Two opposing arrays of ultrasonic transducers emit sound waves that travel toward each other and interfere, creating a standing wave instead of a travelling one. At certain fixed heights in the gap — the pressure nodes — the alternating compressions and rarefactions cancel out almost completely, while at the midpoints between them (the antinodes) the pressure swings violently. Small, light objects placed in the field feel a net force, the acoustic radiation force, that pushes them away from the loud antinodes and traps them at the quiet nodes, where they can hover indefinitely against gravity.
Real acoustic levitators typically run at 20–40 kHz — just above human hearing — and can trap water droplets, insects, small electronic components and even live seeds, which makes the technique useful for container-free mixing and drug-formulation research where touching a surface would contaminate the sample.
Two opposing ultrasonic transducer arrays fire sound waves at each other, forming a standing wave whose pressure nodes act as invisible shelves that hold small beads against gravity.
Pressure nodes sit every half wavelength apart, so raising the frequency shortens the wavelength and packs the nodes closer together, letting particles hop to a new resting height in real time.
Tune the frequency to relocate the nodes, drag amplitude below the trapping threshold to watch particles fall, switch the emitter array size to see the field steady out, and toggle the field off entirely.
Because the radiation force scales with the square of amplitude, halving the drive voltage cuts the levitation force to a quarter — which is why real levitators need a comfortable safety margin above the trapping threshold.