A two step method of enhancing the cavitational effect produced by ultrasonic signals focused at a specific location in a medium includes a first step of using one or more transducers to apply the ultrasonic signals to create a waveform that causes formation of bubbles at the location, and a second step of modulating the amplitude of the ultrasonic signals at a frequency of several Hz to several tens of kilohertz immediately following or coincident with the formation of the bubbles.