摘要:

A new method using spatial Fourier transform has been developed to measure reflection coefficients at oblique incidence. The method involves the measurement of complex pressure distributions on two parallel planes lying close to the surface of a test material and decomposing each of the complex pressure distributions into plane‐wave components by using two‐dimensional spatial Fourier transform. The incident and reflected plane‐wave components on the surface of the test material can be mathematically separated by the use of plane‐wave propagation theory. This separation leads to the determination of reflection coefficients at arbitrary angles of incidence. Investigation has been made into the error due to the finite size of the measurement area to show that the magnitude of the error can be reduced by using a dipole source instead of a monopole source. Numerical examples are given to illustrate the validity of the method.

ISSN:0001-4966    

DOI:10.1121/1.400068

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Three control strategies for the active reduction of an enclosed sound field are experimentally implemented and compared. A strategy of energy minimization successfully reduces the sound field at both resonant and antiresonant frequencies, whereas strategies developed for the control of propagating sound in ducts do not perform well.

ISSN:0001-4966    

DOI:10.1121/1.400353

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper presents a method to predict the vibration response of a group of modes, using a sample of modes of the group. The accuracy of the prediction can be improved when increasing the size of the sample. Numerical calculations made on a beam, driven in longitudinal motion, show the ability of the method to reconstitute the energy response of a group of modes including resonant modes, with a sample of a number of modes close to that in the effective damping bandwidth.

ISSN:0001-4966    

DOI:10.1121/1.400069

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The present paper is an illustration of the application of the integral equation technique to a nonlinear random response problem. This method relies heavily on the effective use of matricesabinitioand appropriate matrix manipulation to get the equivalent linear stiffness matrix which is the crux of the equivalent linearization technique used here. Rectangular isotropic plates clamped laterally and free of in‐plane stresses at the edges are subjected to random excitation. The analysis is done using Von Karman equations and numerical results have been presented.

ISSN:0001-4966    

DOI:10.1121/1.400070

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Measurements of the absorption coefficient of ultrasonic waves have been made within the frequency range of 10–100 MHz in aqueous solutions ofN‐methylacetamide (NMAA) and zinc chloride (ZnCl2). The method used to prepare quasi‐two‐component solutions permitted a change in the ratio of the number of amide molecules to electrolyte molecules, given a constant number of water molecules. The results indicate the occurrence of a single relaxational process in the solutions under investigation and the frequency range adopted. On the basis of the theory of relaxational absorption of sounds, relaxation parameters as well as the enthalpy of activation of the solution in question, which was 16.4 kJ/mol, have been calculated. The character of the relaxational process observed was determined on the basis of the analysis of relaxational curves. The discovered relaxational process in H2O–NMAA–ZnCl2was ascribed to the formation and disintegration of ‘‘solvatomers’’ composed of [NMAA Zn(H2O)3]2+.

ISSN:0001-4966    

DOI:10.1121/1.400071

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The stability of resonant vibration to temperature change is a crucial point to consider the feasibility of the method of modal vibration control of large ultrasonic tools using ‘‘wave‐trapped horns.’’ The stability for aluminum tools through theoretical consideration and numerical simulations is evaluated. As a result, the vibration modes are found inherently quite stable to the temperature change expected to occur in ordinary working environments.

ISSN:0001-4966    

DOI:10.1121/1.400072

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Light diffraction by ultrasonic pulses at normal incidence is studied as well in the Raman–Nath region (low frequencies) as in the Bragg region (high frequencies). Based on a generating function method, an analytical expression for the Raman–Nath‐like diffraction has been derived and compared with earlier work. Only small corrections within its validity region were observed. In order to extend these existing theories toward higher frequencies, numerical expressions for the intensity of the diffracted light waves are obtained by means of the Laplace transform theory. This powerful method leads to the same results for Raman–Nath‐like diffraction and can easily be applied for much higher frequencies, too. Examples are provided showing the frequency dependence of the ultrasound, the influence of the Raman–Nath parameterv, and the spectral composition of the pulse on the diffraction pattern. When the pulse approaches a continuous wave, both theories converge to known results. A general condition concerning the symmetry properties of the diffraction spectrum has been derived.

ISSN:0001-4966    

DOI:10.1121/1.400073

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A normal‐mode model, including absorptive effects and elastic waves in the ocean floor, is presented for a source in the water column and a receiver in the water or the elastic medium. The appropriate boundary conditions between all liquid and solid layers are taken into account, and an absorptive semi‐infinite basement is assumed, which causes the radiating wave‐number spectrum to be discrete. The adiabatic approach has been adopted for the case where the acoustic properties or the boundaries vary slowly with range. Results of these calculations display the importance of even a slight range dependence, and of bottom elasticity in shallow‐center sound propagation.

ISSN:0001-4966    

DOI:10.1121/1.400074

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Beginning with the normal mode expansion for the acoustic field at a fixed receiver due to a source that moves at constant depth on a straight‐line course (not necessarily a radial from the receiver) [K. E. Hawker, J. Acoust. Soc. Am.65, 675–681 (1979)] while emitting a time‐harmonic signal, an expression is derived for the correlations among the modal components of the field generated by such a source whose emission is a statistically stationary random process with nonzero bandwidth. This expression is used to identify the degradation suffered by the modal components of the received signal autocorrelation due to experimental time integration. An illustrative example of a low‐frequency narrow‐band source in a shallow water environment is examined. There, the motion of the source results in Doppler splitting of the single source emission line into a group of modal spectral lines at the receiver. Recently it has been experimentally verified [Glattetreetal., J. Acoust. Soc. Am.86, 680–690 (1989)] that for a ‘‘collision course’’ (one that passes directly over the receiver) these lines are stable, resolvable features of the received spectrum. The source in the example, however, is on a ‘‘near miss’’ heading that passes the receiver at a range of 2 km. This produces a time dependence in the frequencies of the Doppler lines that makes their experimental resolution problematic. It is concluded, however, that some of the structure of this line group should still be experimentally resolvable.

ISSN:0001-4966    

DOI:10.1121/1.400075

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A three‐dimensional (3‐D), second‐order finite‐difference method was used to create synthetic seismograms for wave propagation in heterogeneous media in order to investigate the scattering of elastic and acoustic energy due to topography on the seafloor. The method uses a fully staggered grid in Cartesian coordinates as developed by Virieux [Geophysics51, 889–901 (1986)]. Numerical results were generated for two models: a linear fault scarp on the seafloor, and a flat seafloor containing a rectangular channel. Wave‐front snapshots allow the scattering and focusing of different wave modes with direction to be visualized. Compressional and shear wave backscattering from the sides of the features can be seen together with the trapped compressional wave energy propagating inside the channel. The results illustrate the effects of out of the plane scattering due to simple seafloor topographic features.

ISSN:0001-4966    

DOI:10.1121/1.400076

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP