摘要:

In this paper a theoretical and experimental analysis is proposed for elastic wave propagation in heterogeneous waveguides of a rectangular cross section. A formulation is presented which is based on approximate fields of displacement and stress which satisfy exactly the continuity conditions at interfaces and boundary conditions on lateral faces. Two types of waveguides are envisaged: a so‐called rectangular clad‐core guide and a guide made of three superimposed layers, the materials involved being orthotropic. The linear elastodynamics of these guides is deduced from a two‐field variational formulation of the Hellinger–Reissner type. Dispersion spectra are constructed for both types of waveguides depending on the concentration and relative arrangement of the different materials which build up the guides. Geometrical and material conditions deduced from the values of asymptotic speeds are proposed in order to guarantee the propagation of a certain mode in a prescribed region of the guide. Dispersion gauges are introduced with a view to controlling the dispersion which strongly depends on the nature and arrangement of materials. Experimental results obtained by a resonance method are reported for the two types of rectangular guides. The dispersion observed and the existence of guided modes are reported. Finally, comparison is made between theoretical predictions and experimental results, which proves that the predictions were correct.

ISSN:0001-4966    

DOI:10.1121/1.392768

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An abrupt change of cross section in a circular lossless duct is considered. The analysis includes a nonrigid wall at the interface of both parts of the duct. Whereas previous investigators used iterative algorithms to calculate the behavior of the discontinuity, here a straightforward method is developed which avoids any convergence problems. In particular, large discontinuities can also be examined even if an arbitrary number of higher wave modes can propagate. The solution is given in form of a scattering matrix. For the plane wave mode a simple equivalent circuit is derived. The results are compared to an approximate solution given by Karal [J. Acoust. Soc. Am.25, 327–334 (1953)] and to a measurement. At low frequencies all data agree excellently. Some scattering parameters for different cases (different size of discontinuities, different wall admittances, difference frequency ranges) are represented. It is demonstrated that the magnitude of certain scattering parameters can exceed unity, if the corresponding wave is evanescent.

ISSN:0001-4966    

DOI:10.1121/1.392769

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Earlier low‐frequency results for scattering and reflection by random distributions of relatively arbitrary bosses on rigid or free base planes are applied numerically to completely rigid or free ellipsoidally embossed isotropic and anisotropic surfaces. Illustrative curves exhibit the effects of boss shape and orientation, and of their statistical distribution (pair correlated as well as uncorrelated) on the coherent power reflection coefficient and the reflected phase, and on the incoherent differential scattering cross section per unit area versus angles. In addition, for rigid surfaces, simple approximations are included for the location and magnitude of the pseudo‐Brewster minimum in the reflected power and for the extrema of the forward (specular) and backscattered cross sections, as well as for the location of the zero of the phase change on reflection. We emphasize anisotropic forward and backscattered differential cross sections near grazing for applications to sea–air, sea–bottom, and sea–ice programs.

ISSN:0001-4966    

DOI:10.1121/1.392770

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An algorithm for the reconstruction of the physical parameters of a layer of an attenuating medium is presented. The parameters are to vary only along the thickness of the layer. The input data for the algorithm consist of the reflection coefficients of two incident time‐harmonic plane waves intersecting the layer at two different angles. Using scattered data, the density variation of the medium is determined, as is the spatial variation of its attenuation coefficient. The frequency variation of the attenuation is assumed to be known. The reconstruction algorithm is based on the impediography formula modified to account for non‐normal incident waves and for the attenuation of the medium. Several numerical examples of the algorithm are presented using computer‐generated input data for media with specified density and attenuation variations.

ISSN:0001-4966    

DOI:10.1121/1.392771

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Experimental studies about the reflection of bounded acoustic beams onto plane liquid–solid interfaces show that for an incidence at or near the Rayleigh angle, the reflected field exhibits unexpected properties. In particular, this phenomenon has been observed ten years ago by Neubauer and it is now well understood: The key is the reradiation in the liquid medium of the Rayleigh wave generated by coupling with the incident beam. Theoretical descriptions of this feature have been proposed that are based upon asymptotic or numerical considerations on the velocity distribution along the interface. In this paper, these previous theories are enlarged in the following sense: The incident beam is now defined by its velocity distribution along a plane emitter; the reflected beam is described through its pressure field in a region which is not restricted to the interface; several incident beam profiles are considered. Moreover, the short wave asymptotic analysis is different from the previous ones; here field integral representations are evaluated by means of the steepest‐descent method.

ISSN:0001-4966    

DOI:10.1121/1.392772

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Studies of marine animals such as fish and plankton require knowledge of their density (number of animals per unit volume). Standard acoustical techniques, such as echo integration, give accurate estimates of density, but only with a carefully calibrated system. We have found the statistical nature of the echo useful in making rough estimates of density without a calibrated system. In particular, we are studying the probability density function (PDF) of the maximum (peak) value of the echo achieved in a time gate. One useful property of this PDF is that for the two extremes in which the echoes from the individual scatterers overlap and do not overlap, the PDFs take on drastically different shapes. The difference is so great that one can visually judge from the PDF whether or not the echoes were overlapping. For a given ping duration and sonar beamwidth, there is a critical density above which the echoes overlap (or conversely below which the echoes do not). We use this property to estimate densities of animals. Examples are given in this paper of order of magnitude estimates of the density of fish in Trout Lake, Wisconsin and Lake Michigan, and a lower bound estimate of 1‐cm‐size plankton density in the Gulf Stream Boundary near Cape Hatteras, North Carolina. The technique is shown to be simple and robust. It was quickly implemented without calibration of the system. The technique is good for fast preliminary results and is a good check on the accurate, more involved techniques.

ISSN:0001-4966    

DOI:10.1121/1.392773

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Multiple scattering of elastic waves by randomly distributed cylinders of arbitrary cross section has been considered. Because the pair‐correlation function, as well as the quasicrystalline approximation, has been incorporated in the presented formalism, the effective phase velocity, as well as the coherent attenuation in dense systems, has been investigated. A more efficient scattering formalism (SF) has been employed rather than the exciting field formalism (EF) used earlier by the authors. Closed form expressions for the phase velocity and the attenuation are given in the Rayleigh limit, and numerical results are presented for a wide range of frequencies and concentrations.

ISSN:0001-4966    

DOI:10.1121/1.392774

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A propagator model is presented for acoustic wave propagation through a bubble swarm. Both phase velocity and coherent attenuation are studied as a function of frequency and compared with experimental measurements of Silberman [J. Acoust. Soc. Am.29, 925 (1957)].

ISSN:0001-4966    

DOI:10.1121/1.392775

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Approximate expressions are discussed which are applicable for acoustic quantities in the vicinity of a plane piston source of ultrasound which radiates into an absorbing medium. A particularly useful approach for nearfield calculations combines an expression valid near the axis with another, given by Pierce [Acoustics,AnIntroductiontoItsPhysicalPrinciplesandApplications(McGraw‐Hill, New York, 1981), Chap. 5], which is valid elsewhere. This approach gives reasonable accuracy at relatively low computational cost. Computed plots are presented, showing spatial distributions of the square of the pressure amplitude. Most of the plots are for a source diameter of 1.2 cm, a frequency of 3 MHz, and an absorption coefficient of 0.15 Np/cm; these are representative of conditions for medical applications of ultrasound.

ISSN:0001-4966    

DOI:10.1121/1.392776

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A small telemetry device, called a ‘‘vocalight,’’ was designed for attachment to a dolphin’s head using a suction cup. The vocalight lights up a variable number of light‐emitting diodes depending upon the loudness of sounds received at a hydrophone within the suction cup. If vocalights matched for sensitivity are put on each dolphin within a captive group, observers can identify which dolphin produces a vocalization. Use of vocalights indicates that source levels of whistles from captive bottlenosed dolphins,Tursiopstruncatus, range from approximately 125 to over 140 dBre: 1 μPa at 1 m.

ISSN:0001-4966    

DOI:10.1121/1.392777

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP