Acoustic Levitation

03 February 2009


Acoustic levitation



Instead of hitting a mechanical resonance in the glass with it moving back and forth, I hit a resonance in the air in the glass (a bit like when you blow across a bottle).

The brown tube is holding the speaker I was using.

and as you can see the paper has taken to the air and has started levitating.

This kept me fascinated for a good while, as I was wearing ear defenders (it is very very loud!), and I think I have worked out what is going on.

The air inside the glass resonates - it moves in and pressure builds up, until the air rushes out again, overshoots and the pressure drops again causing the air to rush back in. The size of this vibration changes with position, it will be largest

The levitation is due to the volume of the sound changing around the glass. It will be louder inside the glass than above it. The paper is moved with the air, so in the louder areas with more vibration it moves more than in the quieter areas. This means that if a vibration moves it from a loud area to a quiet area it is now in an area which is vibrating less so the return vibration will be smaller and at the end of the vibration cycle it will have moved. So over lots of vibrations it will have moved towards the quieter area, and if the sound is loud enough this effect is strong enough to counter gravity and will make the paper levitate.

Resonance in glassMovement of the paper
The air resonates, rushing in and out. In some places the size of this vibration is larger than others.The paper moves with the air. This means that it moves further when it is in a place with large vibration than small. So on average the paper moves towards the smaller vibrations.



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