摘要:

Sound scattering from a sphere of arbitrary size is treated theoretically when the acoustic properties of the sphere are near those of the surrounding medium. Closed form analytic expressions are found for the reflectivity and total cross section. These expressions become exact only in the limit as the ratio of the densities and ratio of the speeds approach unity, but it is shown that for ratios as large as 5/4 and probably as large as 3/2 the approximate reflectivity and total cross section compare favorably with results of exact calculation. Calculations of reflectivity and total cross section, which would require weeks if made from the exact solution of the wave equation, may be completed in a few hours using the approximate closed form expressions.

ISSN:0001-4966    

DOI:10.1121/1.1906766

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The reflection of a spherical sound wave from a wall with the boundary conditions expressed in terms of a normal impedance independent of the angle of incidence is treated. It is shown that the integral for the reflected wave can be evaluated exactly in closed form under certain conditions. The solution given for an arbitrary normal impedance involves a slight approximation of the integral. The reflected wave is brought into a form such that it can be considered originating from an “image source” having a certain amplitude and phase. Graphs for determining this amplitude and phase are given in terms of a “numerical distance,” which depends on the normal impedance and the position of the field point. Pressure distributions around point sources for different wall impedances are shown. The limitations in simulating plane wave conditions at a boundary and the corresponding effect on free field methods of measuring acoustic impedance are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1906767

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The nonspecular reflection of plane waves of sound from certain surfaces composed of absorbent bosses (semicylinders or hemispheres of arbitrary impedance) on an infinite plane of ∞ or 0 impedance is considered. Exact solutions are obtained for the problem of the single boss and then extended, subject to the single‐scattering hypothesis, to obtain far field solutions for certain planar distributions of bosses of radii small compared with the wavelength. The results are compared with those obtained previously for non‐absorbent bosses, and it is shown that the effects of the finite impedance are most pronounced in the simple source terms of the scattered components and may lead to either a decrease or an increase in the radiation reflected at the specular angle. Another effect of the finite impedance (for the small finite distributions) is to shift the critical value of the angle of incidence for which the reflection at the specular angle consists only of the specular component—below this value the reflection at the specular angle being a minimum and above it a maximum. For the infinite uniform random distributions it is found that the effect of the bosses is essentially but to change the impedance of the plane—these effective impedances being functions of the angle of incidence and the parameters and distribution of the bosses. The effect of the finite impedance of the bosses is most pronounced for these distributions yielding terms much lager than those previously retained for the nonabsorbent bosses. The results for the analogous distributions of cylinders and spheres are also given.

ISSN:0001-4966    

DOI:10.1121/1.1906768

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The measured transmittivity of a steel plate in water is presented as a function of the angle of incidence and the product of frequency and plate thickness over wide ranges.The normal velocity of the plate surface can attain an amplitude necessary for good transmission only by constructive interference among internal reflections. It is shown that the ideal conditions can be met in a plate of finite width in only a few cases. In general, the conditions for a transmission maximum are the conditions for the existence of appropriate types of stable traveling waves in a plate of infinite extent; these conditions, however, are modified by edge effects. An apparent effect of this modification is to produce changes in the divergence of the transmitted beam and hence in the observed transmittivity.

ISSN:0001-4966    

DOI:10.1121/1.1906769

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The acoustical impedance is expressed in terms of energy functions, and this procedure is applied to interpret the observed decrease in kinetic mass of circular apertures at large amplitudes.

ISSN:0001-4966    

DOI:10.1121/1.1906770

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Report on research of resonant absorbers; the experiments have been carried out with sound waves by normal incidence on the panel, and by random sound. The measured values are compared with the theoretical ones.

ISSN:0001-4966    

DOI:10.1121/1.1906771

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

It is assumed that acoustic background noise is caused by a distribution of random “white” noise sources whose physical mechanism is unspecified. A law expressing the amplitude‐distance attenuation characteristic of the medium is also assumed. Several distributions of noise sources are considered: uniform volume distributions, uniform surface dipole distributions, and two mixed cases. The noise drop‐off with frequency at a point below the surface is found for each case. For an infinite volume of noise sources, this drop‐off is 6 db/octave at all frequencies. It is shown how this simple model can be generalized to other attenuation laws and other spatial and amplitude distributions of noise sources.

ISSN:0001-4966    

DOI:10.1121/1.1906772

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

To obtain information the characteristics of relaxation phenomena in liquids, measurements of ultrasonic absorption were performed in seven binary mixtures having nitrobenzene as one component.The absorption coefficients in some systems (benzene, chloroform) confirm the results obtained previously in mixtures formed by an unassociated, very absorbing liquid and another component with much smaller absorption coefficient. In this case, the absorption coefficient decreases quickly when small quantities of the second liquid are added to the first one. The explanation of this behavior seems to be found in the fact that an increase of the efficiency of collisions between the molecules occurs. When different molecules are added to the high absorbing liquid, they decrease the time necessary to establish equilibrium among the internal degrees of freedom of the high absorbing molecules.In other mixtures the two components have similar absorption coefficients. These systems are of great interest, because the characteristic behavior of the ultrasonic absorption in the mixture then depends on the nature of both pure liquids. If the two liquids are unassociated and their molecules do not interact strongly between themselves, the curve of the absorption coefficientversusconcentration presents a very clear minimum. If, instead, there are strong interactions, the shape of the curve is altered. This would indicate the presence of an additional type of energy loss.In the systems of nitrobenzene and an alcohol, the absorption coefficient, as a function of the mole fraction, has a maximum at an intermediate concentration, just as happens in some mixtures of water and alcohols.

ISSN:0001-4966    

DOI:10.1121/1.1906773

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The results of experiments with frogs under a hydrostatic pressure demonstrate that cavitation is not an important factor in the mechanism of production of paralysis of the hind legs of frog by ultrasonic (frequency one megacycle) irradiation over the lumbar enlargement region of the spinal cord. Experimental results indicate that a linear relation exists between the reciprocal of the minimum exposure time for paralysis and the acoustic amplitude. This result is readily described in terms of a one factor rate process. On the basis of this experimentally determined relation, it is shown that time rate of change of temperature cannot be correlated with the observations. It is concluded on the basis of a theoretical calculation that absorption of ultrasound at interfaces in the spinal cord does not result in minute hot regions.Further work on summation of subparalytic doses, spaced apart at various time intervals, indicates that the recovery process following exposure to a subparalytic dose of ultrasonic radiation may not be a monotonic function of time.

ISSN:0001-4966    

DOI:10.1121/1.1906774

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1906775

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP