Volume 38, Issue 4, October 1965
Index of content:

Ultrasonic Absorption in Aqueous Solutions of High‐Molecular‐Weight Polysaccharides
View Description Hide DescriptionThe ultrasonic amplitude absorption coefficient has been determined in aqueous solutions of four molecular weights (73 000, 186 000, 370 000, and 2 000 000) of dextran, a linear α (1 → 6) anhydroglucose polysaccharide of biological origin, over the frequency range 3–69 Mc/sec at 22°C. It is found that the concentrationfree absorptionspectrum exhibits a remarkable similarity to that of aqueous solutions of beef hemoglobin. The latter finding leads to the conclusion that ultrasonicabsorption in tissues is not dominated exclusively by constituent proteins and that the contribution to such absorption by molecular structures lacking tertiary configurations may be considerable.

Re‐Vision of the Speech‐Privacy Calculation
View Description Hide DescriptionThe articulation index used in telephone communication, the listening equation for sonar, and the acoustical‐privacy calculation of architectural acoustics are all founded on a computation of an excess of signal level over noise level just sufficient to permit some stated detectability. From this viewpoint, data compiled for “Speech Privacy in Buildings” by Cavanaugh, Farrell, Hirtle, and Watters [J. Acoust. Soc. Am. 34, 475–492 (1962)] have been reviewed for possible simplications in their procedure for estimating acoustical privacy. Interrelations are demonstrated among various current methods for rating noise and sound insulation, and a general equation for acoustical privacy is derived. As an example, for the case of “confidential” privacy, 200‐ft^{2} floor area, and raised voice, satisfactory acoustical privacy is to be expected if D+N_{A} = 85 dB, where D is the sound isolation between the rooms (the reduction of sound from one room to the next) as rated by a procedure like that for the sound‐transmission class, and N_{A} is the background‐noise level in the receiving room measured with a sound‐level meter on A‐weighting.

Reaction‐Time Distributions in the Detection of Weak Signals in Noise
View Description Hide DescriptionReaction times were collected for three observers in a YES‐NO signal detection experiment, for two different signal strengths, and three a priori probabilities of a signal occurrence—P (S): 0.2, 0.5, and 0.8. Each server gave 1800 responds under each of the six conditions. A trial ended only when the observer terminated it. Some of the major findings are (1) latencies have Pearson distributions, mainly Beta of the first and second kind; (2) there is an apparent genetic character to the distributions—that is, they resemble each other according to the response (YES or NO) rather than the stimulus (signal or noise alone); (3) at P (S) = 0.2, the mean latency of a false rejection of a signal is least, that of a correct detection greatest, but for P (S) = 0.5, and 0.8, mean latency is least for correct detection, and greatest for correct NO'S, and (4) generally, reaction times are inversely related to the a priori probability of their associated stimulus classes.

Continuity Effects with Alternately Sounded Noise and Tone Signals as a Function of Manner of Presentation
View Description Hide DescriptionAn experiment is reported on continuity effects produced in a longer‐duration, less‐intense noise signal alternating with a shorter‐duration, higher‐intensity tonal signal. The perceived continuity in the noise signal is demonstrated to be significantly affected by the following: (1) the frequency of the tonal signal (200, 400, 1000, 2000, and 4000 cps), (2) the duration of the noise signal (70, 250, and 950 msec), and (3) the manner of presentation (monaural versus dichotic). Monaural thresholds were found to be larger than dichotic continuity thresholds. Continuity effects were demonstrated under dichotic presentation. That is, the noise signal was reported as continuous when interrupted for periods of time that would be clearly perceived if no tonal signal were interpolated the interruptions of the noise. A two factor theory is forwarded to explain the dichotic and monaural continuity effects. The monaural effects are discussed in terms of rate of decay of auditory sensation. The dichotic effects are discussed in terms of facilitation of ongoing neural discharge somewhat more central than the first‐order neurons.

