There is an umbrella sticking up through the sand at the beach on a warm, sunny day. At the crash of the most recent wave, a large gust of wind quickens up the sand, and hoists the umbrella, taking it for a ride high into the air. What is happening here, i.e. the basic physics of the wind/umbrella interaction, is not so difficult to understand. There exists a difference in air pressure: above the umbrella low pressure, beneath the umbrella high pressure. The high pressure wishes to equalize with the low pressure, so the high pressure air moves up and mixes with the low pressure region, carrying the umbrella with it. Thus the umbrella rises.
The phenomenon of wind arises from differences in air pressure in space. Too much air over here compared to there, and the wind moves over there. Sound travels through air in a similar manner. You bang on the table, and the vibrations in your hand and those on the table jiggle the nearby air.... And this is all encapsulated in something called the Gor'kov potential, given by $$U = 2\pi R^3\left(\frac{\langle p^2\rangle}{3\rho c^2} - \frac{\rho\langle u^2\rangle }{2}\right).\label{gorkov}\tag{1}$$ Here, $R$ is the radius of the levitating sphere, $\rho$ the air density, $c$ the speed of sound in the air, and $\langle p^2\rangle$ and $\langle u^2\rangle$ the mean square amplitudes of the sound pressure and velocity, respectively. Similar to Newton's potential in orbital mechanics, or Coulomb's potential in electrostatics, levitating objects subject to the Gor'kov potential reside in positions where $(\ref{gorkov})$ is a minimum. This perturbation in the surrounding air is enough to create a pressure wave in the air, which transmits outward and eventually vibrates your eardrums, which our inbred microphones interpret as a sharp clap. Seeing that sound and wind are just pressure differentials in the surrounding air, it is interesting to ask if sound can induce similar effects as wind. For instance, might sound lift the umbrella?
In comes the phenomenon of acoustic levitation (a.k.a. acoustophoresis), wherein sound floats material objects. The culprit here is that of acoustic radiation pressure. To understand this, consider the following gedanken-experiment. There is a speaker placed down on a table, so that sound projects from the speaker to the ceiling. Suppose the speaker is attached to a function generator, and so outputs a noise with a constant frequency. The speaker operates by vibrating a ``plate'' up and down at the frequency set by the function generator, thus creating regular disturbances in the air, which your eardrums interpret as sound. These disturbances are longitudinal waves, which should be contrasted with transverse waves. Whereas a transverse wave vibrates in the direction perpendicular to the direction along which the wave is moving, longitudinal waves vibrate in the direction parallel to the direction of motion of the wave. Electromagnetic waves, as you will learn later in Physics 51, are transverse. However, ocean waves are longitudinal, as are sound waves. (Click here for a neat GIF contrasting longitudinal and transverse waves.)
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