Volume 30, Issue 12, December 1958

Signal Detection as a Function of Frequency Ensemble. II
View Description Hide DescriptionThis paper presents the results of a second experimental investigation of the detection of a signal in noise as a function of signal ensemble size and ensemble frequency range. Data are presented for the three observers who took part in the experiment. The results obtained are compared with (1) predictions based upon three mathematical models, (2) performance expected of a mathematically optimum detector, and (3) results obtained in the first of these investigations.

Effect of Auditory Cue on Discrimination of Auditory Stimuli
View Description Hide DescriptionThis report presents the results of an investigation dealing with (1) signal discrimination as a function of frequency range of the stimulus ensemble, and (2) the effect of auditory cue on discrimination of auditory stimuli as a function of frequency differences between cues and subsequent stimuli.
Data are presented for three observers. The results obtained are compared with predictions based upon the null hypothesis and a narrow‐band scanning model.

Genesis of Endolymphatic Hypoxia Following Acoustic Trauma
View Description Hide Description(1) Loud bursts of pure tone tend to decrease slightly the hydrogen diffusion rate into the first turn of the scala media of the eupneic guinea pig and markedly in the hyperventilated animal (2) White noise increases the rate of diffusion of hydrogen gas into the auditory cortex of the nonanesthetized cat. (3) Acoustic trauma produces vascular stasis in exposed blood vessels of the hamster cheek pouch.
These results are interpreted as indicating that the endolymphatic hypoxia following loud sound is due to a decrease in the oxygen supply/demand ratio resulting from (a) an increase in metabolism, and (b) no change, or decrease in cochlear blood flow and/or permeability of the stria vascularis.

Studies on the Near Fields of Monopole and Dipole Acoustic Sources
View Description Hide DescriptionIt is shown that when an observer is not farther than one wavelength from an acoustic source, he can determine the distance to the source from measurements of the pressure and of the particle velocity of the medium. The expressions are developed using integrals of the power so that they are relatively insensitive to the pulse shape. Exact expressions are obtainable only in the case of a monopole source. However, a ratio R _{2} is introduced which shows a monotonic dependence on distance in the near field and which is surprisingly insensitive to the nature of the source and to the shape of the wave. Experimental measurements are carried out for a dipole source. Agreement between the theory and the experiment is only moderate because of the difficulty in making sufficiently accurate acoustic measurements.

Intelligibility at High Voice Levels and the Use of a Megaphone
View Description Hide DescriptionA small megaphone was tested for use in direct voice communication high levels of vocal output against backgrounds of noise. Intelligibility scores were determined for 12 talkers, speaking with and without the megaphone, over a range of field voice levels of 70 to 100 db SPL (at 1 m). Nearly the entire over‐all acoustical gain produced by the megaphone was realized in improved intelligibility above backgrounds of white noise and low‐frequency noise. The acoustical gain of the megaphone permitted a lower voice level, and, hence, delayed the deterioration in intelligibility associated with distortions due to shouting. Frequency shifts in the voice spectrum were measured, with and without the megaphone, over a range of vocal efforts from conversational to maximum shout. Control tests demonstrated that the voice‐frequency shifts associated with shouting are not the primary factor in the deterioration of intelligibility; the frequency shifts appear to be only symptomatic of serious vocal distortions in shouting.

Acoustic Absorption Coefficients of the Surface of Laboratory Animals
View Description Hide DescriptionSound absorptionmeasurements on haired mice, hairless mice, rats, and guinea pigs have been made using reverberation chambers. The initial reverberation‐time experiments on haired and hairless mice were conducted in a cylindrical chamber using a Brüel and Kjaer recorder. Later measurements on all types of animals were made in an asymmetrical chamber using an oscilloscope and an electronic counter. The absorption coefficients were measured in the frequency band from 1 to 20 kc. All experiments showed that the absorption coefficients rise between 6 and 20 kc. Those for the haired animals approach 100%. Hairless mice had lower absorption coefficients approaching 20% at 20 kc.

On the Dynamics of a Bull Whip
View Description Hide DescriptionThe production of a cracking sound by a bull whip is discussed. Photographic evidence shows that the crack is produced by the tip of the whip exceeding the speed of sound and giving off shock waves rather than by the whip slapping itself. This evidence includes motion pictures as well as still shadow pictures which show the tip shedding shock waves. There follows a mathematical discussion of an idealized bull whip with a view to the problem of how the free end of this mechanism achieves a high velocity while the end held in the hand moves relatively slowly. A mathematical solution is given in which it is assumed that a discontinuity in tension propagates down the whip.

Pulsation Oscillations of Cavities in Rubber
View Description Hide DescriptionThe oscillation behavior of a spherical cavity in an infinite elastic medium is calculated for the case of an incident spherical dilatational wave entirely reflected at the cavity walls. It is shown that there exists for every Poisson's constant 0⩽σ⩽0.5 a frequency for which the amplitude of the oscillating cavity walls becomes a maximum. It is also shown that the amplitude resonance curves are symmetrical and that, assuming loss‐free material, they have a finite half‐width, which is caused by radiation losses and depends only on the ratio of shear wave and dilatational wave velocity.
Pulsation oscillations of spherical, rotational‐elliptical, cylindrical, and cubic cavities in rubber blocks were examined experimentally. The pulsation oscillations were excited by means of a sound source coupled to the surface of the rubber block. The sound pressure inside the cavities was measured with a probe microphone. The experimental results for spherical cavities are compared with the theory. Good agreement is found with respect to the resonance frequencies.

Efficiency of a Linear Array of Point Sources with Periodic Phase Variation
View Description Hide DescriptionThe radiation efficiency of a uniform linear array of point sources with periodic phase variation is evaluated. Two different types of interference are found depending on whether the characteristic length of the phase variation is greater or less than the wavelength of the sound radiated. In its application to suction roll silencing in paper mills it is found to be less effective than a linear continuous phase variation.

Minimizing Damage from Random Vibration
View Description Hide DescriptionDamage is defined and examples are cited. Illustrative problems of relay contact arm chatter, gyro precession and noise generation, and stress fatigue in a structure are given. It is concluded that there are three means by which damage can be reduced: raising the damage threshold of the item, modifying the transmissibility of the intervening structure, or reducing the severity of the excitation. The examples are an attempt to show specifically how this can be done in practical cases.

On the Fatigue Failure of Structures due to Vibrations Excited by Random Pressure Fields
View Description Hide DescriptionOn the assumption that the forced modes of vibration of a structure, subjected to pressure fluctuations random in time and space, can be approximated by the composition of the motions of the uncoupled natural modes, a general analysis is made using the ideas of vibration theory and spectrum analysis. The power spectrum, and hence the rms value, of any quantity depending linearly upon structural distortions is derived and it involves a quantity (called the “joint acceptance”) concerning the spacewise structure of the pressure field and of the geometry of the modes of vibration. It is shown how this result may be used (on assuming “normal” randomness) to estimate the fatigue life on the hypothesis of cumulative damage.

On the Approximation to the “Infinite” Solution by the Method of Normal Modes for Random Vibrations
View Description Hide DescriptionIf a plane sheet (membrane of plate), which is excited by homogeneous random pressures, is assumed to be infinite in extent then a very direct method of analysis is possible. However, several important applications concern structures which are far from infinite in extent (e.g., the excitation of panels by boundary layer pressure fluctuations, and the problem of structural vibrations due to jet noise). This restriction may be overcome by using the method of normal modes, which is more cumbersome by its very nature. It turns out that if it is the power spectrum of displacement which is required, then the “infinite” approach can be used only when the sheet is large enough for there to be only negligible reflections from its edges. However, if one is concerned only with the average mean square value of displacement (or related quantities), and if the detailed shape of the spectrum is of no consequence, then the infinite solution may be used, regardless of the magnitude of the damping, provided that the gravest modes are not excited.

