Acoustic levitation using standing waves

Most physics teachers will have to demonstrate standing waves at some point in the school year and there are a number of standard demonstrations which can be done with school lab equipment. When teaching about them, I also show videos of standing waves I can’t recreate in the classroom and the one above is a lovely addition to my resources for this topic. This video also reminded me of a piece of art I saw at the Tate Modern several years ago – Kinetic Construction (Standing Wave) – which was the first time I saw a Physics demonstration presented as “art”.

3 thoughts on “Acoustic levitation using standing waves”

  1. Alom,

    Thank you, that is delightful. But it doesn’t really explain what is happening.

    There must be a net force upwards provided by the acoustic field. I can think of several possibilities, but it is not obvious to me how it works. And how far out of the normal range of acoustic intensities one needs to go to see this.

    Can you explain?

    M

    1. I agree with you, Michael, that in a science context this must be considered in terms of force. I do understand that there is no simple, straightforward “story” to tell (as there is with the position of nodes and antinodes) but I would want to use this to reinforce that basic principles apply in complex situations. Namely that we know from the fact that these objects have mass, that there must be a downward force, which must be balanced by an upward force. Therefore the air (the only thing which actually interacts with the objects as far as the sound wave is concerned) must be at higher pressure at the bottom surface of the object than at its top surface (averaged over time). How that happens I can only guess, pressure fluctuations in a sound wave average to the same mean anywhere along the wave.

      I have the impression that this Newtonian approach is often considered boring and “too mechanistic” but it is needed to go beyond the “weirdness is cool” entertainment effect and make science work.
      I find the graphic shown in the video where the object is “trapped” between two sine waves very misleading. Aren’t soundwaves longitudinal?
      Have a look at http://www.youtube.com/watch?v=yd9DgsI95hc
      The fog from the dry ice shows that there are air currents induced by the standing waves which don’t just move the air back and forth as in a straightforward standing wave but in a rotating flow “cell” . This means that there are “pockets” of upward moving air which support the objects “floating” in air.

  2. I noticed a similar effect once when attempting to smash wineglasses with sound
    http://www.thenakedscientists.com/HTML/content/kitchenscience/garage-science/exp/acoustic-levitation/
    My take on it is that it is to do with spatially rapidly changing amplitudes of sound – if you are near the node in a place with high amplitude you will be moved back and forth a certain distance, if this moves you somewhere with a lower amplitude you won’t move back as far, so objects tend to move towards the nodes – as in a Chaladni plate, where if a sand particle is sitting on an antinode it is bounced away from there, but on a node it stays still.

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