摘要:

Coded signals received by a moving receiver or from a moving source exhibit a time‐scaling Doppler effect that can be detrimental to their ability to be correctly decoded. The effect is especially critical for the highly Doppler sensitive pseudo‐random noiseMsequences frequently used to measure propagation travel times. This paper develops a technique to measure dynamic Doppler effects empirically by tracking the phase of the transmitted carrier, and to remove such effects through interpolation and resampling of the signal. These techniques were very effective for simulated signals and will in the future be employed to help resolve the propagation times of the Heard Island feasibility test data collected by the Canadian receivers.

ISSN:0001-4966    

DOI:10.1121/1.410259

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Data from a moving source received by three horizontally separated bottomed sensors in a shallow water environment are analyzed using a broadband matched‐field approach to source localization. The water depth was approximately 150 m, and the low bottom loss in the area results in low spatial coherence of the broadband field. The modeled acoustic fields are constructed via a ray model. Localizations are made in thex‐yplane. In a large search area the time evolution of the ambiguity surfaces produced from the data agree fairly well with simulated surfaces. Ambiguity surfaces are generated successively at intervals of about 1.6 s in small search areas with a fine spatial grid to test the possibility of tracking a source. Results of the source tracking are presented. The analysis also considers the effects of the sea floor on localization in the context of spatial coherence.

ISSN:0001-4966    

DOI:10.1121/1.410260

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A sufficiently strong sound source generates in a thermoviscous fluid, due to nonlinearity, a frequency spectrum consisting of all multiples of the original frequencies and the sums and differences of these multiples. After a certain distance, a shock front is formed because of the energy transfer from lower to higher frequencies. In the case of two original frequencies as a source (the biharmonic case), the damping of high frequencies leaves us at a large distance from the source with primarily the difference frequency. The propagation of plane waves is described by the Burgers’ equation whose solution in the regions before and after the shock formation exhibits significantly different approximate analytical expressions. In this work, an analytical description of the total amplitude in the region after formation of shock in the case of a biharmonic sound source is found. This is a generalization of the well‐known Khokhlov solution for a monochromatic (single frequency) source. This description is turned into a Fourier series which can be specialized into the classical Fay solution for a monochromatic source. From this Fourier series the behavior of the individual frequencies is obtained, in particular the difference frequency which is also examined.

ISSN:0001-4966    

DOI:10.1121/1.410261

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The nonlinear characteristics of an acoustic tube wave propagating along the axis of a fluid‐filled circular borehole in an elastic solid that is locally isotropic but whose properties may vary radially is considered. The analysis is carried out in the quasistatic limit. All terms through quadratic in the amplitude of the wave are considered and the amplitude of second‐harmonic generation and the pressure dependence of the tube wave speed,dVT/dp, are expressed in terms of the fluid and formation nonlinear parameters. The results show that if there is no radial variation of the shear modulus of the solid then both the amplitude of second‐harmonic generation anddVT/dpare independent of the third‐order elastic constants of the solid and nearly equal to those of the fluid alone. If there is a radial variation of the shear modulus then the numerical calculations indicate that both the amplitude of second‐harmonic generation anddVT/dpcan be completely dominated by the nonlinear parameters of the solid. A perturbation theory valid for the case in which the shear modulus is nearly constant is derived that demonstrates that the nonlinear response is scaled by the value of the third‐order parameters of the solid, leveraged by the degree and depth of alteration of the shear modulus.

ISSN:0001-4966    

DOI:10.1121/1.410195

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The propagation of linear, dissipative, time‐harmonic acoustic waves in a broad class of periodic waveguides is investigated theoretically. It is shown that a Floquet‐like theorem is applicable to the system of partial differential equations and boundary conditions that describes the dynamics of a thermoviscous fluid in a periodic waveguide, and that the solution wave functions are therefore Bloch wave functions. Expressions for the parameters that characterize the Bloch wave functions are derived and the band structure of these functions of frequency is investigated. It is found that conditions of axial inversion symmetry (the waveguide may be isotropic or anisotropic) and reciprocity impose substantial restrictions on the allowed Bloch wave solutions.

ISSN:0001-4966    

DOI:10.1121/1.410196

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The propagation of time harmonic acoustic Bloch waves in periodic waveguides is investigated experimentally and the findings compared with those of a theoretical investigation. The waveguides under study are air‐filled rectangular aluminum ducts that are loaded with a periodic array of rigidly terminated rectangular side branches. The two parameters that characterize the Bloch waves, the Bloch wave number and the relative component wave amplitude, are measured and found to compare well with the theory. The band structure that is a well‐known characteristic of Bloch wave dispersion is clearly evident in both parameters, and the features of the band structure are as predicted. The similarities and differences that, according to the theory, are allowed to exist between the forward and the backward traveling Bloch waves are likewise reflected in the measurements.

ISSN:0001-4966    

DOI:10.1121/1.410197

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The shell function method is presented as a new technique to evaluate the acoustic pressure field produced by a practical source of finite area, in a medium having an arbitrary spatially varying sound speed in three dimensions. Instead of initially subdividing the source surface area into many area elements, the shell function method determines the surface element of source area associated with the ray received at the observation point for a specified travel time. Special attention is given to the deficiency of conventional ray tracing for the case of a caustic, where an infinite pressure is predicted. On the other hand, the shell function method takes into account both the convergent state of a sound beam and the corresponding surface element of the source area. As a consequence, the sound pressure at and in the vicinity of caustics can be given an appropriate value directly. Numerical examples are included to show the computational procedure of the shell function method and the ensuing results.

ISSN:0001-4966    

DOI:10.1121/1.410198

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The data, obtained [R. L. Weaver, J. Acoust. Soc. Am.85, 1001 (1989)] on high eigenfrequencies of small aluminum blocks, is reanalyzed using improved fitting procedures. If the results are quantitatively different from those published previously, they do not significantly modify the main conclusion that fluctuations of the spectrum on a small scale (involving a few mean eigenfrequency spacings) are well described by the model of Gaussian orthogonal ensemble (GOE) of random matrices. In addition, it is shown that, on a larger scale, the spectrum is correlated due to the existence of periodic classical ray trajectories. In other words, the large scale oscillations of the spectrum yield information on the size and shape of the aluminum blocks.

ISSN:0001-4966    

DOI:10.1121/1.410199

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

An evaluation of a ray‐tracing volume source formulation of wave scattering is presented. The basis of the formulation is to combine the physically realistic interpretation of propagation as rays with an exact diffractive representation of scattering such as the volume source formulation. The formulation has been used previously in inverse scattering but an evaluation of its accuracy has not been performed for the corresponding direct problem. In this paper the formulation of the technique is formally presented and a numerical evaluation of its accuracy is performed. Results are provided for a variety of cylindrical objects and comparisons are presented with exact results and also results from the Born approximation. The results indicate that indeed dramatic improvements over the Born approximation are possible.

ISSN:0001-4966    

DOI:10.1121/1.410200

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Experiments have been conducted to study the propagation of sound over a rigid surface with a step discontinuity. The Biot–Tolstoy–Medwin (BTM) model of a rigid wedge [J. Acoust. Soc. Am.69, 1060–1064 (1981)] is used to predict the acoustic field directly in the time domain. Experiments carried out with a transient acoustic spark source confirm that: (1) in the region where there are no direct reflected waves, diffracted waves combine with diffracted/reflected waves to produce a smooth transition; (2) the acoustic field around the step discontinuity can be highly directional due to the interference of the direct, the reflected, the diffracted, and the diffracted/reflected waves; and (3) the acoustic field is essentially unchanged if the step discontinuity is replaced with a smooth step whose radius of curvature is less than the characteristic wavelength of the incident sound.

ISSN:0001-4966    

DOI:10.1121/1.410201

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP