Nonlinear acoustic method for bubble density measurements.

An enormously high nonlinear response of a bubble to an acoustic excitation makes nonlinear methods possible and very selective for bubble observation and sizing. Among different nonlinear acoustics methods of bubble detection there is one based on difference frequency generation under the action of two high- frequency pump waves. In this method the nonlinear scattering from a single bubble was studied. In the present paper the nonlinear incoherent scattering of two high-frequency acoustic waves with the difference frequency generation from a bubble cloud was theoretically and experimentally investigated to estimate bubble size distribu- tions. In the experiments two primary waves were produced by focused transducers. One of the primary frequencies was kept constant at 2.25 MHz; another was changed. Thus the difference frequency was varied from 30 to 320 kHz, which corresponds to the resonant bubble radii from 109 to 10 μ m. For the bubble clouds produced in a laboratory tank by electrolysis- and slit-type bubble makers, the bubble densities were well estimated with the present nonlinear acoustic method, respectively. [This work was done at the Acoustic Research Lab. of Sung Kyun Kwan Univ. under support of STEPI, Republic of Korea, and supported, in part, by RFBR (Grant 94-05-16755), Russia.]