An apparatus and method for using ultrasound augmented with microbubbles, thrombolytic drugs or other agents for clot lysis wherein at least one ultrasound transducer generates a plurality of acoustic signals and time, amplitude, phase and frequency modulation of the signals provide more uniform power delivery with fewer gaps in the ultrasound field. Interference patterns from one or multiple transducers are constantly shifted in position. A phased array of transducers may generate a beam that is swept over the area to be treated. In another embodiment, an array of transducers may generate ultrasound at a number of slightly varying frequencies to produce an interference pattern that sweeps in and out through the targeted tissue. A single array may be used to produce both effects simultaneously or separately.