MadSci Network: Physics
Query:

Re: How does ultrasound move things?

Date: Thu Jun 15 13:29:31 2006
Posted By: Tetjana Ross, Faculty, Oceanography, Dalhousie University
Area of science: Physics
ID: 1145812167.Ph
Message:

Hi Robin,

To understand how sound waves might be able to levitate objects or knock people over, we first need to understand what they really are. There's a nice description on the Wikipedia site. I'll summarize here. Sound waves are waves of compression. A passing wave crest is manifested by a change in local air or water pressure. When you listen to your stereo these pressure changes are so small that they're imperceptible to your sense of touch ; if you hold up your hand, you feel no force acting on it. Only the very sensitive membranes in our ears pick up the pressure changes.

If, however, the amplitude of the sound waves is large enough and the object is small and light enough (for example a tiny droplet of some substance) it can be held aloft by these pressure changes. If you're interested in learning more about the physics of levitation, there's a nice article in Science .

While some sonic weaponry is real, the sound is generally used to create discomfort and nausea rather than to knock people over. Weapons that knock people through the air with sound waves are still firmly in the realm of science fiction. See this Wikipedia article for more on sonic weapons.

Cheers,
Tetjana


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