摘要:

The theory of echo formation provides formulas relating echo energy to physical characteristics of the target. Single‐target theory (applicable to counting isolated fish) is extended to the multiple‐target case relevant to schooling fish. An echo‐integrator equation relates fish density to echo energy integrated over a time gate corresponding to the depth channel of interest. Parameters include the equivalent beam angle, the expected backscattering cross section per fish, equipment sensitivity, and a time‐varied‐gain correction factor. Variation of environmental factors (sound speed and absorption) affects the parameter values. More important is the variation of biological factors (fish behavior and physiology) which affects backscattering cross section and target strength. Verification of the echo‐integrator equation, depending upon the linearity principle concerning the addition of signals from randomly distributed multiple targets, is discussed in the fisheries context. The swimbladder is the dominant sound reflector in fish having one. Accordingly, fish targets may be classified as (a) bladder closed, (b) bladder open, or (c) no bladder. Within each category, fish of the same size have similar target strengths. Target strength variation with fish size, water depth, and time is discussed. Experimental target strengths are well scattered even for nominally similar fish. Nevertheless, useful information about fish stocks can be obtained through careful application of acoustical techniques.

ISSN:0001-4966    

DOI:10.1121/1.399285

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Encaged aggregations of live krill in good to pristine condition have been ensonified at 38 and 120 kHz. Concurrent underwater television observations of behavior resemble those made by underwater divers in naturally occurring swarms, with comparably high densities of the order of 104animals/m3. Mean, single‐animal target strengths have been inferred from measurements of echo energy. For aggregations with mean lengths in the range [30,39] mm, the mean single‐krill target strengths are in the range [−88,−83]dB at 38 kHz and [−81,−74] dB at 120 kHz. Collateral measurements on some of the same encaged specimens determined a density contrast of 1.0357±0.0067 and sound‐speed contrast of 1.0279±0.0024, relative to seawater. These numbers have been used with the fluid‐sphere model as stated by Greenlaw [Limnol. Oceanogr.24, 226–242 (1979)] . Computed backscattering cross sections have been averaged over the length distributions of each measured aggregation, resulting in target strength predictions in the range [−86,−80]dB at 38 kHz and [−79,−76] dB at 120 kHz.

ISSN:0001-4966    

DOI:10.1121/1.399282

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Earlier results for multiple scattering by arbitrary configurations ofNobstacles are applied to seven regular arrays of two to six monopole resonators (with radiusa, maximum scattering cross section σc, and normalized resonance frequencyxc=kca). The arrays involvem=1, 2, or 3 different values of the separations of monopole centers, withdas the smallest. For each array, the corresponding scattering amplitude reduces to a sum ofm+1 collective oscillator modesFn(orthogonal in the same sense as the spherical harmonics). The doublet, triangular, and tetrahedral arrays are specified by two modes,F0andF1; the square, pentagonal, and octahedral arrays require a third,F2; and the hexagonal array also requires a fourth,F3. The scattering cross sectionSof each array is less than 2Nσc, with maximum at a frequencyka=xnearxcfor relatively largekd=y=x/p≫x. Ify>2xis small, theFnreduce to simple multipoles in terms of spherical harmonics and elementary functions ofx,p, andxc. ThenShas narrow resonance peaksSn=O(σc) at frequenciesxnrelatively far fromxc(withx0xc); the averages over orientationS̄nadd up toNσc(an average oscillator‐strength sum rule). The separations of the resonance frequenciesxnand the fineness of the peaksSn(nonoverlapping at half‐power) provide distinctive signatures for diagnostic and related applications.

ISSN:0001-4966    

DOI:10.1121/1.399292

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Obliquely incident GaussianP‐wave inputs furnish important test signals for ultrasonic detection of flaws in layered elastic media. Unlike vertically impinging beams, multiply reflected oblique beams eventually are converted into guided modes. The horizontal ranges over which the field is beamlike or modelike must be well understood for selection of a diagnostic scheme with ‘‘good’’ features for detection and identification. An analytical study is conducted here to detail the beam‐to‐mode conversion andP–SVcoupling processes in an aluminum plate. The rigorously based approach utilizes the complex source point (CSP) method for generation of the input beam, and offers ray, mode, and hybrid ray–mode alternatives for best characterization of the wave phenomena. Numerical reference results over a representative range of parameters, which furnish potentials and displacements in various cross sections and on the plate boundaries, are interpreted in terms of beam or mode observables and related to the spectral wavenumber content of the incident signal. These considerations lead to the formulation of certain rules that categorize the detected signal in a manner which may be helpful in the construction of inversion algorithms for flaw detection.

ISSN:0001-4966    

DOI:10.1121/1.399268

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The acoustic behavior of a flexible fibrous material was studied experimentally. The material consisted of cylindrically shaped fibers arranged in a batting with the fibers primarily aligned parallel to the face of the batting. This type of material was considered anisotropic, with the acoustic propagation constant depending on whether the direction of sound propagation was parallel or normal to the fiber arrangement. Normal‐incidence sound‐absorption measurements were taken for both fiber orientations over the frequency range 140–1500 Hz and with bulk densities ranging from 4.6–66.7 kg/m3. When the sound propagated in a direction normal to the fiber alignment, the measured sound absorption showed the occurrence of a resonance, which modified the absorption that was primarily attributed to viscous and thermal effects. When the sound propagated in a direction parallel to the fiber alignment, indications of resonances in the data were not present. From comparing these two sets of data and from considering the material structure, the resonance in the data for fibers normal to the direction of sound propagation was attributed to fiber motion. An analytical model for the acoustical behavior of the material displayed the same fiber‐motion characteristics shown in the measurements.

ISSN:0001-4966    

DOI:10.1121/1.398968

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A theoretical determination of the location and size of blockages in the one‐dimensional duct has been carried out. It is found that the blockage cross‐section area function can be calculated from the measured eigenfrequencies obtained using two boundary conditions, i.e., closed–closed (or open–open) and closed–open ends. When the blockages are small, the area function of the blocked duct can be expressed as a one‐dimensional spatial Fourier transform, whose spatial frequencies are related to the eigenfrequency shifts caused by the blockages in the duct. The method developed can be applied to the detection of the multiple blockages. The experimental results show that the accuracy of the calculated blockage area function is dependent on the number of the eigenfrequencies used. The agreements between the calculated blockage area and the actual blockage are excellent when the half‐wavelength of the eigenfrequency used is greater than the length of the blockage.

ISSN:0001-4966    

DOI:10.1121/1.398914

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In this paper a study of the transient ultrasonic field radiated, in a fluid, by a planar transducer coupled to a lens is proposed. The transient pressure is measured using two small probes of different radii. Experimental waveforms are compared to waveforms that have been calculated using the impulse response method, extended in order to take account of the finite size of the receivers. The transient pressure field is interpreted by the concept of geometrical wave and edge waves. The averaging effect due to the finite size of the receivers, which acts differently on the ultrasonic field components, is put in evidence.

ISSN:0001-4966    

DOI:10.1121/1.398915

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

An extension of standard results for the acoustic force on an object in a single‐mode resonant chamber yields predictions for the onset of oscillational instabilities when objects are levitated or positioned in these chambers. The results are consistent with experimental investigations. The present approach accounts for the effect of time delays on the response of a cavity to the motion of an object inside it. Quantitative features of the instabilities are investigated. The experimental conditions required for sample stability, saturation of sample oscillations, hysteretic effects, and the loss of the ability to levitate are discussed.

ISSN:0001-4966    

DOI:10.1121/1.398916

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Theoretical predictions are made of the effect of an absorbing ground surface on acoustic impulsive waveforms propagating in a homogeneous atmosphere for frequencies below 500 Hz. The lower frequencies of the pulse are enhanced as the effective flow resistivity of the ground surface decreases and as the propagation distance increases. The pulse waveforms and peak amplitude decay observed for propagation distances of 40 to 274 m over grassland were satisfactorily matched by calculations using an assumed effective flow resistivity of 200 kN s m−4. Measurements over snow gave much greater amplitude decay rates, and the waveforms were radically changed in appearance, being dominated by the lower frequencies. These waveforms were satisfactorily matched only when a layered ground was incorporated into the calculations; then, an assumed surface effective flow resistivity of 20 kN s m−4gave good agreement with the observed waveforms and peak amplitude decay.

ISSN:0001-4966    

DOI:10.1121/1.398917

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The theoretical solution for three‐dimensional acoustic propagation in a wedge‐shaped ocean with ideal pressure‐release boundaries predicts a well‐defined beam in the down‐slope direction that diverges as the energy propagates out towards deep water. Outside of the beam, shadow zones are formed, and its spatial extent is determined by the lowest mode of propagation. The purpose of the experimental results presented here is to check the theoretical analyses, as part of an ongoing investigation into three‐dimensional propagation in the ocean environment. The wedge model in this experiment has a pressure‐release bottom and a slope that is around 20 deg. For convenience of analysis, only the lowest mode is excited by locating a source near the cutoff range. It has been found that for down‐slope propagation, the spatial characteristics of the field in a direction parallel to the shore line are in good agreement with the theoretical solution, and the azimuthal extent of the beam depends on the wedge angle. In addition to the evaluation of the exact theory, measurements have been made of acoustic pulse propagation and distortion in the wedge.

ISSN:0001-4966    

DOI:10.1121/1.399302

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP