摘要:

The nearfield flow noise that is generated in the boundary layer of a vehicle in water turns out to be practically, independent of the kind of vehicle that generates the turbulence. Recorded levels depend on the size and shape of the recording hydrophone, even at very low frequencies when the linear dimensions of the hydrophone are small as compared to the convection wavelength of the turbulence. At high frequencies, because of their finite size, all hydrophones become nearfield insensitive and record the radiated noise. No significant difference was observed in the radiated‐noise curve measured at a distance of about 100 ft from a rising buoyant unit, not even at frequencies as low as 150 Hz, whether the unit was of solid wood or had a thin metal shell. The radiation field depends on the second time derivative of the turbulent velocity fluctuations and, consequently, on the unsteadiness and intermittancy of the turbulence in the boundary layer. For a rotating cylinder, the measured radiation field levels approach those predicted by the recently improved Lighthill formula. For a ship, they are higher; for a buoyant unit, which is a small and a very unstable vehicle, the radiated noise becomes particularly great.

ISSN:0001-4966    

DOI:10.1121/1.1911663

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The propagation constant for sound waves propagated through gases confined to tubes is shown to be a function of the specific heat of the gas, a “Reynolds” number, a “Knudsen” number, and the momentum and energy accommodation coefficients of the surface. A numerical solution for the propagation constant agrees with room temperature velocity and absorption measurements in Ar andN2assuming a momentum accommodation coefficient of 1 and an energy accommodation coefficient of 0.9. No effort was made to clean the tube‐wall surface, and no difference could be detected in a stainless steel and a brass tube. The method offers considerable promise for measuring energy accommodation coefficients.

ISSN:0001-4966    

DOI:10.1121/1.1911664

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Cylindrical geometries with axial symmetry are often used to model physical systems. Although such a model materially simplifies calculations, it may ignore significant effects arising from small asymmetries in the system. In order to demonstrate this and to examine the nature of such effects, a model was analyzed consisting of a cylindrical waveguide with perfectly reflecting walls and an isolated point source displaced small distance from the axis of the hole. The subsequent analysis shows two distinct types of arrivals associated with the geometry. Both of these produce strikingly large asymmetries in the motion of the system. These effects are clearly displayed in the closed‐form solutions obtained for the problem and the light that they shed on the nature of the reflection process at a cylindrical interface may well be of significant value for ray‐theory methods in diffraction.

ISSN:0001-4966    

DOI:10.1121/1.1911665

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Available theory indicates that plane waves of sound are reflected and refracted at an interface of relative motion (a velocity discontinuity) between two regions of fluid. If the relative velocity is sufficiently great, three types of reflection occur, ordinary, total, and amplified, depending on the incident wave angle. In the amplification regime, theory predicts resonances. Here the velocity discontinuity replaced by a transition layer of finite thickness separating the two fluid regions. This layer is approximated by two equal velocity discontinuities (Model I) and by a linear velocity profile shear layer (Model II). For one example in the regime of ordinary reflection, the effects of thickness are negligible for thicknesses up to 1/10 of the incident wavelength. For the chosen examples in amplified reflection, extreme reductions in transmission and reflection coefficients occur for a thickness as little as 1/50 of a wavelength. The two models approach (at unequal rates) total reflection and zero transmission at larger thicknesses. These effects are produced by layers of fluid traveling at or near the apparent wave speed parallel to the shear layer. Such layers tend to “insulate” the two fluid regions from each other.

ISSN:0001-4966    

DOI:10.1121/1.1911666

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Strain‐displacement relations are developed for a thin truncated circular conical shell and its reinforcing rings and stringers. Using these relations in an energy approach, methods are developed for determining the resonant frequencies of simply‐supported conical shells in unstiffened, ring stiffened, or ring‐and‐stringer‐stiffened configurations. The stiffeners may be either widely separated or closely spaced. Computed values obtained using these methods are compared with the results of a series of experiments on ring‐stiffened and ring‐and‐stringer‐stiffened conical shells.

ISSN:0001-4966    

DOI:10.1121/1.1911667

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Utilizing linear shell theory, which includes both membrane and bending effects, the differential equations for the axisymmetric, nontorsional motion of a fluid‐filled thin spherical shell are obtained by means of Hamilton's principle. The motion of the fluid is assumed to be governed by the linear wave equation. It is shown that appropriate limiting cases of the frequency equation of a fluid‐filled shell agree with those of the simpler models previously investigated. A description of some of the salient features of the frequency spectrum of such a fluid‐shell system is also given in view of the frequency spectra of the limiting cases.

ISSN:0001-4966    

DOI:10.1121/1.1911668

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Analysis and numerical results are presented for a problem in which a vertical harmonic load is applied to an infinite isotropic elastic plate. Two cases are analyzed: In the first case, an equal harmonic load is applied to the two bounding planes of the plate in a rectangular region, and in the second case, a harmonic load is applied to the bottom face equal and opposite to the harmonic load on the upper face of the plate. In each case, the vertical displacement at the center of the loaded area is derived and numerical results are presented.

ISSN:0001-4966    

DOI:10.1121/1.1911669

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The fluctuations in energy occurring in successive reflections of acoustic pings from the ocean surface have been measured in the acoustic frequency range from 160 to 1360 Hz. The relations of these fluctuations to the independently measured statistics of the ocean waves indicate that the degree of fluctuation is proportional to the effective roughnessRup to a saturation roughness, the distribution of reflected energies departs more and more strongly from normal as the roughness increases, and only surface waves whose wavelength exceeds that of the impinging acoustic ping by a factor considerably greater than I will produce fluctuations in the reflected pings.

ISSN:0001-4966    

DOI:10.1121/1.1911670

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

This paper investigates the vibration characteristics of a small insoluble gas bubble in quiescent blood subject to harmonic pressure fluctuations close to the natural frequency. Pressure fluctuations of about 0.05, 0.1, and 0.2 of the mean pressure are studied. Results are obtained for the pressure variation of the blood in the vicinity of the bubble and for the changes of the mean radius and surface acceleration of the bubble. The resonance curves are calculated. It is disclosed that the effect of viscosity on the response is rather important in blood.

ISSN:0001-4966    

DOI:10.1121/1.1911671

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

It has been previously determined that precompressed onionskin paper may be used as an acoustic decoupler. In our evaluation, compressional‐wave measurements were made on various types of paper, and in particular onionskin typing paper, at 32 and 1000 kHz. At the lower frequency, a composite material technique was used to allow pressurization of the samples. The velocity of sound and attenuation were found to vary nonlinearly with pressure over the range from 0 to 2000 psi. This nonlinearity introduces a nonlinear acoustic loading of the transducer elements and is the main disadvantage of onionskin paper. However, it was confirmed that the onionskin paper should be more stable with pressure than the other papers, provided that it has been properly precompressed. Most of the insertion loss provided by onionskin paper is caused by the mismatch in acoustic impedance. Thus, a fairly wide variety of materials should provide equivalent acoustical properties.

ISSN:0001-4966    

DOI:10.1121/1.1911672

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP