Previously, acoustic levitators worked by sending out sound waves from an ultrasonic transducer to a reflector. The waves bounced back and forth many times between the transducer and reflector. When the two components were just the right distance from one another—the distance had to be a multiple of half a wavelength—a standing wave appeared between them. In a standing wave, multiple waves add up just right so that some parts of the resulting wave are still. These are called nodes. The rest of the wave is extra big. Acoustic levitators make standing waves that produce enough force to hold particles in the nodes. If someone moves the transducer or reflector, however, that force disappears.