摘要:

Previous coupled mode approaches to acoustic propagation in ocean waveguides either (1) apply strictly to the case where interfaces other than the (free) ocean surface are range‐independent, or else (2) they satisfy the boundary conditions at such interfaces approximately. In this paper coupled mode theory is extended to give a solution which exactly satisfies the boundary condition on a range‐varying rigid basement. This is achieved by transforming the coordinate system so that at each range the basement coincides with a coordinate surface, and the partial separation of variables is construed with respect to the coordinate that is normal to that surface. Related work by other researchers is discussed, and the numerical implementation of the theory is described. Results for basements exhibiting both upsloping and downsloping regions are presented and discussed.

ISSN:0001-4966    

DOI:10.1121/1.408376

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The simulated annealing technique of nonlinear optimization [S. Kirkpatrick, C. D. Gellatt, Jr., and M. P. Vecchi, Science220, 671–680 (1983)] is applied to optimization of horizontal and vertical sonar receiver array configurations. Optimizations of horizontal planar arrays to minimize array noise gain in a 2‐D isotropic noise field result in regular geometric arrangements of the array elements. Performance measures are then formulated for arbitrary 3‐D array configurations using either plane‐wave beamforming or linear matched‐field beamforming. These measures, which are based on normal‐mode modeling of average array signal and noise responses, are applied first to optimization of plane‐wave–beamformed vertical arrays. The optimized vertical‐array configurations exhibit less regularity of spacing than do the horizontal arrays, because of the complexity of the acoustic field in the vertical. The results can be interpreted as a compromise between placement of hydrophones at depths with high signal‐to‐noise ratio and separation of hydrophones to reduce the array noise gain. Examples are given for optimization for matched‐field processing based on minimization of average array noise power or average sidelobe power.

ISSN:0001-4966    

DOI:10.1121/1.408377

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The determination of elastic constants from ultrasonic bulk wave velocity data taken in a plane of symmetry is studied. It is shown that the problem is well‐posed, i.e., the solution is unique and stable. The displacement polarization factors are used to define the degree of material anisotropy and represent solutions of the Christoffel equation. Such a representation in a rotated coordinate system helps to derive simple expressions for coefficients describing the sensitivity of ultrasonic waves to different elastic constants. Thus the optimal refraction angles for velocity measurement can be determined simply. The analysis performed shows that the elastic constants reconstructed from velocity data are not affected by the selection of initial guesses used in a nonlinear least‐square optimization (the solution is unique). The solution stability is validated by performing reconstruction with different levels of random scatter in the velocity data. The effect of the angle range used for velocity measurements has been analyzed, and it is shown that, depending on the refraction‐angle range, reconstruction of the longitudinal elastic constants is dominated either by the quasilongitudinal or by the quasitransverse velocity. The desirable range of refraction angles for determination of elastic constants with high accuracy is determined.

ISSN:0001-4966    

DOI:10.1121/1.408378

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Polymer relaxations at the glass transition are often used in damping applications. Questions arise whether there is any limit to the height and width of the damping peak that can be achieved. A related question is whether there are any limits on the combination of these two properties that are achievable. This later question arises because of the experimental observation that the height of the peak is inversely related to the width of the peak. In this paper, these questions are addressed using various analytical models of polymer behavior. Starting with the single relaxation time model and progressing to the Cole–Cole model, the Davidson–Cole model, and finally the Havriliak–Negami (HN) model, height and width predictions are obtained. It is found that the HN model predicts a band of physically possible height and width combinations when using reasonable values of the model parameters. High peaks are narrow and broad peaks are low. For a loss factor peak height of 2, the half‐width must be less than 3 decades; for a half‐width of 8 decades, the peak height must be less than 0.5. A comparison with experimental data for polymers of widely varying properties is in good agreement with these predictions.

ISSN:0001-4966    

DOI:10.1121/1.408355

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The characteristics of optoacoustic pulses produced by photostrictive means are investigated. Since such acoustic disturbances have a nonthermal origin, and can be as short as the inverse of the highest frequency phonons that couple the photostrictive deformation to the lattice, ultrashort (subpicosecond) optoacoustic pulses can be produced. The differences are discussed between cold photostrictive optoacoustic pulses and those produced by thermal means in terms of what may be called a rise time reduction factor that characterizes the relative pulsewidths of the thermal to nonthermal pulses; a criterion for picking materials that would show large rise time reduction factors is given. The strength of a photostrictive optoacoustic pulse, in the high‐intensity regime, is calculated. Since the nonthermal nature of the photostrictive optoacoustic pulse is important in the production of ultrashort optoacoustic pulses, thermalization mechanisms subsequent to laser irradiation of the photostrictive material are examined. It is shown that there is a trade‐off between the strength of the compression and the initial temperature that it exhibits, and an estimate of the expected initial temperature of the compression is given. The characteristics of photostrictively generated optoacoustic pulses and those produced by other means, including nonthermal electrostrictive generation, are compared.