Tonal Consonance and Critical Bandwidth
View Description Hide DescriptionFirstly, theories are reviewed on the explanation of tonal consonance as the singular nature of tone intervals with frequency ratios corresponding with small integer numbers. An evaluation of these explanations in the light of some experimental studies supports the hypothesis, as promoted by von Helmholtz, that the difference between consonant and dissonant intervals is related to beats of adjacent partials. This relation was studied more fully by experiments in which subjects had to judge simple‐tone intervals as a function of test frequency and interval width. The results may be considered as a modification of von Helmholtz's conception and indicate that, as a function of frequency, the transition range between consonant and dissonant intervals is related to critical bandwidth. Simple‐tone intervals are evaluated as consonant for frequency differences exceeding this bandwidth. whereas the most dissonant intervals correspond with frequency differences of about a quarter of this bandwidth. On the base of these results, some properties of consonant intervals consisting of complex tones are explained. To answer the question whether critical bandwidth also plays a rôle in music, the chords of two compositions (parts of a trio sonata of J. S. Bach and of a string quartet of A. Dvořák) were analyzed by computing interval distributions as a function of frequency and number of harmonics taken into account. The results strongly suggest that, indeed, critical bandwidth plays an important rôle in music: for a number of harmonics representative for musical instruments, the “density” of simultaneous partials alters as a function of frequency in the same way as critical bandwidth does.

Acoustic Water Tunnel
View Description Hide DescriptionAn acoustic water tunnel has been constructed that can be used to study various solid‐liquid interface processes, such as heat transfer and scale formation, in the presence of intense sound. The tunnel, in modified form, can also be used to measure the acoustical impedance of various structures, such as porous metal plates, both in the presence and absence of convective flow.

Properties of Transducer Ceramics under Maintained Planar Stress
View Description Hide DescriptionIn order to predict the performance of certain types of deeply submerged underwater transducers, measurements have been made of the properties of piezoelectricceramics as a function of pressure, time, and electric field. Planar compressive stresses up to 35 000 psi were imposed on bariumtitanate and lead zirconate titanateceramic by subjecting thin hollow spheres to uniform external pressure. With increasing stress, the dielectric constant,piezoelectric constant, coupling factor, and elastic compliance decreased by as much as 70%, part of the change being permanent but most of it reversible. The stability of the ceramic properties with duration and number of cycles of the imposed stress varied widely with ceramic composition. Large increases in high‐field dielectric loss were observed at high stress in ceramics usually considered suitable for high‐power projectors. The results indicate that transducers that utilize ceramic under planar stress may suffer significant degradation in performance when deeply submerged.

Coupling between Two Modes of Vibration in a Piezoelectric Resonator
View Description Hide DescriptionTwo coupled modes of vibration are sometimes observed with piezoelectric‐ceramic resonators in which a single mode is expected. The coupling is inferred through the presence in the admittance characteristic of two close resonance peaks; it is also seen through a nonexponential decay of resonant vibrations. The effect is attributed to a perturbation of symmetry (such as a flaw in the material or imperfect geometry) that permits coupling of the principal piezoelectric mode to some other secondary mode whose resonance is close in frequency. To determine the requirements for obtaining coupling, a split‐electrode system was used on a thin, rectangular, ceramic plate to measure the resonant frequencies of the first three extensional modes (length, edge, radial) and first three face‐flexural modes, the dimensions being varied from a length/width of 6:1 to 1:1. Coupling between the length mode and second flexural mode was investigated in detail, using a plate of length/width 3.5:1, resonant at 45 kc/sec. Perturbations of symmetry were obtained by adding or removing mass at a corner; coupling could be produced by a change in mass of the order of 10^{−3} of that of the whole resonator. The experiments show the modification of the electrical characteristics of the principal piezoelectricresonance as the coupling to a secondary mode of the same frequency is increased and, also, the effect of coupling two modes of different frequency. The results are discussed qualitatively in terms of an equivalent electrical circuit.

Distinctive Features and Errors in Short‐Term Memory for English Vowels
View Description Hide DescriptionErrors in short‐term recall of six English vowels (I, ε, æ, U, ʌ, ɑ) were tabulated and related to several distinctive‐feature systems. Vowels were embedded in two contexts: /l[ ]k/ and /z[ ]k/. Subjects were instructed to copy items as they were presented, followed by recall of the entire list of (six) items. Perceptual errors were excluded from the recall error matrix by scoring for recall only correctly copied items. The rank‐order frequency of different intrusions in recall of each presented vowel was almost perfectly predicted by a conventional phoneticanalysis in two dimensions: place of articulation (front, back) and openness of the vocal tract (narrow, medium, and wide). The error matrix also supported the assumptions that the values of openness are ordered in short‐term memory and that the correct value on the openness dimension is more likely to be forgotten than the correct value on the place dimension. The study suggests that a vowel is coded in short‐term memory, not as a unit, but as a set of two distinctive features, each of which may be forgotten independently.

