Levitation is a phenomenon in which an object appears to 'float' in mid-air without wires or strings. Acoustic levitation is levitation using sound waves that counteract the pull of gravity and make an object float in mid-air.

The Principles behind Acoustic Levitation

Sound is a disturbance or vibration that moves through a medium (air, liquids or solids) in the form of waves. The source of sound is any object that moves or rapidly changes shape which causes the air around the object to vibrate.

Consider a bell that is struck, causing a vibration. As one side of the bell moves out, it compresses the air molecules beside it, causing a 'high' pressure area – a compression – which corresponds to the 'peak' of a sound wave; as the side of the bell moves back, it pulls molecules apart and causes a 'low' pressure area – a rarefaction – which corresponds to the 'valley' of a sound wave. The bell repeats the whole process, causing a continuing series of compressions and rarefactions.

Sound is also reflective. Throw a sound at a surface and it will come back in a direct line; this is the principle behind echoes and sonar. Sound that is reflected results in interference or an interaction between compressions and rarefactions. Compressions that meet other compressions strengthen each other; compressions that meet oncoming rarefactions balance out.Acoustic Levitation

Sometimes, interference and compressions combine to form standing waves which appear to vibrate or move in segments or to shift back and forth rather than travel from place to place. This creates an illusion of sound waves 'standing still' – the standing waves.

Standing waves have defined nodes (areas of minimum pressure) and anti-nodes (areas of maximum pressure). An object placed in the nodes of a standing sound wave will 'float' in mid-air as the sound waves counteract gravity.

Acoustic Levitation Process

An acoustic levitator has two parts – a transducer or sound source and a reflector which 'bounces' the sound wave back. The acoustic levitator creates a standing wave; the object being levitated is 'placed' within the area of the sound waves and will eventually find the calm area of the nodes where it will float. Of course, ordinary sound waves do not exert sufficient force to float objects. Extremely intense sound is needed to produce the sound waves for acoustic levitation; usually this is over 150 decibels.

Uses of Acoustic Levitation

Current uses for the process include the manufacture of very small microchips, analysis of chemicals or compounds that cannot be contaminated by contact with a container, and the study of foam physics.

Gravity pulls out liquid from foam, destroying it; 'floating' foam placed in an acoustic levitator allows researchers to study how it performs such tasks as cleaning ocean water. Acoustic levitators can help in the manufacture of very small microchips by tuning or adjusting the sound waves that 'float' molten materials. The proper tuning of the sound waves can create a perfect sphere from molten material or deposit plastics on specific areas of a microchip.