depending on the measurement technique employed. It is interesting that in recent measurements, using direct source‐to‐receiver systems where the sound field is approximately a spherical wave,r values of about 1.2 are obtained. This approaches the result expected from classical kinetic theory for rotational relaxation.
25(1953); http://dx.doi.org/10.1121/1.1917655View Description Hide Description
The propagation of longitudinal elastic waves in a solid circular cylinder is a problem of long standing. If flexural and torsional waves are excluded, then it is found there are three distinct types of propagation. These three types are referred to as the Young's modulus, the shear type, and the dilatational type. For the last two types there are an infinite number of modes. The Young's modulus type of propagation is characterized by the fact that as the radius to wavelength ratio a/λE approaches zero, the velocity approaches for the zero mode. The shear type is characterized by the fact that as a/λs becomes large the velocity approaches . The dilatational type of propagation is characterized by the fact that as a/λd becomes large the velocity approaches . Only the zero mode for the Young's modulus type has been previously discussed. The higher modes for the shear and dilatational types exhibit cut‐off frequencies below which there is attenuated propagation. As the frequency approaches the cut‐off frequencies from above, the phase velocity becomes infinite and the group velocity approaches zero. As a/λE becomes large, the velocity for the Young's modulus type approaches the Rayleigh surface wavevelocity. The shear and the dilational types differs from the Young's modulus type in one significant way; there is no zero mode for the dilational type, so that unattenuated propagation does not exist below the lowest cut‐off frequency. Applications to ultrasonics, crystal systems, and noise isolation problems are discussed.
25(1953); http://dx.doi.org/10.1121/1.1917656View Description Hide Description
Starting with the nonlinear boundary layer equations for a viscous incompressible fluid, a perturbation calculation is carried through with the intention of investigating terms of third and fourth order. The investigation is motivated by the belief that these higher order terms might give a clue to the behavior of acoustic streaming flows as the amplitude of the exciting oscillations is increased. The calculations are rather laborious and involve several simple extensions of the perturbation technique. The resulting expression for the stationary flow is applied to several examples including the flow near a cylinder and is seen to indicate a reversal in the direction of circulating flow when the intensity reaches a certain level, in agreement with experimental observations.
25(1953); http://dx.doi.org/10.1121/1.1917657View Description Hide Description
Measurements have been made of the attenuation of high amplitude sound waves which have assumed the sawtooth‐repeated shock form. The range of sound amplitudes was from 0.08 to 0.30 atmosphere peak to peak. The fundamental frequencies used were between 400 to 1200 cycles per second. A siren was the source and a bariumtitanate movable probe microphone the receiver. Both 1‐ and tubes were used. When reciprocals of the sound pressure amplitudes are plotted against the number of wavelengths, straight lines result in accordance with the theory of Rudnick. The slopes are considerably less than those predicted by theory and this will be discussed.
25(1953); http://dx.doi.org/10.1121/1.1917658View Description Hide Description
The scattering of a sound wave in a medium undergoing shear flow confined to a finite region is investigated under the assumption that the total velocity field is everywhere small compared to the velocity of sound. Formulas are obtained for the angular distribution and frequency distribution of the scatteredwave in terms of the four‐dimensional Fourier transform of the shear velocity field. The cross sections for the scattering of a plane wave of frequency ω by a shear flow of given scale and spatial structure go typically as ω4 M 2, where M is a characteristic Mach number of the flow. The coupling between the shear and longitudinal velocity fields has a tensor character such that the scattering vanishes at 180° and at 90°. The spectrum of the scattered sound wave is very sharp in the forward direction and becomes broader at larger scattering angles. Explicit expressions for the cross sections are obtained for the case of scattering from a region of isotropic turbulence. When the frequencies of importance in the turbulence are small compared to the frequency of the incident sound wave, the average differential scattering cross section can be expressed directly in terms of the energy spectrum of the turbulence.
25(1953); http://dx.doi.org/10.1121/1.1917659View Description Hide Description
A fluid jet in a nonviscous incompressible medium is capable of sustaining a number of vibrational modes in analogy with electromagnetic waves in a wave guide. This paper enumerates and classifies modes that would be excited by small disturbances at the orifice of a circular jet. By means of perturbation theory these modes are shown to be of the form , where In and Kn are modified Bessel functions and Φ is the velocity potential. Physically these represent waves propagating down the jet which increase in amplitude from the source of the disturbance until jet rupture occurs and turbulence results.
25(1953); http://dx.doi.org/10.1121/1.1917660View Description Hide Description
Since the ray theory does not give the correct range dependence for the fluctuation of the intensity of radiation propagated in a medium in which the index of refraction varies randomly, a wave theory solution of the problem has been formulated. By means of the Born approximation to the wave equation, the coefficient of variation (ratio of standard deviation of pressure amplitude to mean pressure amplitude) has been evaluated. Using Liebermann's data, the results are compared with Sheehy's experiments, and excellent agreement is found. A discussion of the ray theory will also be given.
25(1953); http://dx.doi.org/10.1121/1.1917661View Description Hide Description
In the treatment of the propagation of radiation in an inhomogeneous medium bounded by a rough surface, it is frequently necessary to consider both reflection and refraction effects. A method of treating such problems was presented at the meeting of the Acoustical Society at San Diego last fall. By using a Laplace transform technique it has been possible to obtain information on a wide variety of problems of experimental interest. One of the outstanding characteristics of the solution is that in many cases the attenuation of the field is but weakly (logarithmically) dependent on the surface parameters. This may explain why it has been so difficult to demonstrate the dependence of attenuation in acoustic experiments upon the state of the sea surface. The predicted attenuations at large ranges for various sea surface models will be presented. In general, they show that the attenuation is reduced for surfaces which scatter more energy into rays making larger angles with the surface (but which still remain trapped within the surface layer).
