Cell disruptions using ultrasonic energy transmitted through a flexible interface into a liquid region has limitations because the motion of the vibrating tip is not completely transferred into the liquid. To ensure that some degree of contact will be maintained between the ultrasonic horn tip and the flexible interface, the liquid must be pressurized. The pressure conditions that yield consistent coupling between the ultrasonic horn tip and the liquid region were explored in this study by using an analytical model of the system and test fixture experiments. The nature of the interaction between the horn tip and the flexible interface creates pulses of positive pressure rises, increase in temperature, streaming flow, and almost no cavitation in the liquid. There was sufficient energy to create a cloud of microspheres, or beads, that maintain a consistent pattern of ballistic motion in the liquid. The sonication was found to be repeatable by studying video recordings of bead motion and was shown to be statistically consistent using measurements of temperature rise. Sonication of bacterial spores to obtain measurements of released nucleic acid and SEM images of damaged spores were used to verify the effects of liquid pressure on the horn—interface—liquid coupling.