摘要:

The ultrasonic absorption coefficient has been measured in single benzene crystals along the principal crystallographic axis. The range 6–38 MHz and at temperatures between 170° and 250°K. All three cases indicated a relaxation frequency near the middle of the frequency range (different in each case), and the values along thea‐ andc‐axes suggest the presence of other relaxation frequencies. The measurements have been compared in detail with the theories of Liebermann and Danielmeyer, but the agreement is far from satisfactory.

ISSN:0001-4966    

DOI:10.1121/1.1914462

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The analytic solution for a Rayleigh‐type surface wave on a linear viscoelastic half‐space is obtained. The general viscoelastic solution predicts several properties of the surface wave distinct from those predicted by elasticity. The velocity, absorption coefficient, and displacement field are described theoretically.

ISSN:0001-4966    

DOI:10.1121/1.1914463

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The method of multiple scales is used to derive the equations that describe the spatial and temporal variation of the amplitudes and phases of a wave packet propagating in slowly varying hard‐walled or lined ducts. The analysis is carried out for rectangular as well as circular ducts. These equations are statements of the conservation of energy. For large admittance or high‐frequency modes, an approximate expression is obtained for the attenuation. This expression shows that all possible acoustic modes are attenuating. The results also show that decreasing the cross‐sectional area can lead to elimination of some of the acoustic modes.

ISSN:0001-4966    

DOI:10.1121/1.1914464

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The surface Helmholtz integral equation (SHIE) for the acoustic pressure on an object whose surfaceSvibrates harmonically with a given normal velocity, is known to fail by nonuniqueness at certain “characteristic” frequencies. These frequencies are identical to the eigenfrequencies of the homogeneous Dirichlet problem for a volume of fluid enclosed by the surfaceS. This paper deals with nondegenerate characteristic frequencies, that is, frequencies at which exactly one independent solution of the interior homogeneous Dirichlet problem exists but no nontrivial solutions of the interior homogeneous Neuman problem exist. The problem is reformulated in terms of operators on the Hilbert space of functions which are square integrable overS. The eigenvectors of the (skew‐Hermitian) Green's function operator for the interior Neumann problem provide an orthonormal basis for the space and, hence, the unknown Green's function operator for the exterior Dirichlet problem can be completely specified by its matrix elements with respect to these eigenvectors. The nonuniqueness failure of the SHIE is found to result from the indeterminacy of a single element of this infinite matrix. An expression for the indeterminate matrix element is obtained directly from the SHIE by a perturbation technique yielding a formal solution of the problem.

ISSN:0001-4966    

DOI:10.1121/1.1914465

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Morse's equation for the radiation impedance of a vibrating rigid piston is extended to give the generalized radiation impedancesZmnlof all modes in circular and annular ducts which have arbitrary wall admittance β and which terminate in an infinite baffle. The equation forZmnlis reduced to a single infinite integral which is a function of the mode radiation directivity factors. An infinite matrix equation is derived which relates the generalized mode reflection coefficientsRmnlto the radiation impedances. Results are presented which show that the duct‐wall admittance is a significant parameter in the effect of duct‐termination reflections and that mode‐coupling reflection effects may be more important than direct‐mode reflection effects.

ISSN:0001-4966    

DOI:10.1121/1.1914466

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The Bernoulli‐Euler theory of transverse beam vibration, suitably extended to take into account internal beam damping, is used to derive closed‐form expressions for the mechanical driving‐point impedance and force transmissibility of two types of nonuniform cantilever beams that are driven at their free ends by a sinusoidally varying point force. The beams are truncated and of rectangular cross section; they have a linearly tapered depth and have either (1) constant breadth or (2) parabolically varying breadth. Representative computations of the frequency dependence of impedance and transmissibility are plotted for beams having the same length and mass as an equally long and equally massive uniform reference beam. Significant attenuation or amplification of force transmissibility is observed, depending on the proportions of the beams and on whether their depth tapers towards their fixed or free end. The measured resonance frequencies of a set of nonuniform aluminum (6061‐T651) cantilever beams agree closely with the predictions of theory.

ISSN:0001-4966    

DOI:10.1121/1.1914467

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The vibratory patterns of a circular plate were studied systematically by means of real‐time holography and also by time‐averaged holography. The present real‐time method, an extension of the original method of Stetson and Powell (1965), allows one to view reconstructions of vibratory patterns directly and observe their changes as the frequency and/or amplitude of the signal are being varied. The contrast obtained is much poorer than that given by the time‐averaged method. Nevertheless, the real‐time method permits one to pinpoint specific mode patterns for later recording by time‐averaged holography. In a given situation, time‐averaged holography records approximately twice as many alternate bright and dark fringes as the real‐time method.

ISSN:0001-4966    

DOI:10.1121/1.1914468

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The radiation from a spherical source vibrating with an arbitrary, axisymmetric, time‐harmonic velocity distribution while positioned at an arbitrary point within a fluid sphere, which is embedded in another infinite fluid medium, is computed. This configuration is an idealization of a spherical acoustic lens, with focal point inside the lens, when used as a sound projector. The translational addition theorem for spherical wave functions is used to express series of wave modes centered at one sphere in terms of modes centered at the other, thereby facilitating the task of satisfying the boundary conditions. The radiation load on the source and the farfield directivity pattern are evaluated for representative values of the many parameters that characterize the problem, such as the wavelength size of the lens and source, the relative characteristic impedances and sound velocities of the two fluid media, the position of the source, and the velocity distribution on the source sphere. In general, no great increase in the radiation resistance is noted when the source is at the focal point of the lens. The focal points for monopole and dipole sources are usually not coincident. The size of the source has little effect upon the farfield directivity pattern.

ISSN:0001-4966    

DOI:10.1121/1.1914469

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Shallow‐water attenuation loss was studied under varying oceanographic conditions within a standard area. By using combined ship‐aircraft measurement methods, sound‐propagation characteristics were ascertained in conjunction with relevant environmental parameters. The selected sea area had only very slight lateral variation for all environmental characteristics permitting good control of measurements. Evaluations to date indicate typical frequency‐response differences for attenuation, with or without sea‐surface participation in sound propagation. With bottom participation only, the magnitude and frequency response of the attenuation have a first approximation consisting of a model with the bottom represented only as a quasiliquid smooth stratification of the complex sound velocity and density.

ISSN:0001-4966    

DOI:10.1121/1.1914470

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Our results are always parameterless, that is, explicit functions of the problem variables found without using rays, ray equations, or ray parameters. Whenc= sinhz, we find a series solution of the reduced wave equation▿2u + ω2c−2u = 0characterizing radiation from a point source at (x0,y0,z0) in three dimensions. Ignoring the shadow region(x − x0)2 + (y − y0)2 > π2, we use formal methods to find the first three terms ofaseries for the radiation solution, beginning with the termexp[iτ(ω2 − 1/4)1/2]{sinρ/[ρc(z)c(z0)]}1/2 cschτ, τ defined by coshτ = (coshz coshz0 − cosρ)/(sinhz sinhz0)andρ = [(x − x0)2 + (y − y0)2]1/2. In all of two‐dimensional (x, z) space except the shadow region|x − x0| > π, formal methods and reexamination of Cohn's Riemann functions show thatQλ(coshτ) is an excellent approximation to the radiation solution. HereQλis a Legendre function,λ = 1/2 + i(ω2 − 1/4)1/2, andρ = x − x0. Special and limiting cases of the above give analogous results forcequal to coshz, cosz,ez, z, and 1. We obtain Pekeris' exact solutions whenc(z) =z, and the classical solutions whenc(z) = 1.

ISSN:0001-4966    

DOI:10.1121/1.1914471

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP