摘要:

Measurements and analyses are presented of the backscattering of 420‐kHz sound by 43 individual animals of representative zooplanktonic and micronektonic taxa. Direct measurements of an individual’s target strength were made with a commercial dual‐beam sonar system in an enclosure filled with filtered seawater deployed off a dock at Friday Harbor, Washington. The dependence of target stengths upon individual length, wet weight, and dry weight was investigated. In addition, the ‘‘target strength’’ and statistical variations of echo amplitude due to variations in shape and orientation of the organism were compared with acoustic scattering models involving different shapes (the general shapes of the sphere, and straight and uniformly bent finite cylinders were used along with attempts to take into account roughness). It was found that: (1) backscattering cross sections are proportional to volume of the organisms rather than area as would be predicted by a sphere scattering model, (2) mean target strength based on average backscattering crossection is best described by the bent cylinder model whose modal series solution is truncated, and (3) the fluctuations of the echo amplitudes are well described by the Rice probability density function whose shape parameter is related to the randomly rough straight cylinder model. These extensive studies showed conclusively that the elongated animals scattered sound more like elongated targets than spherical ones, thus demonstrating the need for models more sophisticated than the spherical ones routinely used to date. The data and model analyses provide a basis for devising future acoustical data acquisition and processing techniques for bioacoustical field studies.

ISSN:0001-4966    

DOI:10.1121/1.400077

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper gives the first two moments of a wave field scattered by grazing incidence on a moderately rough surface. The expressions are derived for normally distributed surfaces with arbitrary spectrum, and are valid at depths that are large compared with the surface height. It is demonstrated that the first moment has a weak dependence on the surface fluctuation spectrum. The first moment is compared with Monte Carlo simulations, and gives close agreement. It is also shown that for a given degree of surface roughness the first moment retains the flat‐surface reflection property of being determined by the distance from an ‘‘image source,’’ i.e., the sum of the depths of source and receiver.

ISSN:0001-4966    

DOI:10.1121/1.400078

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

There is a general formulation by Nyborg that accounts for the streaming inside the viscous boundary layer on a solid surface in the presence of a sound wave [W. L. Nyborg, J. Acoust. Soc. Am.30, 329 (1958)]. Using the streaming velocity at the edge of the viscous layer from Nyborg’s theory as a slip boundary condition, the streaming pattern at large outside the layer for various geometries is calculated. Compressibility of the first‐order wave motion is retained, such that its effect is reflected in the boundary condition for the secondary flow, although the latter is considered as incompressible. For the case of a cylinder or a sphere situated at a velocity antinode of a plane standing wave, it is found that the streamlines are closed loops as a consequence of compressibility. If the solid body is displaced from the antinode, the vortex pattern becomes asymmetric. A weak viscous drag acts on the object in the direction opposing the displacement. As a reaction, a weak net flow arises in the direction of the displacement. These findings are consistent with observations, and are absent in previous theoretical treatments of the problem, in which the oscillating flow is assumed to be uniform and incompressible.

ISSN:0001-4966    

DOI:10.1121/1.400079

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The acoustic radiation generated by an impulsive point source in a continuously layered fluid with depth‐varying parameters is investigated theoretically with the aid of the modified Cagniard method. Using a one‐sided time Laplace transformation with a real positive transform parameter, a Fourier transformation with respect to the horizontal space coordinates and appropriate one‐sided Green’s functions, the system of transform‐domain differential equations in the depth coordinate is rewritten as a system of integral equations that can be solved by a Neumann iteration. The modified Cagniard method leads to space‐time expressions for the relevant iterates that physically are representative for the successively reflected waves. This iterative method is shown to be convergent in the time domain for any continuous and piecewise continuously differentiable depth profile in the inertia and compressibility properties of the fluid. To show the generality of the method, the fluid is assumed to show anisotropy in its volume density of mass, which is the kind of anisotropy that shows up in the equivalent medium theory of a finely layered fluid. The continuously refracted waves emitted by the source and the singly, continuously, reflected waves are discussed in detail. With this technique, no difficulties arise with ‘‘turning rays,’’ as is the case in the asymptotic ray theory of the (real) frequency‐domain analysis of the problem.

ISSN:0001-4966    

DOI:10.1121/1.400080

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A continuous Legendre transform spanning the polar angle (θ) domain −∞<θ<∞ embodies the essence of a nonconventional integral representation derived here for source‐excited time‐harmonic pressure fields in the presence of a thin elastic spherical shell immersed in different interior and exterior fluids. Avoiding the conventional Sommerfeld–Watson transform route, the formulation identifiesdirectlythetravelingwaveswith their multiple encirclements of the shell by extending the θ domain from its conventional 0≤θ≤π range with periodicity constraints into the unbounded (multi‐sheeted) domain without these constraints. Periodicity for the closed shell is recovered by summing contributions from an infinite array of image sources located in the ‘‘nonphysical’’ portion θπ of the angular space. The rigorous solution is obtained by synthesis over a complex spectral continuum. Various alternative representations are derived from it, with special attention given to those that lend themselves to ray‐acoustic asymptotics. The rigorously derived ray acoustic constituents include incident and geometrically reflected ray fields as well as surface guided ray fields. The latter are excited by phase matching of the incident ray field to the traveling wave modes in the shell, and they reach the observer by phase‐matched detachment [see also P. L. Marston, J. Acoust. Soc. Am.83, 25–37 (1988)]. The phase matching applies to directly excited leaky waves as well as to waves that decay initially into the fluid; the latter are excited from an exterior source by evanescent tunneling. Transitional effects, where simple ray theory fails, are discussed as well, as is the special case of far‐field plane wave scattering. The ray parametrization for the canonical closed spherical shell will play an essential role in subsequent treatment of deformed and truncated shell geometries [cf. L. B. Felsen and I. T. Lu, J. Acoust. Soc. Am.86, 360–374 (1989)].

ISSN:0001-4966    

DOI:10.1121/1.400081

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper presents a step‐marching method for a wave equation solution in inhomogeneous media. The solution obtained is the transmission signal. The method expands the solution in terms of plane waves, which during propagation, are mixed by the medium inhomogeneity. Like the parabolic equation (PE) method, the present method is step marching in space and thus allows for efficient solutions. From comparison to analytic solutions, the present method is more accurate than the parabolic equation method away from the major propagation direction.

ISSN:0001-4966    

DOI:10.1121/1.400082

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A general model is developed to characterize the effect of an ultrasonic measurement system on the experimental determination of ultrasonic scattering as a function of angle and frequency. The model includes arbitrary emitter beams and detector apertures as angular spectra of plane waves. Arbitrary emitted pulses and detector time gates are incorporated through frequency spectra of temporal harmonics. A transformation of variables is employed to express the spectrum of the measured pressure as a product in wave space of a system function and the Fourier transform of the medium variations. The mean‐square value of the measured pressure spectrum is similarly expressed as a product of the squared magnitude of the system function and the power spectrum of the medium variations. The measured quantities are shown to become scaled values of intrinsic scattering characteristics when the system function weight is concentrated relative to the medium characteristics in wave space. The assumption of an indefinitely long detector gate is used to represent the system function as a product in which one factor is a beam function dependent on spatial frequency and the other factors are dependent on temporal frequency. Beam‐function calculations as well as calculations of second moments and overall beam weight are made for identical Gaussian‐shaped emitter and detector apertures to illustrate the blurring and weighting effects of measurement system beam patterns as a function of scattering angle. The moment calculations are shown to identify circumstances when the medium variation function can be factored out from under the integral and the measurement represented as a simple product of the medium properties and a measurement system weight. The results may be used to design scattering experiments in which degradations due to system effects are within acceptable limits.

ISSN:0001-4966    

DOI:10.1121/1.400083

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The acoustic response of a rectangular cavity to speaker‐generated excitation through waveguides terminating at orifices in the cavity walls is analyzed. To find the effects of orifices, acoustic pressure is expressed by eigenfunctions satisfying Neumann boundary conditions as well as by those satisfying Dirichlet ones. Some of the excess unknowns can be eliminated by point constraints set over the boundary, by appeal to Lagrange undetermined multipliers. The resulting transfer matrix must be further reduced by partial condensation to the order of a matrix describing unmixed boundary conditions. If the cavity is subjected to an axial temperature dependence, the transfer matrix is determined numerically.

ISSN:0001-4966    

DOI:10.1121/1.400084

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The perturbation analysis method of multiple scales is applied to the problem of linear sound propagation in a rectangular waveguide with viscous and thermal dissipation effects at the boundary. In the past, the method of multiple scales has been applied to the corresponding problem of finite level fluctuations in a waveguide. The second‐order solution (representing the first correction due to nonlinearity) cited in these investigations would however preclude the solution of the linear problem, as it grows without bound with the axial coordinate of the waveguide. In this investigation an alternate solution at second order is proposed, which is bounded throughout the spatial domain of the duct. This property of the alternate solution allows the linear problem to be solved, including the region arbitrarily close to the cutoff frequency for higher‐order modes. In addition, the uniform solution may be directly applied to the analysis of finite level fluctuations, with consequences as yet unknown.

ISSN:0001-4966    

DOI:10.1121/1.400085

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This work is concerned with the numerical modeling of elasto‐acoustic problems applied to thin shells and specifically curved plates. A finite element formulation of the elastic problem is coupled to a variational boundary element solution of the acoustic problem. This solution to the acoustic problem, proposed by Mariem and Hamdi [J. B. Mariem and M. A. Hamdi, Int. J. Num. Methods. Eng.24, 1251–1267 (1987)], is implemented using high‐order isoparametric elements, and attention is given to techniques of accelerating the numerical implementation for solutions at multiple frequencies. These techniques include a reduction in the problem size through the use of symmetry and, more importantly, the interpolation of the fluid impedance matrix within the frequency regime. The advantages in using this variational formulation are, first, the manner in which a highly singular integral operator is made amenable to numerical approximation, second, its application to nonclosed thin shells, and, third, its numerical implementation leads to the formulation of a symmetrical fluid matrix.

ISSN:0001-4966    

DOI:10.1121/1.400086

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP