Acoustic levitation

Acoustic levitation is a method for suspending matter in air against gravity using acoustic radiation pressure from high intensity sound waves.

It works on the same principles as acoustic tweezers by harnessing acoustic radiation forces. However acoustic tweezers are generally small scale devices which operate in a fluid medium and are less affected by gravity, whereas acoustic levitation is primarily concerned with overcoming gravity. Technically speaking, dynamic acoustic levitation is a form of acoustophoresis, though this term is more commonly associated with small scale acoustic tweezers.

Typically, sound waves at ultrasonic frequencies are used thus creating no sound audible to humans. This is primarily due to the high intensity of sound required to counteract gravity. However, there have been cases of audible frequencies being used. There are various techniques for generating the sound, but the most common is the use of piezoelectric transducers which can efficiently generate high amplitude outputs at the desired frequencies.