Elementary Results for Scattering by Large Ellipsoids
View Description Hide DescriptionElementary high‐frequency approximations for scattering by an arbitrary body are specialized to ellipsoids and applied in detail for source and observation points at distances large compared to the scatterer's size. It is shown, for example, that, if the direction of observation is varied in a fixed plane containing the direction of incidence, then except for the near‐forward direction (“shadow region”) the first approximation for the far‐distant field for a given ellipsoid is specified by a “universal curve.” In particular, if the fixed plane contains an axis of the ellipsoid, then there is a unique curve for the intensity versus the angle measured from the reflection of the direction of incidence in the contained axis. For the special case of the spheroid, the field on the “axially specular” cone (the cone defined by the angle of incidence with the axis of rotation) remains constant until one gets near the shadow region, and the fields on adjacent cones (i.e., with generators at other than the specular angle with the axis) vary slowly with the azimuthal angle; for many practical purposes, we may neglect the slowly varying effects and approximate the reflected intensity pattern in the vicinity of the axially specular cone by a conical lobe of revolution. Numerical examples are given for a family of spheroids; e.g., we determine contours of constant intensity on a plane containing the source as functions of the spheroid's distance from the plane, etc.

Plane‐Wave Scattering from a Rough Surface with Correlated Large‐ and Small‐Scale Orders of Roughness
View Description Hide DescriptionA rough surface is assumed to be composed of a slowly varying large‐scale roughness and a rapidly varying small‐scale roughness, and it is assumed that these are correlated. The exponential forms of both the autocorrelations of these surfaces and their crosscorrelation were used. This paper shows that the return from such a composite surface for a plane‐wave incidence can have either one of the following terms contribute a major portion of the total return power: namely, the large‐scale roughness term, small‐scale roughness term, and the crosscorrelation term. It is therefore concluded that contribution due to each term must be evaluated before neglecting its effect on the total return power. These results are equally applicable to acoustic and radar cases.

Reflections from Lossy Media
View Description Hide DescriptionA simple analysis shows that, when a transient plane wave in an elastic medium is reflected at a plane boundary with a lossy medium, the transient waveform of the reflection is affected by the loss parameters of the second medium. If the attenuation in the second medium is small, and if the ρc products of the two media are matched, then the reflected waveform is the convolution of the incident waveform, with the integral of the Fourier transform of attenuation as a function of frequency. Thus, attenuation for a lossy solid or liquid can be obtained by this external‐pulse technique. Where attenuation is some simple function of frequency, its Fourier transform is some recognized generalized function. Sample waveforms have been observed using airborne sound in specially prepared tubes; good qualitative agreement with predicted waveform was obtained.

Rotationally Symmetric, Transient Response of a Small‐Angle Truncated Conical Shell owing to a Moving Pressure Front
View Description Hide DescriptionA thin, truncated conical shell is analyzed for the initial transient response due to the passage of a pressure front of constant velocity and amplitude. The principle of virtual work is applied to a segment of the shell, between two adjacent generatrices, treated as a beam on an elastic foundation. The ends of the shell are considered to be simply supported. The natural or free modes of vibration are approximated using an 8‐term series of functions each satisfying the boundary conditions, and the resulting modes are used in conjunction with time‐dependent coefficients to formulate the forced response.

Equivalent Electrical Network for the Transversely Vibrating Uniform Bar
View Description Hide DescriptionThe equivalent network for the transversely vibrating uniform bars under various end conditions are given, the value of each element of the networks is determined, and graphs showing the frequency characteristics of each element in the matrices for the bars are given. Some considerations such as the effect of the neglect of the distributed mass are given. The analysis by the method of normalized functions is briefly explained by citing a few examples.

Investigation of Structure‐Borne Active Vibration Dampers
View Description Hide DescriptionThe use of active feedback elements to produce damping of multiresonant structures has been extended to include the case in which the damping‐force generator is supported by the structure being damped. A theoretical damping equation is developed and experimental verification is obtained for the particular case of a uniform aluminum beam damped at its midpoint. In this structure‐borne configuration, the active damper is potentially applicable to a wider variety of vibration problems because external support for the damping‐force generator is no longer necessary. When applied to the uniform beam, an experimental structure‐borne damper demonstrated damping bandwidth in excess of 10 kc/sec and equivalent loss factors of up to 0.22 for resonant frequencies below 2 kc/sec.

Long‐Range Sound‐Propagation Study in the Southern Ocean—Project Neptune
View Description Hide DescriptionAn experiment to determine some characteristics of long‐range underwater sound propagation was undertaken by the U.S. Naval Ordnance Laboratory in April 1964. In this experiment, Project Neptune, sound signals were dropped at various ranges from a listening station in Bermuda. To supplement this and other stations, one was established, by the New Zealand Naval Research Laboratory, off southern New Zealand to record the sound signals dropped on the final phase between Cape Town, South Africa, and Perth, Australia. The recorded energy was analyzed in ‐oct bands to determine the transmission‐path characteristics for low frequencies. The signal envelopes were found to differ in shape from the usual solar case, and the attenuations were much larger than previously obtained for either RSR (refracted‐surface reflected) or sofar propagation. These differences may be explained in terms of the different velocity structure of the Southern Ocean from that sound in the Atlantic or Pacific Oceans. In particular, the thermocline is not as pronounced as in temperate or tropical regions and is absent south of the Antarctic convergence. Thus, the energy was transmitted by a mixture of RSR and sofar modes, little sofar energy arriving from those shots whose tracks crossed the Antarctic convergence.

Harmonic Distortion of Spherical Sound Waves in Water
View Description Hide DescriptionThe problems arising from the generation of harmonic distortion by underwater sound devices are discussed. Expressions are derived for the harmonic distortion of spherically propagating finite‐amplitude sound waves, and distortion is calculated for a typical high‐intensity sound source. The calculation illustrates that finite‐amplitude effects can produce a significant amount of second‐harmonic distortion in a practical situation.

Deep‐Ocean Sound Attenuation in the Sub‐ and Low‐Kilocycle‐per‐Second Region
View Description Hide DescriptionAn experiment was conducted to provide an empirical evaluation of attenuation in sea water in the frequency range of 354–3540 cps. Recordings were made of solar shots detonated every 5 miles along a 500‐mile track. The receiving elements were located near the axis of the sofar channel. An analysis is outlined both in terms of received energy spectrum and transmission loss as a function of range. The experimental data have been statistically fitted by least‐squares methods to a mathematical model of the form , where H _{0} represents the transmission loss in excess of inverse first‐power spreading. These results have been combined with those from a subsequent experiment, conducted in the same geographical area, that extended the frequency coverage to 112 cps. The regression line fitting the joint set of coefficients to an upper limit of 1780 cps is given by . The present evaluation is compared with other results, and an apparent systematic anomaly is discussed.

Influence of Shear Flow on Sound Attenuation in a Lined Duct
View Description Hide DescriptionA theoretical and experimental study of the influence of shear flow on the attenuation of sound in a lined duct is presented. Both upstream and downstream propagation are considered. Solutions of the linearized equations for acoustic‐wave propagation in flow, based upon both uniform and power‐law models of the mean‐flow profile, are compared with attenuation measurements in a duct having two opposite side walls lined with a porous fiberglas® blanket for a frequency‐geometry range kδ⩽1 and midstream Mach numbers M_{l} <0.2. Here, k is the plane wavenumber and δ is the aerodynamic boundary‐layer thickness. Both profile models yield results in close agreement with experiments at low frequencies, kδ<0.1. For intermediate and high frequencies, 0.1 <kδ<1, the uniform‐flow model fails, as expected, since it can only account for the convective effects of the flow upon attenuation of sound. It was not expected that the power‐law model, which seemingly accounts for the effects of both convection and refraction within the shear layer upon the sound wave, would yield results much the same as those obtained for the uniform‐flow profile, and thus fail in various degrees for this frequency range. Also, scattering of sound by turbulent flow does not appear to be strong enough to account for discrepancies between theory and experiment. As a result, the problem of accurately predicting the effects of refraction upon soundattenuation in the range 0.1 <kδ <1 remains unsolved. The uniform‐flow model, for which solutions are easily obtained, proves useful from an acoustical‐engineering point of view.
 LETTERS TO THE EDITOR


Rating Scales and Two‐State Threshold Models
View Description Hide DescriptionThe use of rating‐scale isosensitivity data for testing discrete, two‐state detection models is admitted to be inappropriate, unless additional assumptions are made. It is suggested that simple assumptions may, in fact, be made by which this model may maintain its basic description of sensory processes and yet make predictions about rating results. The predictions are shown to produce the same isosensitivity curves (ROC curves) as did the original, two‐choice version of the model. These functions do not describe the results of rating experiments.
