Introduction
Unless you live in a vacuum, which would be physically impossible since you couldn't breath, you're always in the presence of sound. Whether this is sound you can hear is another question, given that as humans have a limited audible range. Sound is a daily part of our lives and we need it to communicate, listen to music, or even to view a sonograph of an unborn child. So what is sound? Sound is defined as a pressure wave generated by vibrations, such as the rapid movement of your vocal cords, which requires a medium to travel. Sound is commonly viewed as a longitudinal wave whose oscillations, compression and expansion, are parallel to the direction the wave is travelling. This is in contrast to transverse waves, in which oscillations are perpendicular to the direction of travel. If you happen to have a slinky, lay it on the ground and push it inwards. This will create a pulse, where the slinky is compact. This describes the motion of a longitudinal wave. The propagation of sound can be thought of as a mass of particles, where as they collide with one another, transfer momentum, causing the wave to travel [1]. This can be seen below [2].
When two sound waves meet in space they interfere with each other. A wide range of interference patterns may occur depending on the frequency, phase and amplitude of the wave. One possibility is a standing wave, a wave that looks as if its not moving. These waves have pressure changes only in certain points in space, with the others having zero intensity.
The Physics
What is Acoustic Levitation? Have you ever been to a loud rock concert, up and close near the huge speakers? If you have you may have felt sound. The fact that sound has a physical presence, in the form of acoustic pressure, is what allows for acoustic levitation. Acoustic levitation involves the use of sound waves to counteract the effect of gravity. There are two primary types of acoustic levitation, standing wave acoustic levitation, and near-field acoustic levitation [3-4].Here we will only talk about standing wave acoustic levitation. Acoustic levitation has some advantages over other types of levitation, such as magnetic and optical, due to its ability to act on all objects. Problems with acoustic levitation include the need of a media to propagate through and the levitation of macroscopic objects. Here we will look at a basic acoustic levitator including some of the principles involved.
Standing Wave Acoustic Levitation A common acoustic levitator usually includes a transducer and a reflector. The transducer, or vibrating surface that produces the sound wave, similar to that of a loudspeaker, produces a sound that reflects off the reflector. From the first section, you should know that if the two are placed at the correct distance and the initial wave has the desired frequency, a standing wave could be formed. In some cases two waves are individually produced in order to make the standing wave. For acoustic levitation, the orientation of the wave must be parallel to the force of gravity. Standing waves, made up of regions of maximum and minimum pressure have the desirable characteristics for acoustic levitation. A material at a nodal point, region of zero pressure, has the ability to be levitated. Objects located at these unique points will experience a downward force due to gravity but as it is pushed down will encounter atoms moving in the opposite direction that provide an upward force. This keeps the object a location slightly below the nodal point. Additional requirements are needed, such as non-linear sound waves, but understanding this requires higher levels of physics [5]. Interestingly, this method has been used to levitate small animals such as ladybugs and ants [6].
Summary
Acoustic Levitation can be performed on any media that has the required weight and size ratios given powerful enough sound wave emitters. Although its applications will probably never be seen by the public as most uses are restricted to factories and research laboratories, the advancements and products that can utilize the process of acoustic levitation will certainly influence our lives.
