摘要:

The axisymmetric wave equation is solved for the problem of torsional elastic waves impinging on a penny‐shaped crack the periphery of which is assumed to be infinitely sharp. Using Hankel transforms, the problem is reduced to the solution of two simultaneous integral equations of the Fredholm type. The proposed method of solution permits an examination of the complete scattered‐wave field at points both near to and far from the penny‐shaped plane of discontinuity. In elastodynamics, however, it is the nearfield stress solution that is of chief interest. To this end, the singular nature of the local dynamic stress field is determined in elementary closed form, while the magnitude of this stress field, which can be adequately described by a singularity parameterk3, is calculated numerically. The important results are that (1) the stresses are singular of the orderr1−12asr1→ 0 at the diffracting edge of the crack and (2)k3is found to be proportional to the material constants, the crack radius, and the wavelength or frequency of the incoming waves. A knowledge of this parameterk3is essential to a clear understanding of the propagation of cracks through structural components undergoing torsional oscillations, since its value has been known to control the stability or instability behavior of cracks.

ISSN:0001-4966    

DOI:10.1121/1.1911253

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Previous theoretical work on the scattering of sound pulses by a spherical metallic shell in water is extended to two areas of practical interest; (a) linear frequency‐modulated (fm) pulses, and (b) higher frequencies where the wavelength in water is much less than the exterior radius of the shell. Cross‐correlation techniques are applied in the analysis of the echo structure. Computations are performed for a water‐filled aluminum shell with a ratio of interior to exterior radius of 0.95. It is shown that the frequency dependence of the steady‐state farfield echo intensity at high frequencies has the same kind of irregular behavior as that found previously at lower frequencies. Echo pulses are computed for linear fm pulses with different pulse lengths and frequency sweeps. A cross‐correlation scan of each fm echo pulse with its incident pulse shows a series of strong correlations within the echo. The first of these indicates that the first part of the echo corresponds to a reflection of the incident pulse from the outer surface of the shell. Another strong correlation appears to arise from a reflection from the back surface of the shell, particularly at high frequencies. It does not seem that the remaining correlations can be readily matched with simple travel paths within or around the shell.

ISSN:0001-4966    

DOI:10.1121/1.1911254

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The theory of acoustic radiation, extended from the theory of acoustic scattering, is shown to be applicable to the acoustic‐field study of planar transducer or hydrophone arrays. The general nature of the approach is shown to give a new look at the problem of synthesis and the method of studying vibrations acoustically. In the synthesis problem, shading of amplitudes over both space and time is proposed.

ISSN:0001-4966    

DOI:10.1121/1.1911255

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

We show, using the case of the parabolic velocity profilec02/c2 = 1 −α2z2for an underwater acoustic duct, that the normal‐mode series of the problem contains a coherent part which dominates, and explicitly exhibits the phenomenon of range focusing, at a range ofR− 2π/α and its multiples. Owing to the symmetry of the profile, additional focal points are given byR/2 and its odd multiples, situated as far below the channel axis as the source is above, while the even‐multiple focal points lie at the height of the source. The coherent part of the series is given in analytic form, and is shown to approximate a delta function in the depth variable at the focusing ranges.

ISSN:0001-4966    

DOI:10.1121/1.1911256

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Research into the dispersive characteristics of underwater sound propagation has been approached in several ways. In the macroscopic approach, the medium is characterized as a stochastic time‐varying filter. This paper considers the synthesis of an optimum detection system from this approach. The joint moments of the signal processes from an array ofKreceiving elements are then easily determined, given a knowledge of the joint moments of the impulse responses of the medium. A method for synthesizing the optimum processor containing realizable time varying filters is presented for the case when the transmitted signal is deterministic and the medium noise and channel impulse responses are Gaussian. These filters are obtained by considering a “serial” form of the optimum detector where the signal inputs in (0,T) are grouped serially into a single signal having durationkTsec. This form of the processor is amenable to standard techniques for forming either an analog or digital realizable set of filters. A technique is then employed for partitioning each of these filters intok(k+1)/2 realizable filters whose impulse responses occur in (0,T). This partitioning and the subsequent time translation results in a realizable signal processor.

