Johns Hopkins Univ., Baltimore, MD
The enhancement of frequency transformation of acoustic waves (in particular, in parametric arrays) by bubbles added to water has been discussed for about a decade. However, the resonance losses usually degrade the efficiency of the process. Here, the possibility of using small, nonresonant bubbles is considered and the effect by exploiting the resonance of the whole bubble layer is enhanced. It is assumed that a plane wave is incident on such a layer and the transmitted and reflected fields, including the low-frequency waves are investigated, both analytically and numerically. For bubble volume fractions (beta)(approximately equal to)10[sup -3], at a frequency ratio of about 10 and for pump amplitudes of the order of 10[sup -1] atm, the power of the low-frequency signal (including waves radiated in both the directions) may reach 10% of the total power of the incident biharmonic wave. This is a very good efficiency for processes of this type. Further improvements are difficult due to the formation of shocks. [Work supported by the Science Opportunities and Ocean Acoustics Programs of ONR.] [sup a)]Permanent address: Inst. of Appl. Phys., Russian Acad. of Sci., Nizhny Novgorod, Russia.