Ordinarily, a cavity in a liquid is unstable. But it can be stabilized by the alternating cycles of compression and expansion than an acoustic field produces, provided the sonic amplitudes and frequencies are properly chosen. The study of coherent SL, now under way at UCLA under the direction of Professor Seth Putterman, has yielded some remarkable results. Each pulse contains about 105 photons, which are emitted in less than 50 pico seconds, that is, in about 10-11 seconds. This is the dynamical Casimir effect.
Related effects appear for other geometries, and for dielectric bodies instead of conductors.
A bubble in water is a hole in a dielectric medium. Under the influence of an oscillating acoustical field, the bubble expands and contracts, with an intrinsic time scale that may be considerably shorter than that of the acoustical field. Owing to the large fractional change in bubble dimensions that may occur, the relation between field and source could be highly nonlinear, resulting in substantial frequency amplification. The mechanisms that have been suggested for cold fusion and sonoluminescence are quite different. But they both depend significantly on nonlinear effects.