ISSN:0001-4966    

DOI:10.1121/1.1911257

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

A signal‐processing technique for pulsed sonar systems is described in which the sound field is treated as a series of point targets or highlights. Only the phase information of the narrow band echo signals is utilized, the outputs of the individual transducers being infinitely clipped and encoded directly in digital form. Subsequent operations performed on these numbers have the effect of steering the system to point in the direction of the strongest signal which illuminates the array at that particular instant. A linear phase‐difference pattern is looked for, and numerical parameters are extracted that are a measure of how closely the received pattern matches the expected one. Signal detection is achieved by a hierarchy of thresholds operating sequentially on these parameters. Field trials of an experimental equipment in a shallow reservoir are described, and the results and measurements presented. The technique has also been analyzed and simulated on a digital computer, and a quantitative comparison between the efficiency of the proposed system as a signal processer, with that of conventional amplitude methods, is included.

ISSN:0001-4966    

DOI:10.1121/1.1911258

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Underwater sounds from migrating gray whales were recorded from a bottom‐mounted hydrophone array. Sound‐source locations were based upon arrival‐time differences and received levels. Visual tracking corroborated sound data.Moanswere the most common of more than 231 whale sounds recorded in the presence of at least 218 whales. Moans lasted 1.5 sec; their source level was about 126 dBre0.0002 dyn/cm2at 1 yd; and they ranged from 20–200 Hz. Underwaterblowsounds from surface exhalations were 1.25 sec long, and they ranged from 15–175 Hz. Infrequentbubble‐typesignals, lasting 0.7 sec, were about 112 dBre0.0002 dyn/cm2at 1 yd, ranging from 15–305 Hz.Knocksounds were as high as 350 Hz at sound‐pressure levels up to 116 dBre0.0002 dyn/cm3at 1 yd. Gray whales were soniferous during the day and night. The average swimming speed of lone migrators was 5.5 kt, based on sound tracks. No characteristic behavior could be associated with sound production other than blow sounds during exhalations.

ISSN:0001-4966    

DOI:10.1121/1.1911259

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Various ultrasonic properties of whitefish (Coregonus dupeaformis) tissues have been determined. The sound velocity, impedance, and absorption were measured at 22°C with pulse echo techniques for the frequencies of 1, 2, 3, 7, and 11 MHz. The average impedance and absorption of fresh whitefish myomere with a fat content of approximately 8% were 1.634 × 105cgs units and 0.7 dB/cm/MHz, respectively. No significant velocity or absorption anisotropy was observed for the muscle tissues. The effects of freezing on velocity and absorption were investigated. The mean velocity in thawed flesh was only 0.8% higher than in fresh flesh, but increases up to 100% or more were noted in the absorption. This increase depended on the rate at which freezing proceeded; a longer freezing period generally resulted in greater absorption.

ISSN:0001-4966    

DOI:10.1121/1.1911260

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The sounds produced in the nests of hornets were studied with the aim of determining their origin, physical parameters, and significance. For this purpose, hornet nests were placed in a specially built, acoustically insulated chamber, and the sounds produced within the nests were recorded and analyzed. Sounds were recorded from both adult hornets as well as from the late‐instar(large) larvae. Those produced by the adults were mainly air borne and were identified as ventilation and food‐gnawing sounds, while those by the larvae were for the most part solid borne and were interpreted as “hunger signals.”

ISSN:0001-4966    

DOI:10.1121/1.1911261

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Research on cell suspensions, single cells, macromolecular suspensions, and model (nonliving) systems reveal a variety of changes brought about by sound in the absence of gross‐heating or transient (“collapse”‐type) cavitation. Many of these phenomena are explainable in terms of acoustic streaming, radiation pressure, and other characteristics of ultrasound that arise from nonlinearity. Commonly, the sonic effect depends on nonuniformity in the sound field. Interestig results are obtained with techniques by which one can set up nonuniform vibration in the walls or membranes of individual cells. In ultrasonic beams acting on tissue, it is possible that variations may arise from gradients in the incident field, or from inhomogeneities in the tissue, which scatter sound. Gaseous pockets or bubbles would be especially effective as such inhomogeneities.

ISSN:0001-4966    

DOI:10.1121/1.1911262

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP