Index of content:
Volume 27, Issue 3, May 1955
27(1955); http://dx.doi.org/10.1121/1.1907916View Description Hide Description
The motion of the gas in and near a tube open at one end and closed at the other by a piston driven in nearly sinusoidal motion is investigated. Experiments disclose that when the frequency of the piston motion is near the fundamental acoustic resonance frequency of the gas column an intermittent air jet associated with a system of vortex rings is expelled from the tube's open end. This results in an appreciable average thrust on the tube which, however, is much less than might be expected from purely acoustic considerations. A theoretical analysis has been made which takes account of the intermittent vortex and jet formation, as well as of the acoustic radiation field. The analysis leads to formulas for the average thrust and power dissipation and to a relation between the driving frequency and the amplitude of the gas motion at the open end. An empirical parameter κ which represents the fraction of energy dissipated in vortex motion appears in these formulas. It is found possible to evaluate this parameter in terms of the observed thrust at resonance. From this datum the curve of thrust versus driving frequency is calculated and is found to agree well with experiment.
27(1955); http://dx.doi.org/10.1121/1.1907918View Description Hide Description
Recent experimental studies revealed a close connection between spontaneous excitation of organ‐pipe oscillations by flames propagating in tubes and flame front instability characterized by cellular structure. On the basis of these observations a dominant mechanism of vibratory flame movement is proposed, consisting of the following two steps:
1. As a consequence of acceleration instability, the oscillation of the gas column alternatingly distorts and flattens the flame front, thus creating the observed vibration‐induced cell structure.
2. The distortions of the flame front cause in turn periodic changes of flame surface area and therefore of burning rate. Spontaneous oscillations are excited if the phase relation between burning rate variation and pressure oscillation is favorable.
In this paper only the first step is analyzed. The linearized treatment yields stability limits in terms of dimensionless parameters representing amplitude and frequency of gas column oscillation and wavelength of flame front distortion. The properties of the flame front entering into the analysis are the ratio of densities of unburned and burned gases, the burning velocity of the plane flame front, and a characteristic length that determines its stability in the absence of oscillations. Results in qualitative agreement with experimental observations are derived.
27(1955); http://dx.doi.org/10.1121/1.1907920View Description Hide Description
The sound fields induced in a real gas by boundary temperature variations are examined to illustrate how the pressure, temperature, and vorticity modes of motion interact. The pressure mode describes the irrotational propagation of longitudinal disturbances, the temperature mode, convective heat diffusion, and the vorticity mode, the diffusion of vorticity introduced at boundaries by the no‐slip condition (boundary layer). The plane sound wave due to an instantaneous wall temperature rise is a sharp pulse, traveling at sonic speed and proportional to the inverse fourth root of the acoustic Reynolds number based on distance traveled; its thickness grows as the square root of elapsed time. Along a wall, it generates a boundary layer whose friction coefficient is proportional to the inverse square root of the acoustic Reynolds number based on distance from the front. The resulting effective wall slope induces a secondary circular pressure pulse generated by the foot of the plane wave; because this source moves at sonic speed, a pressure singularity appears there, due to the contributions piled up since the beginning of the motion.
27(1955); http://dx.doi.org/10.1121/1.1907922View Description Hide Description
Some sample experimental results relating to the nature and sources of noise in jets are presented. Previous work is extended to include more detailed information about the jet structure. In this regard noise data are compared for jets having somewhat different turbulence profiles. High temperature jets are noted to have different radiation patterns and to be more efficient noise radiators than low temperature jets.Noise data for the high temperature supersonic jet of a rocket engine are included for comparison with those of subsonic jets. Some experiments have shown the noise levels at the low frequencies can be markedly reduced by the use of grids in the jet stream.
27(1955); http://dx.doi.org/10.1121/1.1907924View Description Hide Description
The extension of classical statistical mechanical methods to non‐Hamiltonian systems is discussed. The statistical equilibrium of a confined adiabatically compressible fluid is examined on the basis of a generalized phase space representation, and it is found that there exists a tendency toward equipartition of energy between compressive and vorticity modes which is modified by the viscous dissipation forces. The application of the method to determining nonequilibrium energy transfers between acoustic modes and vorticity modes is discussed.
27(1955); http://dx.doi.org/10.1121/1.1907926View Description Hide Description
An approximate method is developed for the estimation of the acoustic power frequency spectrum of the soundgenerated from isotropic turbulence. The method is based on a hypothetical model for the sound sources, originally produced by Lighthill, consisting of an assembly of quadrupoles extending over the region of turbulence.
27(1955); http://dx.doi.org/10.1121/1.1907928View Description Hide Description
An analysis of sound production by vortices in air jets is given. Specifically, the flow of air through sharp‐edged orifices is considered, and it is shown how the results of Anderson on the excitation of jet‐tones under these conditions may be explained. The analysis also covers excitation of resonant cavities by the jet, a phenomenon giving rise to pipe‐tones.
27(1955); http://dx.doi.org/10.1121/1.1907929View Description Hide Description
This paper presents some recent data on the acoustical behavior of certain components of engine test cell structures. These data include information on (a) the directivity patterns of vertical stacks and (b) the noise reduction provided by some water‐spray systems, (c) the Soundstream absorber, and (d) a 90° vertical bend.
27(1955); http://dx.doi.org/10.1121/1.1907931View Description Hide Description
It is difficult to ascertain the absolute loudness of typing noise by objective measurement. However, it may be practical to use such measurement to determine the loudness level difference in a comparison of the sounds from two typewriters. The requirements that must be met by an accurate measurement technique depend on the degree of accuracy required. Information pertaining to the accuracy necessary has been obtained by employment of two loudness matching procedures over a period of several months. The purpose of the tests was to determine the difference in sound‐pressure level of typing noise necessary to enable a person to identify reliably the louder of two typewriters operated simultaneously in a quiet background. It was found that under the conditions adopted for the investigation, differences in sound‐pressure level had to be nearly one decibel before they could be recognized 50 percent of the time.
Characteristics of typing noise other than pressure level are discussed. The opinion is expressed that these will, in many cases, not cause significant measurement error, and that difference in loudness between typing noises can often be measured with practical accuracy by means of a sound‐level meter.
27(1955); http://dx.doi.org/10.1121/1.1907933View Description Hide Description
Description of the development of an earplug with soft elastic walls and a viscous core malleable at body temperature. The problems of suitable material, shape, and size are considered as well as the shape and size adaptability. People have found these earplugs sufficiently comfortable to sleep with, and they ensure a sound attenuation equal to or better than any known earplug. One size fitted 90 percent of the ears of the 25 people tested sufficiently well.
27(1955); http://dx.doi.org/10.1121/1.1907935View Description Hide Description
To study the prevalence of heating losses among submariners, and their possible causes and consequences, the NEL warble tone hearing test was administered to 1053 submariners. Heating losses of less than 7 db at 500 cps and 5 db at 1000 cps could not be measured due to ambient noises in the testing areas. At the other test frequencies (2000, 4000, and 7000), any positive value of hearing loss could be measured.
It was found that approximately 10 percent of all men tested had high frequency heating losses of 18 db or greater (at 4000 and 7000 cps). A comparison of the hearing losses of enginemen to those of all others as a function of time on boat, for each frequency, showed that in general the enginemen had greater losses at all frequencies although there were minor reversals. The amount of loss was slightly less than expected from previous findings on industrial workers.
27(1955); http://dx.doi.org/10.1121/1.1907937View Description Hide Description
Stimulus variables influencing the fusion of interrupted white noise have been re‐examined in a single trained observer using repetition rate as the independent variable. Critical auditory decay times have been calculated for all observed fusion thresholds under the assumption that the cue for the perception of minimal flutter is a decay of one detectable step in the loudness of the interrupted signal during the off interval. These critical decay times are satisfactorily described by the value 120 milliseconds (independent of intensity) when the burst duration in the interrupted signal is longer than a critical value (5 milliseconds for this listener), but for shorter burst durations the decay times are much lower and exhibit a relationship to burst duration and intensity. A shift in the population of neural units activated under the latter conditions is postulated as an explanation.
The range of individual differences in flutter fusion thresholds under standard conditions has been examined by the use of a tape recorded test requiring repeated discriminations of interrupted from continuous noise. From a sample of 46 normal individuals a population of threshold scores may be postulated having a mean critical fusion frequency of 82.1 interruptions per second with a standard deviation of 19.4 when the interrupted noise has a sound‐time fraction of 0.90 and an intensity of 70 db re 0.0002 dyne/cm2.
27(1955); http://dx.doi.org/10.1121/1.1907939View Description Hide Description
The accuracy of discrimination of small changes in sound level was examined as a function of the magnitude, temporal, and dimensional aspects of the variation between two reference testing conditions. Precision of sound level discrimination declined as: the magnitude of the sound level difference between the two reference conditions increased and as the frequency of occurrence of shifting between the two reference conditions increased. Of lesser importance was the temporal interval between successive discriminations, variation in the audio‐frequency of the reference signals, and the predictability of the to‐be‐presented testing conditions. The results are considered in terms of a preliminary analysis of psychological dimensions in auditory discrimination.
27(1955); http://dx.doi.org/10.1121/1.1907941View Description Hide Description
The auditory thresholds of the cat have been obtained by the conditioned response method. Tests were made for frequencies from 62.5 to 60 000 cps. The upper limit of the cat's hearing is in the neighborhood of 60 000 cps. A comparison of threshold curves for cat and man suggests that the cat may be slightly less sensitive than man for frequencies of 500 cps and below. Sensitivity for the range 500 to 4000 cps is almost the same. For higher frequencies the hearing of the cat is distinctly superior to that of man.
27(1955); http://dx.doi.org/10.1121/1.1907943View Description Hide Description
A set of parameters that yield a simple yet reasonably accurate description of the articulation of vowel sounds is developed. The articulatory description is potentially useful in a speech band‐width compression system based on the coding of articulatory data. The parameters give information on the position of the tongue constriction, the size of the constriction formed by the tongue, and the dimensions in the vicinity of the mouth opening. An electrical analog of the vocal tract is utilized to obtain experimental relations between the articulatory parameters and the formant frequencies. Contours of vowel articulation are derived from these data. The relation of the contours to classical phonetics is discussed.
27(1955); http://dx.doi.org/10.1121/1.1907945View Description Hide Description
Recorded nonsense syllables (CVC) were fed to the headsets of listeners via two channels simultaneously, a direct line and a line that introduced controlled amounts of delay from 0.130 to 0.33 second. The amount of delay was varied in 0.03‐second increments and the effect of this variable upon the reception of the syllables was measured. Although the intervals of delay affected the reception scores differentially, no time of delay in the double presentations yielded intelligibility scores that were higher than the scores obtained through a single, direct playback of the stimuli.
27(1955); http://dx.doi.org/10.1121/1.1907947View Description Hide Description
27(1955); http://dx.doi.org/10.1121/1.1907949View Description Hide Description
A unifying principle is presented whereby all finite lumped systems can be analyzed by a single technique, no matter how complex the system may be. This central principle is the existence of a linear graph which is isomorphic to the dynamical properties of the elementary parts of the system and which shows how the parts are connected together.
An analysis of a lumped system begins with a conceptual decomposition of the system into a number of simple parts, each of which has known dynamical properties. It is shown that if two types of scalar variables can be found such that (1) they define, with the aid of system parameters, the dynamical properties of each part of the system, (2) the first type obeys the MESH LAW, (3) the second obeys the INCIDENCE LAW, and (4) physical dimensions of appropriate products of the two types are consistent, then there exists a linear graph isomorphic with the dynamical properties of the system as described by the variables so selected. Trivial additions to the isomorphic graph yield a schematic diagram for the system. It is then shown that the algebraic properties of the schematic diagram can be used to generate in a straightforward manner a set of equations, on either the nodal or mesh basis, describing the dynamics of the system as a whole. The isomorphic graph is also the key ingredient in establishing complete analogy. Formal procedures for establishing complete analogies are presented.
27(1955); http://dx.doi.org/10.1121/1.1907951View Description Hide Description
An analogy is developed between the paths of sound rays in fluids undergoing shear flow and electron trajectories in magnetic fields. If the Mach number of the flow is small, and the characteristiceddy size large compared to the sound wavelength, the ray paths coincide with electron trajectories in a magnetic field everywhere parallel and proportional to the vorticity vector. The ratio of the magnitude of the magnetic field to the electron momentum is determined by e H/Pc 0 = ψ/c where H = magnetic field,P = electron momentum, e = electron charge, c 0, c = velocities of light and sound, ψ = vorticity. Mathematically the analogy is implied by the similarity between the eikonal equation for the sound rays and the Hamilton‐Jacobi equation for the electrons. Simple illustrations of the correspondence are given.
27(1955); http://dx.doi.org/10.1121/1.1907953View Description Hide Description
This paper reports the results of measurements of the attenuation and velocity of ultrasonicwaves in various liquid media as a function of the intensity level at the detector of the radiation. The attenuation in distilled water and dilute solutions of acetic acid and sodium acetate is found to increase almost linearly with the acoustic excess pressure amplitude in the range from 2 to 7 atmospheres, for frequencies of 4, 5, 7, and 9 megacycles/sec. The attenuation in glycerine shows a similar increase when correction is made for the unavoidable rise in temperature of the medium. The rate of increase is found to be practically independent of frequency in the range employed, thus disagreeing with the prediction of R. D. Fay's theory of the attenuation of high‐amplitude plane waves. No change of velocity with source intensity has been observed.