- Session C. Hearing
Recent Developments in the Selection of Candidates for Training in the Discriminative Detection of Auditory Signals25(1953); http://dx.doi.org/10.1121/1.1917662View Description Hide Description
The uncanny ability of some musicians in detecting and differentiating acoustic signals has been frequently noted. The present study reveals that attitude toward music, even in the layman, can likewise play an important role in the selection of personnel for such tasks. Furthermore, it is shown that certain other vocational interests are highly correlated, both positively and negatively, with that of the musician. Intelligence and personality factors are given due consideration. Eighteen such vocational fields, as well as intelligence and personality factors, have been found to be significant at the one percent level, and fifteen more at the five percent level. Evaluation and interpretation of these findings point toward new sources of candidate material to be used in the discriminative detection of auditory signals.
25(1953); http://dx.doi.org/10.1121/1.1917663View Description Hide Description
The purpose of this study was to explore the variability of the auditory threshold under experimental conditions, for comparison with the known variation in repeated clinical audiometric tests of the same subject. The literature on the reliability of audiometric methods is surveyed briefly. An experiment is reported in which the absolute threshold for pure tones of 1000 cycles per second was studied. Nine normal ears were studied in one to five sittings each. Each sitting involved ascending and descending presentation of the stimulus tones. Intensity was varied in one decibel steps. The subject was required to signal whether he heard each tone. The proportion of “yes” responses at each intensity level did not differ significantly between ascending and descending series. Psychometric functions were constructed by fitting the phi‐gamma function to the data. Functions were computed for each sitting of each subject and for all sittings of each subject. No significance could be attributed to differences among the sittings of any subject. A psychometric function was computed which represents the total subject group. This function is the cumulation of a normal distribution with a standard deviation of 6.72 db. From this function a theoretical distribution of the average results of the type of series used in clinical audiometry is derived. The standard deviation of this distribution is 5.57 db for either ascending or descending audiometric series. The dispersion of audiometric tests of the same subject as reported in the literature is in reasonable agreement with the theoretical prediction, if allowance is made for the use of several series at each frequency in the clinical test. It is concluded that the fitting of the phi‐gamma function is a satisfactory procedure for the study of the psychometric function for absolute auditory thresholds. This function (for a single subject) does not vary significantly over a period of several weeks. It is concluded also that the variation in clinical audiometric tests of the same subject is attributable to the variability of the subject's sensory behavior, rather than to the nature of the clinical test. The variability which has been observed over periods of several months is demonstrable in much shorter periods.
25(1953); http://dx.doi.org/10.1121/1.1917664View Description Hide Description
In the problem of identifying speechsounds by means of measurable acoustical parameters, it appears that no single parameter can give a reliable identification. Hence, the statistical techniques of discriminatory analysis are appropriate to determine the optimum combination of several parameters. These techniques have been applied to the identification of vowels, where the parameters used are those fractions of the total power which are contained in certain selected frequency channels. For these parameters, a statistical distribution related to the multinomial has been assumed. This leads to a simple linear classification function whose coefficients are the logarithms of the mean values of the respective parameters. Preliminary results have been obtained which show considerable success in identifying vowels.
25(1953); http://dx.doi.org/10.1121/1.1917665View Description Hide Description
Using earphone presentation and electrical lag lines which allow precise control of time delays, measurements were made of interaural time difference thresholds for pure tones and noise bands. The time difference threshold is here defined as the minimal change in interaural time difference which elicits a perceptible shift in the location of the sound image. Results present the discrimination threshold for interaural time delay as a function of the psychophysical method of measurement, band width and center frequency of the signal. Under optimum conditions, the mean interaural time difference threshold for the five best listeners is about six microseconds. Under similar conditions, the mean threshold for 23 listeners is about 12 microseconds.
25(1953); http://dx.doi.org/10.1121/1.1917666View Description Hide Description
It is generally known that, except for the difference in frequency characteristic, bone‐conducted sound produces the same sensation as airborne sound. On the other hand, the transmission of acoustic vibration to the cochlea is quite different. The air‐borne sound enters through the oval window and the bone‐conducted sound causes compressions and dilatations of the whole inner ear. It does not seem obvious that the vibration pattern of the cochlear partition is the same in both cases; yet Békésy has shown that bone‐conducted sound can be entirely compensated by airborne sound. This indicates that the behavior of the cochlear partition is the same, independent of the way in which the sound is introduced into the inner ear. Several investigators have used Békésy's findings, which have been confirmed in other experiments, to draw conclusions regarding the mechanics of the cochlea. The classic resonance theory and the so‐called telephone theory have seemed to provide the best explanation; however, these theories disagree with other experimental results. Consequently there has been no completely satisfactory solution to the problem of cochlear behavior in bone conduction. It is shown in the present paper that the cochlear partition vibrates in the same way whether stimulated by airborne sound or by bone‐conducted sound, independent of the factors determining the pattern of vibration. The cochlear partition behaves in the same way even when the vibration is delivered through an artificial opening placed somewhere along the wall of either the scala vestibuli or the scala tympani.