ISSN:0001-4966    

DOI:10.1121/1.408356

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

It is known that an elastic cylindrical lossless shell immersed in an acoustic fluid can guide acoustic energy longitudinally without attenuation. In fact, for the physical and geometric parameters within suitable ranges, different surface wave modes, with different angular dependence around the cylinder, can be supported by the structure. These modes propagate longitudinally with subsonic phase velocities, and are evanescent in the transverse direction. The concomitant dispersion relationships, which provide the phase velocity versus frequency, are here established by using a thick shell model, in the form of a transverse resonance equation. Numerical results indicate that for frequencies greater thanka≥∼5 the surface wave propagation tends to become isotropic. This effect is manifested by the change of the shape of the dispersion curve, relating longitudinal and circumferential wave numbers which turns into a circle from the ‘‘figure 8’’ characteristic of the low‐frequency regime. The intensity vector field in the fluid and in the shell are studied for different surface wave modes, at different frequencies with the aim of understanding the mechanism of the power flow. For the axisymmetric case it is found that in the lower portion of the frequency region of existence of the mode, the power flowing in the fluid is substantially larger than in the shell, whereas at higher frequency the situation is reversed. For nonaxisymmetric modes the power flow within the shell is higher than in the fluid with the exception of then=1, mode for which in a limited frequency region the power transported within the fluid is slightly higher. Also for the intermediate frequency range, i.e., ∼2

ISSN:0001-4966    

DOI:10.1121/1.408357

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The far‐field and near‐field solutions for the radiated acoustic pressure from a line‐driven fluid‐loaded, rib‐stiffened thin elastic plate have been obtained. The plate has been configured to have two sets of rib stiffeners, though the formulation given has been extended to include additional rib‐stiffener sets. The stiffeners composing a given set are identical and are spaced periodically with distancel. However, one set of stiffeners is shifted by an amount Δ from the other set. In this manner, portions of the plate may be configured with periodic repeating sections of arbitrary lengthlhaving aperiodic rib spacing. The stiffeners exert reactive forces upon the plate, but not angular moments. Fluid loading is included on the upper surface of the plate while the lower surface is unloaded, except for a time harmonic line force applied normal to the lower surface. The far‐field radiated pressure has been obtained using the method of stationary phase. Numerical integration techniques were used to obtain the near‐field acoustic pressure.

ISSN:0001-4966    

DOI:10.1121/1.408358

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Fully three‐dimensional Green’s functions are important for synthesizing the acoustic scattering response from submerged spherical shells due to excitation not only by actual radiating sources without special symmetries but also by induced sources on the shell which can characterize the effects of truncations or loadings of the empty undisturbed shell prototype. As in a previous two‐dimensional (2‐D) study of scattering due to axisymmetric (azimuthally independent) source configurations [J. M. Ho and L. B. Felsen, J. Acoust. Soc. Am.88, 2389–2414 (1990)], the goal is the derivation of self‐consistent hybrid ray‐mode asymptotic algorithms from appropriaterigorousalternative spectral representations; hybrid ray‐mode algorithms have been shown to be well matched to the wave phenomena which generate the scattered acoustic fields in the fluid in the mid‐ and high‐frequency rangeka≳5, wherekis the wave number in the fluid and ‘‘a’’ is the mean radius of the shell, and also under pulsed excitation. The presentation here deals with the rigorous spectral foundation, asymptotics, and ray‐acoustic interpretation of the rich variety of generic 3‐D wave phenomena encounteredonthe shell surface, which must be understood in order to treat the general case of arbitrary source and observer locations in a follow‐up paper.

ISSN:0001-4966    

DOI:10.1121/1.408359

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The phase trend of a transfer function (TF) has a nearly constant group delay in a reverberant sound field under high modal overlap conditions. This phase trend is predictable from the number of nonminimum phase zeros of the TF. This paper investigates the phase variability from such a phase trend in a reverberant TF. These phase fluctuations, which are generally due to random occurrences of both poles and zeros, can be expressed using the group delay, which is the first derivative of the accumulated phase. Theoretical analysis of the group delay implies that its fluctuations have a ‘‘1/f’’ type of spectrum and that the variance of the group delay is independent of the frequency band and decreases as the damping of the system increases. The results are confirmed by the power spectrum and correlation analysis using measured TF records. Accumulated phase variances are also investigated on the basis of the Δ statistic [F. Dyson and M. Mehta, ‘‘Statistical Theory of the Energy Levels of Complex Systems. IV,’’ J. Math. Phys.4, 701 (1963)]. The variances increase in linear proportion to the frequency interval of interest, as the frequency interval increases; however, they increase in proportion to the square of the frequency interval when the frequency interval is short.

ISSN:0001-4966    

DOI:10.1121/1.408360

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The scattering from quadratic residue diffusers has been predicted using methods based on the Helmholtz–Kirchhoff integral equation. The methods consisted of two boundary integral methods, and two more approximate solutions utilizing Kirchhoff’s boundary conditions. The predictions were compared to measurements made in both the near and far fields. An accurate boundary integral method which works below and above the well cutoff frequency has been found. The limitations of the more approximate methods have been defined. The approximations behind the quadratic residue diffuser’s design have been tested. The important assumption of a phase change local reacting admittance over each well face was found to be good over a wide range of frequencies. The frequency limit of this approximation has also been defined.

ISSN:0001-4966    

DOI:10.1121/1.408361

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP