摘要:

A modified form of the Eyring equation is derived which explicitly takes account of the anisotropy of a sound field due to the deviation of the energy reflection (absorption) coefficient of individual surfaces from the mean value for all surfaces. When all surfaces have the same energy absorption coefficient, the equation simplifies to that of Eyring; whenin additionthese absorption coefficients are small, the usual simplification to the Sabine equation is valid. Using a simplified version of this modified Eyring equation, one can show that the effective absorption coefficient of a single absorptive surface in a hypothetical reverberation room is a complicated function of the location and total area of randomly oriented scattering panels. Apart from any correction for diffraction, the effective absorption coefficient is also a function of the area of the absorptive surface. In typical situations, this theory predicts that the effective coefficient will be greater (0 to 40%) than the energy absorption coefficient. In general terms, this agrees with measured values. In extreme situations not usually found in reverberation rooms, effective absorption coefficients are predicted to vary more widely and to be either greater or less than the energy absorption coefficient.

ISSN:0001-4966    

DOI:10.1121/1.1912704

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The generation of thermal acoustic waves by a heated metallic film on the surface of a dielectric crystal at low temperatures is discussed. It is shown that even if the waves produced correspond to a uniform distribution of directions of wave vector, there may be a large enhancement of energy flow in some crystallographic directions compared to the average. This effect arises because of elastic anisotropy. Explicit expressions are given for the enhancement expected in the principal directions of cubic crystals.

ISSN:0001-4966    

DOI:10.1121/1.1912705

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

For an infinite plane elastic wave which strikes the plane interface separating two semiinfinite isotropic media, the calculation of the complex reflection coefficient,R*, for varying angles of incidence, θ, is not difficult. In the past, calculations have been made for a number of lossless media and, consequently, an important facet of theR*−θ curve for real materials has been overlooked. The total reflection, |R*|=1, which occurs at the longitudinal‐ and shear‐wave critical angles is well known, but the appearance of a minimum (sometimes a zero) inR* is not and its existence defines a third critical angle, sometimes inappropriately called the Rayleigh‐wave angle, at which a wave with large surface components is generated. During experiments with beams of acoustic waves, there is an apparent lateral displacement of the reflected beam at the third critical angle which manifests itself markedly only when there is a near‐zero in |R*|. Notwithstanding the calculations of Schoch [Ergeb. Exakt. Naturw.23, 127–234 (1950)] and the experimental measurements of others apparently to the contrary, it is strongly suggested that no actual lateral displacement occurs and that re‐radiation from the region outside that ensonified by the incident beam gives rise to the apparent lateral displacement. Some outcomes of the investigation are discussed and of these the more important are: (1) Rayleigh and other interface and surface waves in real media are degenerate; (2) no interface wave can exist independently at the junction of two nonideal media; (3) Huyghens's principle in its elementary secondary‐wavelet form does not apply to lossy media; (4) the usefulness of the sensitivity ofR* to minute changes in elastic parameters at the Rayleigh angle as a measuring tool has a wide application; (5) elliptical polarization of both shear and longitudinal waves can occur, owing to boundary influences in lossy refracting media. The real part of the propagation vector lies in the plane of the ellipse.

ISSN:0001-4966    

DOI:10.1121/1.1912706

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The transformation of acoustic impedance can be accomplished either by a controlled variation of wavefront area as the wave progresses, or by transmission of the waves through a medium in which there is a gradient of density or elasticity. The second concept does not seem to have been exploited despite the fact that the effective density of a fluid medium can be increased by introducing suitable arrays of obstacles, and the bulk modulus of a liquid decreased by compliant inclusions. The present analysis shows that for plane waves a medium in which density varies as (B+mx)−2or elastic modulus as (B−mx)2, wherexis distance andBandmare constants, is analogous to the familiar exponential horn. Simple analytical solutions are also found for radial variations of density or elasticity which transform the behavior of sectoral and conical horns into the acoustic equivalents of exponential horns. Since the wave equation is separable in both cylindrical and spherical coordinates, these solutions are exact. Three examples of artificial fluid structure are given and the relationships between array size, bandwidth, and damping are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1912707

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The frequency dependence of the velocity of Rayleigh waves propagating along rough surfaces is examined from the viewpoint of the mass loading of a smooth surface. Predicted values are given for examples of idealized surfaces on a solid of Poisson's ratio 0.29.

ISSN:0001-4966    

DOI:10.1121/1.1912708

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Closed‐form expressions are presented for the driving‐point impedance, transfer impedance, and force transmissibility of circular and annular plates with clamped boundaries. The plates are internally damped and are driven either by a sinusoidally varying central point force or by one or more concentric ring forces of variable radii. The circular plates are mass loaded or elastically restrained at their midpoints, or are loaded by an ideally rigid annular mass (rib) of arbitrary radius. Computed results illustrating the frequency dependence of impedance and transmissibility are described and their physical significance is discussed. Added mass is very effective in increasing plate impedance levels and in blocking force transmission to the plate boundaries, particularly when the mass is positioned directly beneath the driving force.

ISSN:0001-4966    

DOI:10.1121/1.1912709

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

By taking account of second‐order coupling between stress and strain, modified equations of elastic motion have been derived which include the effects of static surface forces and body forces upon the propagation of elastic waves in isotropic materials. Anisotropy of the kind found in hexagonal crystals is shown to occur when an isotropic substance is acted upon by a uniaxial static stress. A brief discussion of some inferences to be drawn from these results is given.

ISSN:0001-4966    

DOI:10.1121/1.1912710

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The magnetic‐field dependence of the complex Young's and shear moduli are determined for a variety of nonferromagnetic metals. An experimental technique using capacitive excitation of torsional and longitudinal modes in low‐loss systems is described and the criterion for frequency resolution is discussed. Contributions to the magnetic‐field dependence of the elasticity due to the charge carriers and dislocations are considered.

ISSN:0001-4966    

DOI:10.1121/1.1912711

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The equations describing an elastic isotropic Cosserat continuum are presented. Solutions of these equations for the various modes of plane harmonic waves in an infinite medium are briefly discussed. The analysis then concentrates on the development and interpretation of the surface‐wave solution for straight crested waves on a Cosserat half‐space. It is found that a wave analogous to the Rayleigh wave of the classical elasticity theory exists, except that it is a dispersive wave in this theory. The phase velocity of the surface wave may either increase or decrease with frequency, depending on the relative magnitude of the micromaterial moduli.

ISSN:0001-4966    

DOI:10.1121/1.1912712

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The three‐dimensional transient interaction of spherical acoustic waves with a cylindrical elastic shell is investigated using the concept of fictitious Riemann surface. Integral transform techniques are used. The transform double integrals with respect to the two shell coordinates are evaluated asymptotically by the method of “critical points.” Simple expressions for the early‐time responses of the shell and the scattered wave are obtained. The results of this investigation show that the effects of the curvature of the incident wave are quite significant with respect to the bending moments of the shell.

ISSN:0001-4966    

DOI:10.1121/1.1912713

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP