Described are embodiments that employ ultrasonic energy to selectively lyse larger adipose cells in a suspension containing adipose cells of different sizes resulting in a suspension in which the only viable cells are the small adipose cells and stem cells. Embodiments provide for generating an acoustic standing wave field of sufficient intensity and proper geometry, that high shear stress is induced on the cell membranes of cells larger than a predetermined size. The remaining small adipose cells can be physically separated from the suspension after the suspension is subjected to the acoustic standing wave field.