Small pieces of styrofoam become trapped between areas of higher pressure induced by high frequency sound waves. Ultrasonic transducers placed at the top and bottom are basically speakers that emit sound waves at a frequency of 43,000 Hz, which corresponds to a wavelength of 8mm for dry air. If the transducers are positioned just right standing waves are produce with nodes of low pressure at half wavelength intervals- and low mass objects like these foam pellets will be suspended against gravity when placed near one of these nodes.