Susan K. Numrich

Time–frequency analysis of acoustic scattering from elastic objects

Some characteristics of an insonified object are generally present in the signal carried by the returning scattering wave. As an echo is the result of the wave interaction with a material structure, such a response is distinctive for a body of given shape and composition. Traditionally, to express the information content in the echo signature, either the frequency response (transfer/form function) or the time signature (impulse response) is employed; however, for a detailed study of the scatterer’s structure, a joint time and frequency analysis is performed. The aim of this analysis is to develop a simple processing algorithm for extracting the prominent features, which can then be used to determine the physical parameters of the object. The approach is based on the modified version of the Wigner distribution function (WDF) and utilizes an image‐processing technique to depict the outstanding highlights of the scatterer’s response in a two‐dimensional time and frequency display. The physical parameters of ...

Calibration technique for acoustic scattering measurements

Tungsten carbide spheres are used as calibration targets in laboratory acoustic scattering measurements. Though the steady‐state response of any metal sphere in water greatly differs from a rigid body return, over almost the entire frequency spectrum, the rigid body and elastic returns can be separated in a short pulse, broadband experiment. This rigid body echo can then be used as a reference to normalize the scattering returns from targets of interest.

Agent-directed simulation: challenges to meet defense and civilian requirements

The aim of this panel session is to point out the importance of agent-directed simulation, as a scientific concept and technological possibility, to enhance the potential of simulation in both civilian and defense applications. The members of the panel (organized by Dr. Ören) are: Dr. Erol Gelenbe, Dr. S. K. Numrich, Dr. Adelinde Uhrmacher, and Dr. Linda Wilson. The position statements of the panel members are given separately. Ören bases his arguments on the NATO Modelling and Simulation Master Plan. He points out the need to proactively advance simulation science and technology to satisfy the requirements of the sophisticated defense applications. He stresses that, among other methodological advance possibilities, the three categories of agent-directed simulation have to be properly developed and/or tailored for defense applications. Gelenbes interests include goal-directed knowledge processing abilities of agents in hostile environments. Numrich stresses on the need for command and search agents in defense applications. Uhrmacher states challenges for the users and the simulationists on the need of agents for modelling and agents for testing. Wilson covers four key challenges to agent-directed simulation that are: security, standards in communication, computer resources, and system management and monitoring.

Comments on the alpha-particle model of 12C and 16O

Abstract Is is noted that the simple α-particle model of 12 C and 16 O, when applied to recent electron scattering data for these nuclei which greatly extend the region of momentum transfer, still provides a partially satisfactory fit. In particular, for elastic scattering by 16 O the model also predicts the recently observed second diffraction minimum which so far could be explained by more detailed theoretical models only.

Calibration method for acoustic scattering measurements using a spherical target

A method for calibrating acoustic backscattering instrumentation utilizing spherical body as a standard target. A spherical body made of high specific acoustic impedance material, such as tungsten carbide, is positioned a given distance from a source/receiver transducer which is energized to produce a short acoustic pulse directed toward the sphere. Acoustic signals reflected from the sphere are detected by the transducer and processed in the time domain to separate the rigid portion of the return from the elastic portions. The rigid portion is corrected for the transducer to sphere distance, the reflectivity of the sphere, and for the radius of the sphere. The resultant corrected signal represents the incident acoustic pulse produced by the transducer.

Acoustic scattering from finite cylindrical elastic objects

The acoustic scattering from infinite elastic cylinders or spheres is well known. It is possible to characterize these targets by their resonance spectra. The resonances are established by the generation of surface waves that propagate around the circumference of the targets. The resonances originate from the phase matching of repeatedly circumnavigating surface waves. Experimentally, it is possible to characterize a target with a complicated shape, but it is not easy to explain the spectra theoretically because the geometry is not separable and the usual analytical methods to calculate the far‐field pressure cannot be used. In this paper, resonance spectra and angular diagrams obtained from a target consisting of a finite cylinder with hemispherical endcaps are obtained experimentally. To explain the resonance spectra, an integral phase matching condition is used. Upon incidence normal to the cylinder axis, resonances due to the phase matching of surface waves traveling along a circumference or along a m...

Acoustic ringing response of the individual resonances of an elastic cylinder

The results of a theoretical and experimental study concerning the ringing response of the individual resonances of a submersed elastic cylinder, insonified at normal incidence by sinusoidal pulses of relatively long duration, are presented. This ringing consists of a series of superimposed responses, comprising the specular reflection, and a succession of creeping waves which at resonance add in phase to synthesize the ringing response of an individual elastic‐body resonance, provided the pulse spectrum is sufficiently narrow; off resonance, the ringing disappears. Analytically and experimentally obtained echoes from cylindrical targets support this interpretation and are correlated with complex‐frequency poles of the scattering amplitude.

Finite-difference time-domain simulations of wave propagation and scattering as a research and educational tool

In this article we introduce the finite‐difference time‐domain (FDTD) method to solve the wave equation. The FDTD algorithm is a useful tool to study wave propagation and scattering processes because the wave fields, which are computed at each time step, can be displayed to produce animated visualizations during the computation. The FDTD algorithm is simple to implement, easily parallelizable, and well suited to solving wave propagation and scattering problems with complicated boundaries and nonuniform media. In the FORTRAN 90 programming language the basic code is only a few lines, and the computation time is independent of problem complexity. In this article we derive the basic FDTD algorithm and reinterpret it in terms of an interacting cell model and heuristically derive various features that normally require detailed analysis. Finally we show how to implement boundary conditions at the subgrid level.

Selective observation of elastic‐body resonances via their ringing in transient acoustic scattering

Experiments recently carried out at the University of Le Havre, France [G.Maze and J. Ripoche, Rev. Phys. Appl. 18, 319 (1983); J.Acoust. Soc. Am. 73, 41 (1983)] and at the CNRS laboratories, Marseilles, France [C. Gazanhes et al., Rev. CETHEDEC 70, 1 (1982); Acustica 52, 265 (1983)] regarding the scattering of acoustic transients from submerged elastic bodies exhibit in some instances the ringing of individual resonances of such target objects when insonified by tone bursts of long duration. This phenomenon is interpreted in terms of the interference of multiply circumnavigating circumferential wave trains, and its connection with the complex‐frequency poles of the scattering amplitude f (ω) is established. The poles are visualized via a contour plot of ‖ f (ω)‖ in the complex‐frequency plane.

Complex-Frequency Poles of the Acoustic Scattering Amplitude, and Their Ringing

An elastic body immersed in a fluid will ring when isoni- fied by sound whose frequency is the same as one of the resonances of that body. Correspondence has been established between these normal mode resonances of the body and the individual circumferential waves predicted by creeping wave theory. Insonifying the target by a rela- tively long sinusoidal wave train, with a narrow spectrum centered around, or away from, a selected resonance frequency, results in a series of superimposed responses consisting of the specular reflection and succession of creeping waves arriving after repeated circumnavi- gations of the body which, at resonance only, add in phase to generate the ringing response. Echoes from spherical targets are analyzed in this fashion and are also associated with the resonance poles in the complex frequency plane, obtained by us in the form of contour plots.