The effectiveness of particle - liquid separation by ultrasonic radiation forces depends on the acoustic energy density distribution in the standing-wavefield. The energy distribution lit an ultrasonic particle-separation device was analyzed to assist continued optimization and design efforts. Measurements of the energy-density distribution in the liquid using a microscope-based imaging system were compared to laser interferometer measurements of the velocity-amplitude distribution on the transducer and reflector surfaces of the ultrasonic separator. The energy density followed the same trend as the surface velocity, being highest near the resonator center and approaching zero near the walls. The energy density gradients and local gridlike reflector-amplitude variation had characteristic lengths of 1.4 mm. These results suggest that the energy-density distribution in the liquid is a defined function of the dimensions, imposed boundary conditions and physical properties of the reflector and transducer. This understanding provides a practical basis for developing a mathematical model of cell aggregation and retention, potentially enabling the design of resonators with predetermined energy density distributions for specific particle aggregation, separation, and fractionation applications.