Vibrations of a System with a Finite or an Infinite Number of Resonances
View Description Hide DescriptionThe study is based on the general differential equation of a mechanical system. Driving force and solution are expressed as a series of natural functions of the dissipationless system. Dissipation is taken into account by introducing complex elastic constants. Each Fourier coefficient of the driving force is written as the product of an excitation constant and the total driving force. The excitation constants can then be included in the mode parameters of the system and the solution, written as the product of the total driving force and the “mechanical impedance” of the system. The solution is exact and useful at the lower frequencies. A simple solution for higher frequencies can be derived by replacing the series by an integral and evaluating this integral. Three important parameters can thus be obtained: the driving‐point impedance that describes the velocity of the driving point and the reaction of the system to the driving force, the effective impedance that represents the average velocity amplitude over the system, and the effective dissipation resistance with respect to this average‐velocity amplitude.
The expression derived for the high‐frequency driving‐point impedance then turns out to represent, also, the general background level at lower frequencies. The solution for the driving‐point impedance in the intermediate frequency range is obtained by adding to this background the contribution of the natural mode that has its resonant frequency closest to the frequency of the force; the effective impedance, on the other hand, is given with good approximation as the impedance of the mode whose resonance frequency is closest to the frequency of the force; and the effective dissipation resistance is given by the resistive component of this mode impedance.
The mode parameters are computed for rods, membranes, plates, and shells for three situations—a point force, a linear force distribution, and a force distribution given by a Fourier integral. The results make it possible to handle complicated mechanical systems with almost the same ease as simple mass points, and to compute their sound radiation.

Sound Radiation of a System with a Finite or an Infinite Number of Resonances
View Description Hide DescriptionThe sound fieldgenerated by a composite vibrator consists of a wattless near field that decreases very rapidly with distance and an energy‐carrying radiation field. For radiators more than half a wavelength apart, the sound fields are spatially incoherent. The radiation field of a complex sound generator can therefore be computed by adding up the energy contributions of its various radiating elements. The radiating elements can be grouped as vibrators that are small in comparison to the wavelength, vibrators without nodal lines that are large in comparison to the wavelength, and vibrators with a nodal line pattern.
The radiation resistance of vibrators without nodal lines can, for most practical purposes, be stated with sufficient accuracy in terms of that of the equivalent sphere. The sound radiation of vibrators with nodal lines can be attributed to two causes. For the low‐order modes of vibration, the contributions of the zone of positive and negative amplitude do not compensate one another entirely, and the radiation resistance, though small, is different from zero. Such modes may therefore be expected to contribute to the sound pressure if they are excited in their resonance range. For the high‐order modes compensation is complete, but a point force or a force distribution along a line or over a finite area also excites to forced vibrations many low‐order modes with very few nodal lines. Since the distance between the nodal lines is greater than the acoustic wavelength in the surrounding medium, the radiation impedance for these modes is very nearly equal to ρc. Since these modes are forced to vibrate at frequencies above their resonant frequency, the resistive component of their impedance being negligible in comparison to the reactive component, this sound pressure is independent of the damping of the system. The radiation impedance is computed for the cases mentioned.

Contour Vibrations of Thin Rectangular Plates
View Description Hide DescriptionAn approximate theory based on an energy principle has been applied for contour vibrations of rectangular plates and compared with experimental data of ADP Z‐45° cuts with a satisfactory agreement.
The existence of a special mode, which has attracted little attention heretofore, is shown and its connection with the complex branches of the dispersion equation is discussed.
 LETTERS TO THE EDITOR



Notes on a Technique for the Determination of High‐Frequency Hearing Thresholds
View Description Hide DescriptionIn an attempt to develop a method for measuring auditory acuity for high‐frequency tones, the authors discuss pilot experiments in progress using an electromagnetic method for setting the eardrum into vibration (alternating magnetic fields acting on a permanent magnet fixed to the eardrum).

Loudness and Loudness Level
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