Three-Dimensional Mid-Air Acoustic Manipulation
Acoustic levitation (also: Acoustophoresis) is a method for suspending matter in a medium by using acoustic radiation pressure from intense sound waves in the medium.
Sometimes ultrasonic frequencies can be used to levitate objects, thus creating no sound heard by the human ear, such as was demonstrated at Otsuka Lab, while others use audible frequencies. There are various ways of launching the sound wave, from creating a wave underneath the object and reflecting it back to its source, to using a (transparent) tank to create a large acoustic field.
Acoustic levitation is usually used for containerless processing which has become more important of late due to the small size and resistance of microchips and other such things in industry. Containerless processing may also be used for applications requiring very-high-purity materials or chemical reactions too rigorous to happen in a container. This method is harder to control than other methods of containerless processing such as electromagnetic levitation but has the advantage of being able to levitate nonconducting materials.
By 2013, acoustic levitation had progressed from motionless levitation to controllably moving hovering objects, an ability useful in the pharmaceutical and electronics industries. A prototype device involved a chessboard-like array of square acoustic emitters that move an object from one square to another by slowly lowering the sound intensity emitted from one square while increasing the sound intensity from the other.
There is no theoretical limit to the force induced by acoustic levitation given a sufficient sound pressure level, but current systems have lifted at most a few kilograms.
Acoustic levitators are used mostly in industry however some products are commercially available to the public.