In this study, the elastic constants of a titanium diboride (TiB2) single crystal were measured using resonant ultrasound spectroscopy. In contrast to previous work, the current results are consistent with the measured elastic constants of TiB2 polycrystals. In addition, the crystal anisotropy of TiB2 was examined. The current data show that the elastic properties of TiB2 are much more isotropic than previously considered.

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.

The earliest known reference to the mode-locking, or entrainment, of two maintained oscillators is Christiaan Huygens' description of two pendulum clocks "falling into synchrony" when hung on the same wall. We describe an analogous phenomenon in acoustics-the mode-locking of two thermoacoustic engines which have their cases rigidly welded together, but which are otherwise uncoupled. This "mass-coupling" might compete with acoustic coupling when the latter is used to enforce antiphase mode-locking in such engines, for vibration cancellation. A simple theory relating the phase difference between the engines in the locked state to the corresponding ratio of their pressure amplitudes is in excellent agreement with theory and numerical simulations. The theory's prediction relating the phase difference to the engines' natural frequency difference is qualitatively confirmed by experiment, despite larger experimental uncertainties. The mass coupling is relatively weak compared to the aforementioned acoustic coupling, and in general occurs in antiphase, so we conclude that mass coupling will not interfere with vibration cancellation by acoustic coupling in most circumstances.

We report observation of a new mixture-separation process: an insonified mixture of helium and argon in a narrow duct spatially separates along the acoustic-propagation axis. We measure mole-fraction differences across the ends of the duct as large as 7%. We measure initial separation flux densities as high as 10(-3) M2 c, where M is the acoustic Mach number and c is the sound speed. This initial separation flux, as a function of both the amplitudes and the relative phasing of the pressure and velocity oscillations in the duct, agrees well with a recent theory involving oscillating thermal diffusion in the acoustic boundary layer.

The problem of two self-maintained acoustic resonators that can ‘‘mode-lock’’ by means of a mutual coupling will be considered; in particular, the effects of varying coupler parameters (such as the diameter of a coupling duct) on the relative phase and amplitude of the oscillations in the two resonators will be explored. Experimental data, numerical simulations, and analytical predictions will be compared. The results will be used to help design a scheme for canceling structural vibrations in a thermoacoustic engine, by rigidly attatching two such engines together and selecting the appropriate coupler, such that the engines are forced to oscillate in antiphase. The interplay between acoustic coupling and ‘‘mass coupling’’ (coupling through the rigid structure) will also be discussed. [Work supported by the Office of Fossil Energy in the US DOE.]