Experimental observation of subharmonic oscillations in Infoson bubbles

This paper reports the result of an experimental study of nonlinear emission from gas-filled bubbles with particular emphasis on the subharmonic emission. The gas bubbles are Infoson, a contrast agent for use in echocardiology consisting of small gas-filled microspheres with a mean diameter of approximately 4 μm. Pulsed signals with center frequencies of 3.5 and 4 MHz were transmitted through a cloud of bubbles and the level of the subharmonic component was measured as a function of the level of the exciting signal. No sharp threshold, as expected from theory, is found. The amplitude of the subharmonic response increases, however, approximately as the cube of the driving pressure when the driving pressure is in the region of 50–100 kPa.