摘要:

Wave propagation in a semi‐infinite, nonuniform, elastic thin‐walled tube filled with incompressible, inviscid fluid is considered. A one‐dimensional problem, with the tube and fluid initially at rest and a pressure disturbance applied at the end, is solved for different axial variations of the tube properties. Formal asymptotic techniques are applied to the linearized partial differential equation governing the pressure and simple progressing wave and high‐frequency solutions are obtained for Heaviside and oscillatory‐pressure boundary conditions, respectively. These asymptotic series solutions terminate for certain parameter variations to give exact solutions for the pressure. Numerical results are presented and some interesting observations concerning the accuracy of the formal results are made.

ISSN:0001-4966    

DOI:10.1121/1.381483

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The problem of acoustic radiation from a pair of spherical sources vibrating with time‐harmonic velocity distributions which are constant in strength and axisymmetric about the same axis is considered. In particular, the modification of the radiation load on one source due to the presence of the other is evaluated. Numerical results are presented for the four cases of equisize sources which are either in phase or 180° out of phase from one another and vibrating in either the pulsating (monopole) or oscillating (dipole) modes. For sources which are small relative to the wavelength, the results are essentially as predicted by previous analyses of point sources, with the exception of the in‐phase dipoles where, for small wavelength separations between the sources, the radiation resistance of one dipole is not doubled by the presence of the other. As the wavelength size of a source is increased, the near presence of another source affects the reactive component of the radiation loading more than the resistive component.

ISSN:0001-4966    

DOI:10.1121/1.381476

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

As observed microscopically, bubbles in a standing‐wave acoustic field move to the pressure nulls and oscillate, in elliptical orbits, about a common axis. The orbits of individual bubbles are equally spaced about 100 μm apart along the axis, forming a linear array. These arrays form in tap water for peak pressure amplitudes of 10–20 bar, but persist at levels as low as 1.5 bar. The 7‐μm diameter of the bubbles is approximately the theoretical size for resonant air bubbles in water driven at the 986 kHz frequency of the field. The radius of the orbits is approximately inversely proportional to the pressure gradient at the pressure null, with proportionality constant 0.25 bar. In a simplified model of the orbit phenomenon the expected proportionality constant is given by (2)1/2ps, wherepsis the threshold pressure amplitude for rectified diffusion and has the value 0.18 bar. Calculations ofps, based on current theories for rectified diffusion, are in substantial agreement with the this value. The period of the orbital motion is about 1 msec for an orbit radiusBof 35 μm, and increases to about 6.5 msec forB=100 μm. Two adjacent bubbles are synchronized in their orbits approximately 90° out of phase, as required for stability of the arrays under Bjerknes forces.

ISSN:0001-4966    

DOI:10.1121/1.381494

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The force on a small sphere due to radiation pressure in a plane wave field was measured and was found to be in excellent quantitative agreement with L. V. King’s predictions [Proc. R. Soc. London Ser. A 147, 212 (1934)]. Reynolds numbers as high as 2500 were used.

ISSN:0001-4966    

DOI:10.1121/1.381486

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The theory of nonlinear elasticity is applied to a study of mode coupling of elastic waves at a particular frequency, called the critical frequency, in a long circular wire. It is assumed that the wire is of homogeneous isotropic elastic material and that the nonlinearity of medium (the effects of higher‐order elasticity) is primary rather than that involved in the Lagrangian stress and strain tensors. The latter is suggested from the fact that the third‐order elastic constants are of larger order of magnitude than the Lamé constants. The method of multiple scales is employed to obtain a system of equations which describes the behavior of the amplitudes involved in the mode coupling. The analysis of the equations shows that nonlinear mode coupling can occur at the critical frequency and that, except at this frequency, the wave undergoes only a phase shift. Further, progressive wave solutions show that the two wave amplitudes can be expressed in terms of Jacobian elliptic functions, and energy exchange between two modes takes place. Under special conditions, these periodic solutions degenerate to the solitary or shocklike solutions.

ISSN:0001-4966    

DOI:10.1121/1.381500

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The propagation of the fundamental and harmonically generated longitudinal elastic waves is treated by means of an asymptotic iterative procedure directly in the governing nonlinear differential equation. Explicit results are obtained for the steady‐state, spatial growth of the second and third harmonic and the depletion of the input wave. As expected, the analysis indicates that the amplitude of theNth harmonic depends on all the elastic constants up to orderN+1. However, the forms for the amplitudes obtained in the asymptotic solution reveal that, for the known range of ratios of elastic constants of successively increasing order and propagation distances commonly encountered in harmonic generation experiments, only the quadratic nonlinearity, which depends on the second‐ and third‐order elastic constants, is required to accurately account for the experimental results. In addition, the amplitude dependence of the phase velocity of the fundamental longitudinal wave is determined.

ISSN:0001-4966    

DOI:10.1121/1.381501

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A numerical technique for the propagation of sound in the ocean where velocity is a function of both range and depth has been developed. The technique is not restricted to the narrow‐angle (parabolic) approximation and it reduces to an exact solution for a homogeneous media. The given field is transformed into a sum of plane waves via an FFT. This transformed field is propagated without approximation through a homogeneous space represented by an average wave number for that space. Updating the average wave number provides for Snell’s law bending in range. The variations from the average velocity are accounted for by summing deviations from the nominal phase to develop a group of direction‐sensitive phase correction masks. A weighted group of plane waves centered about each direction are inverse transformed for multiplication by the phase mask with the results summed. The choice of overlapping weights in the transformed space provides an approximate continuous phase correction mask for all directions. The functional relationship of range‐step size to frequency, angular spectrum, and velocity profiles are developed. Several results for a 1500‐m channel are shown.

ISSN:0001-4966    

DOI:10.1121/1.381502

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Measured backscattering spectra for preserved specimens of three zooplankters (copepod, euphausiid, sergestid shrimp) are compared to fluid sphere scattering models. Quantitative agreement is found at lowkafor all species, however the copepod results are increasingly higher than predicted forka≳1 and the euphausiid and sergestid shirmp are found to be directional scatterers. A fluid prolate spheroid model is proposed fo the latter species.

ISSN:0001-4966    

DOI:10.1121/1.381503

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The effects of a combination of sound‐speed and current fluctuations on propagation of a cw signal in a deep‐ocean model are analyzed. The mean sound‐speed structure is assumed bilinear, and the channel boundaries are horizontal. The horizontally independent sound speed oscillates with a 12‐h period, while the spatially uniform currents consist of quasisteady and diurnally varying components. The total acoustic field for surface‐reflected–bottom‐reflected rays is investigated for dependence on time, source‐receiver separation, and environmental parameters. Multipath propagation is demonstrated for larger propagation ranges, for which case the number, depths, and occurrence times of amplitude fades are shown to be very sensitive to parameter changes. Where the total field is dominated by one ray, contributions from combined sound‐speed and current fluctuations to phase are investigated. In both cases, phase generally can be influenced by both sound‐speed and current fluctuations, but the former more effectively influence amplitude.

ISSN:0001-4966    

DOI:10.1121/1.381504

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This paper presents an approach to the characterization of the low‐frequency ambient‐noise continuum. A tutorial presentation of several statistical tests is given, followed by the application of these tests to the characterization of narrow‐band low‐frequency ambient noise as a function of space, time, and frequency. Measurements of temporal stationarity, spatial homogeneity, and frequency homogeneity are presented.

ISSN:0001-4966    

DOI:10.1121/1.381505

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP