摘要:

The boundary‐value problem for plane‐wave scattering by a spherical cap is formulated in terms of either the radial velocity in the aperture or the equivalent‐vortex strength of the shell. The resulting complementary integral equations are used to construct complementary variational expressions for the scattering cross section and also are solved approximately by Galerkin's method. Numerical results are given for the scattering cross section, say σ, with special reference to the Helmholtz resonator (small aperture) and the hemispherical shell. The function σ(k), wherekis the dimensionless wavenumber based on the radius of the sphere, rises initially ask4(Rayleigh‐scattering regime) to a first peak, σ(k0), and then executes a decaying oscillation about the asymptotic value, σ∞(twice the transverse area intercepted by the incident wave);σ(k0) = O(1/β)as the polar angle of the aperture, β, tends to zero, whilstσ(k0) = 2.13σ∞for the hemispherical shell.

ISSN:0001-4966    

DOI:10.1121/1.1912714

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Illustrations are given of information carried in the waveforms and frequency spectra of acoustic emissions. The effects of multiple reflections and resonances are discussed. A model of the acoustic‐emission source wave is developed, and arguments are given why this wave should be a pulselike function, rather than an oscillatory function of stress. Further use of this model may allow more quantitative treatment of emission amplitudes, energies, and spectra. Experimental results show the use of two instruments for the evaluation of emission spectra. The feasibility of using frequency analysis to obtain information about source events is demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.1912715

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A theoretical analysis of the propagation of the coherent, or average, acoustic wave in a turbulent medium is presented. The analysis is based on the smoothing method developed by Keller and others. The dispersion equation is derived for a statistically homogeneous medium for three special cases: waves in a fluid at rest with density and temperature fluctuations, high‐frequency waves in a fluid in turbulent motion, and waves in an isentropic irrotational turbulent flow. This equation is solved approximately for the propagation constant of high‐ and low‐frequency waves by assuming that the medium is statistically isotropic. From the propagation constant, the propagation speed and attenuation coefficient are obtained. The results indicate that generally the propagation speed of the coherent wave is less than the average sound speed of the medium. The results also indicate a similarity between the effect of turbulent velocity fluctuations and the effect of inhomogeneities the thermodynamic properties of the medium on the attenuation of high‐frequency waves, whereas there is apparently a fundamental difference between the effect of velocity fluctuations and the effect of thermodynamic inhomogeneities on the attenuation of low‐frequency waves.

ISSN:0001-4966    

DOI:10.1121/1.1912716

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The acoustic field of a plane wave incident on an elastic cylinder may be decomposed into two parts, one representing circumferential waves that propagate around the cylinder, the other part obeying in the high‐frequency limit the laws of geometrical optics. The latter part is studied in this paper, and is decomposed into the specularly reflected wave and into a series of transmitted waves which traverse the cylinder either directly along a secant, or undergo additional internal reflections with and without mode convention (compressional to shear type, orvice versa). Wavefront loci and amplitudes of these waves are calculated using the saddle‐point method, and the corresponding reflection and transmission coefficients are shown to reduce, for the case of a large cylinder, to the known expressions for a flat elastic half‐space in contact with a liquid.

ISSN:0001-4966    

DOI:10.1121/1.1912717

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Volume backscattering strengths averaged over four depth intervals and at the frequencies 3.5, 6, 8, and 12 kHz were measured at a number of stations in the South China Sea and the eastern region of the equatorial Indian Ocean. Measured values ranged from −91 dBrem−1, which occurred at 3.5 kHz, to −61 dBrem−1, which occurred at 8 kHz. On the average, the scattering was a maximum in the depth interval 0

ISSN:0001-4966    

DOI:10.1121/1.1912718

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Sonite is a material used for acoustic decoupling in deep‐ocean sonar transducers. Its density, bulk modulus, sound velocity, and attenuation constant have been measured as a function of precompression and ambient pressure. The sound velocity and attenuation constant were measured at 40 kHz and 12°C, utilizing a water‐filled impedance tube. By using a delay line technique at 1 MHz, the material was found to exhibit transverse isotropy. Sample preparation and measurement techniques are described.

ISSN:0001-4966    

DOI:10.1121/1.1912719

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Closed‐form solutions have been developed by Wood [J. Acoust. Soc. Amer.47, 1448–1452 (1970)] for the exact computation of the true distancedand direction θiof a sound source relative to the observer in an unbounded constant‐gradient medium. Wood's solutions are given in terms of the sound speedcat the receiver depth, the apparent direction θ0, the one‐way travel timet, and the positive gradientg. All of these quantities are measurable at the receiving point. Wood makes use of the Pekeris triangle formulas to arrive at these solutions; however, it is of interest to know whether and how the refraction correction equations can be derived from the conventional ray propagation and time equations in a constant‐gradient medium. Such an alternate derivation is presented in this article. Also, a few applications illustrating the utility of Wood's solutions are presented.

ISSN:0001-4966    

DOI:10.1121/1.1912720

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A desired goal of either ray acoustics or normal‐mode analysis in underwater sound propagation problems is the prediction of the intensity at some field point. The ray‐bundle approach in ray acoustics is a technique which has been employed to predict intensity. By formulating the ray‐path problem in terms of a differential equation, rather than in terms of a range integral, a second differential equation can be obtained which describes the continuous variation of intensity along the ray. An extension of the calculation of intensity to allow for variation of the speed of sound with range as well as depth is easily accomplished. It is shown that, for the intensity function to be continuous, a sufficient condition on the velocity function is that it be twice continuously differentiable except at caustics where the theory breaks down.

ISSN:0001-4966    

DOI:10.1121/1.1912721

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A general approach is developed to compute the time‐dependent velocities of transducers within an array resulting from a set of prescribed electrical inputs. It is noted that the effect of the dynamic response of a transducer of any complexity and the effect of the time‐dependent mutual interactions are included within the approach which is based on a Green's function solution to an initial boundary value problem. The solution of the initial boundary value problem is reduced to a set of time‐domain‐coupled convolution integral equations which can be easily solved on a digital computer. In addition to presenting a clear physical understanding of the transient behavior of arrays, the approach leads to a more basic understanding of array operation and limitations caused by its transient response. Specific application of the approach to the analysis of planar arrays mounted in rigid baffles is discussed and related to earlier work. Finally, a simple numerical example is presented to verify the approach and offer insight into the behavior of coupled systems exhibiting array‐type characteristics.

ISSN:0001-4966    

DOI:10.1121/1.1912722

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A method of computing acoustic‐ray paths is developed. The velocity of sound is approximated by a function of depth that has a continuous gradient and allows one to evaluate the range and travel time integrals in closed form. The advantages of this technique over others are discussed, and some interesting phase effects are illustrated.

ISSN:0001-4966    

DOI:10.1121/1.1912723

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP