Acoustic levitation uses a resonant ultrasonic standing wave to suspend small objects; it is used in a variety of research disciplines, particularly in the study of phase transitions and materials susceptible to contamination, or as a stabilization mechanism in microgravity environments. The levitation equipment used for such research is quite costly; we wanted to develop a simple, inexpensive system to demonstrate this visually striking example of standing waves. A search of the literature produced only one article relevant to creating such an apparatus, but the authors’ approach uses a test tube, which limits the access to the standing wave. Our apparatus, shown in Fig. 1, can levitate multiple small (1-2 mm) pieces of expanded polystyrene (Styrofoam) using components readily available to most instructors of introductory physics. Acoustic levitation occurs in small, stable equilibrium locations where the weight of the object is balanced by the acoustic radiation force created by an ultrasonic standing wave; these locations are slightly below the pressure nodes. The levitation process also creates a horizontal restoring force. Since the pressure nodes are also velocity antinodes, this transverse stability may be analogous to the effect of an upward air stream supporting a ball.

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A demonstration of acoustical levitation
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2.
An excellent demonstration and explanation can be found at https://www.youtube.com/watch?v=0K8zs-KSitc.
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E.
Jones
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How an air stream can support a cupcake
,”
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4.
We used 20-mm piezo disks available through Amazon for about $1.00 each.
5.
If a function generator is not available, a simple 555 timer astable circuit can be used to create an essentially square wave with the desired fundamental frequency (see, for example, the Texas Instruments LM555 datasheet). The amplifier and piezo have poor responses to the higher harmonic frequencies of the square wave, so the fundamental frequency will dominate the mechanical output of the piezo.
6.
If an oscilloscope is unavailable, some digital multimeters are capable of measuring AC voltages for frequencies up to 100 kHz, though less costly models often can only measure up to 1 kHz. The height of the reflector also can be set by trial and error: there should be a resonant spacing within a range of about λ/2, though the ability to adjust this distance by a few tenths of a millimeter is helpful.
7.
Because of the relatively low power of the piezo driver, only objects with very low densities can be levitated. We found the polystyrene from packing peanuts and coffee cups to be too dense.
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