摘要:

A new pulse‐echo technique using real‐time digital averaging is described for measuring the phase velocity in solids up to 50 MHz. The technique can measure the transit time between any two echoes to an absolute accuracy of 0.1 nsec and measure small changes in transit time to a sensitivity of 20 psec. Experimental examples are given based upon ultrasonic velocity measurements in a single crystal superconductor.

ISSN:0001-4966    

DOI:10.1121/1.1913077

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

In this paper we report the generation of ultrasonic waves in solids, at frequencies varying from 10 to 200 MHz, using a dielectric transducer. It is shown that the dominant process involved is the effect of electrostatic forces acting directly on the propagation medium, and not piezoelectricity or electrostriction of the central dielectric.

ISSN:0001-4966    

DOI:10.1121/1.1913078

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The wave equation for elastic waves in piezoelectric crystals is found, including the third‐order terms in the expansion of the thermodynamic potential in the strain and electric field. The effective nonlinear parameters for a general crystal are obtained. In the general case these parameters depend on the second‐order and third‐order coefficients: nonlinear piezoelectric, electrostrictive, and linear electro‐optic. These coefficients are considered for piezoelectric and nonpiezoelectric crystals. Some estimates are given of the values of the nonlinear coefficients.

ISSN:0001-4966    

DOI:10.1121/1.1913079

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

It is shown that simple elastic media in motion exhibit new properties, owing to the effective compressional and shear wave velocities produced by the motion. Presently, we consider scattering of a compressional plane wave by (1) a half‐space moving parallel to the interface and (2) a cylinder moving along its axis. In both cases the assumed boundary conditions correspond to good contact at the interface. Computational results are given for the scattering amplitudes as a function of velocity and angle of incidence.

ISSN:0001-4966    

DOI:10.1121/1.1913080

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The solution of the wave equation with a source term extended in space and moving as a rigid body with translation and rotation is expanded about an interior point in terms of point multipoles, which are expressed as spatial derivatives of the Liénard‐Wiechert solution for a monopole. An application to sound radiation from moving rigid bodies is given using the theory of Ffowcs‐Williams and Hawkings. As examples, the radiation fields of a rotating ellipsoid and a sphere in accelerated rectilinear motion are estimated.

ISSN:0001-4966    

DOI:10.1121/1.1913081

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The problem of the transmission of sound in a duct with very thin shear layers at the walls is treated by an inner expansion method. The results show that the formulation of the problem of the transmission of sound in a duct with a shear layer at the wall converges, in the case of a vanishingly thin shear layer, to the formulation of the same problem when uniform flow is assumed and the wall boundary condition is that of continuity of particle displacement.

ISSN:0001-4966    

DOI:10.1121/1.1913082

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The transmission of acoustic disturbances from the interior of the jet through the mean velocity profile and into the farfield is studied in detail. The noise generator is taken to be a sequence of transient acoustical point sources traveling with the local fluid in the idealized jet. The idealized jet is two‐dimensional, and extends to infinity upstream and downstream with velocity profile independent of streamwise position. For the limited set of examples considered the following conclusions apply: (1) Velocity profile has a large effect on the magnitude of the noise radiated to the farfield. (2) Much of the farfield noise, especially at low Strouhal numbers, originates not as true waves but in the form of acoustical disturbances within the jet which are not radiating energy. (3) At subsonic velocities, the characteristic lobes appearing in a polar plot of farfield mean‐square pressure approach the downstream axis as frequency decreases. This somewhat resembles experimental trends, but many other factors must be considered before a valid comparison can be made.

ISSN:0001-4966    

DOI:10.1121/1.1913083

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The shadowing function, defined as the probability that a point on a rough surface will be illuminated by radiation from a source at fixed height above the surface, is considered. An exact expression for this probability is derived and evaluated approximately when the surface elevation is a normal random process. The effect of surface correlation is considered and shown to be appreciable. Several numerical computations of the shadowing function are presented.

ISSN:0001-4966    

DOI:10.1121/1.1913084

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The scattered field calculated when a plane acoustic wave is incident upon an ideally rigid sphere is considered as a composite and resolved into two parts. One part, which includes the specularly reflected contribution to the field, arises out of scattering by the bright hemisphere—the scattering sphere being divided into two regions by the boundary of the geometrical shadow. The second part, owing to scattering by the shadow side of the sphere, includes the major part of the radiation due to creeping waves. This separation is consimilar with the separation of the scattered field that is made in the creeping‐wave formulation of scattering theory. The separation is made here, however, by treating the scattering sphere as a spherical radiator and solving a pair of boundary‐value problems by classical means. Each of the boundary‐value problems is related to one part of the scattered field. Both parts are found to yield scattering components that are related to “image pulses” such as are predicted when scattering problems involving transient signals are analyzed using theory based on the Kirchhoff approximation. It is found, however, that the image‐pulse‐type returns, which arise out of the exact classical theory, cancel when the two separate parts of the scattered field are added together, so that, in the complete scattered field, no image‐pulse‐type return is detectable. It is also found that a secondary creeping‐wave component originates in the scattering from the bright side of the sphere. This result indicates that the generation of creeping waves may not be a phenomenon taking place solely at the geometrical shadow boundary. Moreover, it is found that the part of the scattered field which yields the specularly reflected return can be very closely approximated by the scattered field that would be predicted by calculations incorporating the Kirchhoff approximation. Interference between the various components, identified as a result of the separation, explain a number of features of the scattering behavior observed when long pulses are incident on the sphere.

ISSN:0001-4966    

DOI:10.1121/1.1913085

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

A theory is developed to describe the propagation of sound waves in a rigid isotropic and homogeneous porous medium filled with a compressible fluid, including both the effect of viscous dissipation and the effect of thermal conduction. An expression for the conductance as a function of the frequency is derived by assuming a capillaric model. A comparison is made with independent experimental results, and close agreement with the theoretical results is obtained. A discussion of the structural parameters, including a description of a method for determining their values at a given frequency, is also made.

ISSN:0001-4966    

DOI:10.1121/1.1913086

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP