Volume 106, Issue 4, October 1999
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
106(1999); http://dx.doi.org/10.1121/1.427926View Description Hide Description
A new method for solving diffraction problems is presented. It is based on the use of Gaussian diffraction theory. Comparison of the Gaussian beam expansion and Fourier series expansion reveals that the Gaussian expansion is a more general and more powerful technique. The new method combines the Gaussian beam superposition technique [Wen and Breazeale, J. Acoust. Soc. Am. 83, 1752–1756 (1988)] and the numerical (finite-element)solution to the parabolic equation [Huang, J. Acoust. Soc. Am. 84, 1405–1413 (1988)]. The new method is capable of solving for the sound field even in an inhomogeneous medium or in the presence of reflection, as shown by computer modeling. The source may be a Gaussian source or a distributed source. Calculated results are compared with experimental results in a laboratory water tank.
106(1999); http://dx.doi.org/10.1121/1.427927View Description Hide Description
Quantitative experiments have been performed to measure the harmonic generation of narrow-band Rayleigh waves in aluminum. Based on a comparison of several methods, the Sokolinskii comb transducer was selected as an excitation source. Results are presented for narrow-band Rayleigh waves (center frequency 9.85 MHz) generated by the comb and detected with a path-stabilized Michelson interferometer. The on-axis, out-of-plane displacement amplitude was measured as a function of distance from the source. Maximum out-of-plane displacements greater than 7 nm were observed. An existing quasilinear theory that included both diffraction and attenuation was used to interpret the data. Values of the nonlinearity parameter 0.21–0.23 predicted displacements in good agreement with the data and were consistent with values for calculated from literature values of the third-order elastic moduli of aluminum.
106(1999); http://dx.doi.org/10.1121/1.427928View Description Hide Description
The interaction of a sound wave in a compressible fluid in contact with solid boundaries produces thermoacousticeffects. A century ago, Lord Rayleigh gave a general qualitative explanation of the acoustic power production by unsteady heat transfer to compressible fluid in terms of the relative phase of the pressurewave and the heat transferred to the fluid. In quantitative terms his interpretation is equivalent to saying that a wave is promoted if the absolute value of this relative phase is less than π/2; otherwise the wave is attenuated. In this paper, an experimental investigation of the attenuation or promotion of an acoustic wave as a function of an axial temperature gradient at the wall of the waveguide is presented. Using the techniques previously developed [J. Acoust. Soc. Am. 103, 1532–1537 (1998)], acoustic temperature and pressure oscillations measurements in the thermal boundary layer of an acoustic standing wave in air were made. The results reported here experimentally confirm the validity of Rayleigh’s interpretation of the power production for this case. This confirmation is more direct than any previous evidence.
106(1999); http://dx.doi.org/10.1121/1.427929View Description Hide Description
Oscillating thermal diffusion in a sound wave in a mixture of two gases is remarkably effective for separating the components of the mixture. We consider this separation process in boundary-layer approximation, with zero temperature gradient and zero concentration gradient along the direction of sound propagation. In the boundary layer, the combination of thermal diffusion with the oscillating temperature gradient and oscillating velocity gradient leads to second-order time-averaged fluxes of the two components of the mixture in opposite directions, parallel to the wave-propagation direction. The oscillating thermal diffusion also adds to the dissipation of acoustic power in the boundary layer, modifying thermal-relaxation dissipation but leaving viscous dissipation unchanged.