Volume 20, Issue 3, May 1948
Index of content:
20(1948); http://dx.doi.org/10.1121/1.1906367View Description Hide Description
A number of special techniques are described which are necessary in preparing the cochlea. Using these, the elasticity of the cochlear partition is determined as a function of locus along the canal. It is demonstrated that the production of undertones is very improbable because of the form of vibration in the cochlear partition and because the basilar membrane possesses no inherent stress.
20(1948); http://dx.doi.org/10.1121/1.1906368View Description Hide Description
An experimental hearing aid, two representative commercial models, and a high fidelity audio‐amplification system were explored in a series of systematic articulation tests with hard‐of‐hearing subjects as listeners. The experimental hearing aid was designed to determine to what extent certain desirable design objectives could be incorporated in a portable hearing aid. A technical description of the experimental instrument is given in an appendix. The paper describes the experimental methods and testing equipment employed and summarizes the results in the form of articulation curves obtained from six hard‐of‐hearing listeners. The method adequately brings out performance differences in the four instruments and shows the superiority of the experimental hearing aid over the commercial instruments for most subjects.
20(1948); http://dx.doi.org/10.1121/1.1906369View Description Hide Description
The effect on intelligibility of eliminating either the high frequency speech sounds or the low frequency speech sounds was determined by standardized articulation testing procedures under the special circumstance of a background of white masking noise. In general, it was found that intelligibility increased as the frequency range and the intensity level of the speech signal were increased. Contours are presented which describe the interrelations among (1) the frequency range of the speech signal, (2) the intensity level of the speech signal, and (3) articulation efficiency.
For each of the experimental conditions, the articulation index—a measure developed by the Bell Telephone Laboratories—was computed. It was found that the scatter of the computed indices, when plotted against the scores actually obtained, was of the same order of magnitude as the intrinsic variability of typical articulation scores. This result indicates that the computational procedures are an acceptable substitute for time‐consuming articulation testing when an approximate assessment of the relative efficiencies of a series of communication systems is required and when a background noise is encountered like the white noise of this study.
It was found that the relative contribution of the various speech frequencies was not constant, but rather was a function of the intensity of the speech signal relative to the constant white noise masking signal. More specifically, as the intensity level of the speech signal was increased, the relative contribution to intelligibility of the higher speech frequencies increased.
20(1948); http://dx.doi.org/10.1121/1.1906370View Description Hide Description
This paper gives the results of measurements of the absorption coefficients of panels made from birch plywood, using the standardized test procedure of the Acoustical Materials Association. In the range from 128 to 512 cycles measurements were made at interoctave frequencies in order to take account of the marked resonances shown. Effects of thickness of the plywood, of the air space back of the panels, and of absorbent backing were investigated, as well as that of damping material applied to the plywood. Measurements were also made on splayed assemblies of panels placed vertically against the wall of the reverberation chamber. The measurements showed that prefabricated portable panels of the type investigated afford an easily applied means of varying the absorption‐frequency characteristics of rooms of moderate size.
Secondary Pressure Pulses Due to Gas Globe Oscillation in Underwater Explosions. I. Experimental Data20(1948); http://dx.doi.org/10.1121/1.1906371View Description Hide Description
Pressure waves emitted by the oscillating gas globe in underwater explosions of T.N.T. have been recorded at depths great enough to render small the perturbation effects due to migration of the bubbles under the influence of gravity. The first eight periods of oscillation have been measured and the pressure‐time curves analyzed in order to obtain peak pressure, positive impulse, and energy flux resulting from the first two pulses. The scaling of pressure pulse parameters with charge size is examined.
Secondary Pressure Pulses Due to Gas Globe Oscillation in Underwater Explosions. II. Selection of Adiabatic Parameters in the Theory of Oscillation20(1948); http://dx.doi.org/10.1121/1.1906372View Description Hide Description
A summary is given of the theory of pulsation of a stationary gas globe in an infinite incompressible fluid. If three parameters appearing in the formulation are selected by fitting the equations to three independent experimental results, it is shown that the theory fits the remaining experimental results and can be used to predict bubble pulse properties over wide ranges of the independent variables.
20(1948); http://dx.doi.org/10.1121/1.1906373View Description Hide Description
The greater part of the absorption of sound in aqueous suspensions of small spherical particles can be attributed to the viscous drag between the fluid and the particles in the sound field. The absorption resulting from this process is found to agree with that obtained by Lamb in another manner. The applicability of the theoretical result to suspensions of irregular particles is examined by means of measurements on sand and kaolin suspensions, using a pulse‐reflection method at megacycle frequencies. Approximate agreement with the idealized theory is found as the particle size, viscosity, and frequency are varied.
20(1948); http://dx.doi.org/10.1121/1.1906374View Description Hide Description
A description is given of a radial vibrator used in underwater sound signaling. A technique is presented for providing a plastic cast to protect the windings which serves to improve rather than decrease the potential efficiency. Data are presented to show the operating characteristics of these sound sources.
20(1948); http://dx.doi.org/10.1121/1.1906376View Description Hide Description
A common method of making transducers with broad frequency responses is to use ensembles of resonant subelements of slightly different frequencies. The theory, design, construction, and tests of a transducer of this type are presented in this paper. Six subelements of three different natural frequencies about 6 percent apart were used. The subelements consisted of magnetostrictive laminated nickel stacks, which were mechanically isolated from each other but linked by a common electrical winding. The relationships between the amplitude and phase responses of the unit as functions of frequency were determined experimentally and compared with the theory. The efficiency of transformation of energy was determined for the frequencies of maximum response. All the characteristics of the stepped‐frequency transducer are compared to those of a less resonant, single‐frequency transducer of comparable breadth of frequency response. It is found that the stepped‐frequency transducer can be no more efficient than its single‐frequency equivalent, and that its amplitude and phase response characteristics average about the same as the single‐frequency equivalent but are more irregular. In the stepped‐frequency transducer there are two frequency intervals in which the phase may remain stationary or actually decrease with an increase in frequency, but these intervals occur at the amplitude response minima. It is concluded that in practical transducer design slightly resonant single‐frequency elements are superior to those made of ensembles of more resonant subelements of several different frequencies.
20(1948); http://dx.doi.org/10.1121/1.1906377View Description Hide Description
An experimental method has been developed for measuring the reflection coefficient of the ocean bottom for supersonic sound at grazing incidence. In this method the geometry is controlled so that the resultant of the direct and reflected rays passes through consecutive maxima and minima at the hydrophone. The amplitude reflection coefficient is given by the expression , where M is the amplitude of a maximum and m is the amplitude of the following minimum. Using this method, with a grazing angle of nine degrees, the following values were obtained for the reflection coefficients at 17 stations near Woods Hole: mud—0 to 0.20, mud‐sand—0.20 to 0.40, sand‐mud—0.40 to 0.60, sand—0.40 to 0.85, stony—0.50 to 0.85. It is observed that the reflection coefficient depends not only on the median size of the particles making up the bottom, but also on the sorting of the particles and on the bottom topography.
20(1948); http://dx.doi.org/10.1121/1.1906378View Description Hide Description
A periodic point source in medium 1 is at the center of a spherical shell of medium 2 which is surrounded by medium 3. The densities, sound speeds, and shell radii are arbitrary. For this case an exact, explicit expression for the sound pressure is obtained. If the shell radii become infinite but the thickness remains constant, and if media 1 and 3 are the same, the solution reduces to that of Rayleigh for plane waves normally incident on a flat plate. If the shell radii become infinite and if media 2 and 3 are the same, the solution yields the acoustic Fresnel formulae for normal incidence of a plane wave on a plane interface between two half‐infinite media. For finite radii, with media 1 and 3 identical and the shell thickness small compared to a wave‐length, a simpler form for the solution is obtained. This solution is found to agree with the approximate solution of H. Primakoff and J. B. Keller for the sound fields reflected and transmitted by a thin shell of any shape, when their result is specialized to the present case.
20(1948); http://dx.doi.org/10.1121/1.1906379View Description Hide Description
The paper illustrates certain techniques common to acoustics and supersonic aerodynamics. The basic equations describing hydrodynamical phenomena taking place in a supersonic, inviscid stream are developed. After linearization, the equations are solved for the velocity potential resulting from an arbitrary source distribution in the plane z − 0. The integration of the results for two‐dimensional flow is carried out, and the application to various aerodynamic problems is cited. As a second illustration, an analogy, attributable to von Kármán, between two‐dimensional flow caused by a distribution of acoustic oscillators along a line having arbitrary time histories and the steady three‐dimensional flow about a thin wing is described. Using the techniques of Fourier integrals, this analogy is used to compute the wavedrag caused by a symmetrical wing at zero lift.
20(1948); http://dx.doi.org/10.1121/1.1906380View Description Hide Description
This paper discusses the dynamical theory of the impact of a felt hammer upon a string at a distance α from one fixed end such that the reflected wave from the other distant fixed end does not reach the striking point during the time the hammer is in contact with the string. The Heaviside operational method leads to a solution in the form of a series consisting of incommensurable terms with damping coefficients increasing with the order of the term and tending to a limiting value. In the case of a hard hammer the damping coefficients decrease, and the terms are very much less incommensurable. These are the essential differences between the two cases that lead to smoothening down of the pressure and time curve between the hammer and the string in the case of a felt hammer.
20(1948); http://dx.doi.org/10.1121/1.1906381View Description Hide Description
Dedicated to the late O. M. Corbino in 1939, this institute lost, during the war, its original building and most of its facilities, which included newly constructed reverberation and free‐field rooms for large‐scale acoustics research. Most of the comprehensive acoustics library was salvaged, together with some pieces of equipment. Operations were renewed in 1945 in a portion of the Italian National Research Council building at Piazzale delle Scienze, Rome. The group numbers about ten research scientists, some of whom are members of the staff of the University of Rome, and about ten service personnel. The program features mainly ultrasonics, vibration studies of solids, and acoustic instrumentation.
The ultrasonics program is characterized by the exploitation of light diffraction methods for measurement. In some recent developments, the usually employed standing waves between a quartz crystal and a reflector have been replaced by a pair of oppositely directed parallel beams of progressive waves from two quartz plates driven at the same frequency by a common source. The new device is free from the high precision requirements for parallelism and operates continuously over a fairly wide range of frequencies without mechanical adjustment.
Ultrasonic velocity and absorption in several liquids and liquid mixtures are being studied from 1 to 10 megacycles, as a function of temperature. Velocity is now determined by measurement of wave‐lengths in a progressive beam which is illuminated stroboscopically by the two‐crystal cell described above. For absorption measurements, a monochromatic light line is passed through a single progressive ultrasonic beam and masked to allow a single‐order diffraction line to fall on a photo‐cell. The measured light intensity is proportional (within certain limits which can be checked) to the ultrasonic intensity which can then be measured as a function of distance to determine absorption.
Dynamic properties of wood and metal are being studied by vibrating small samples and measuring the elastic and damping coefficients from resonant behavior. The basic tool is a condenser which drives the sample by electrostatic force and which also frequency modulates a high frequency carrier to provide a measure of vibration amplitude. The output is presented oscillographically to display force and displacement on the two axes of the scope. Samples are suspended on fine stretched wires and driven in transverse or longitudinal modes of vibration. These simple and effective means for studying internal damping and elasticity as dependent upon composition and temperature should prove useful in solid‐state physics studies.
Instrumentation studies include tube resonance methods of acoustic impedance measurement, frequency dividers, and multipliers for stable sound and ultrasonic sources, earphone testing, and microphone calibration.
The I.N.E.A.C. has issued some eighty reports and papers, which have appeared in Ricerca Scientifica, Alta Frequenza, Atti dell 'Accademia Nazionale dei Lincei, Journal of the Acoustical Society of America, and other journals. A Bibliography, annotated with abstracts or summaries of several of the papers, is included in this report.
- LETTERS TO THE EDITOR