How does ultrasound work?

TED explains how ultrasound works.

In a dark cave, bats can’t see much. But even with their eyes shut, they can navigate rocky topography at incredible speeds. This is because bats aren’t just guided by their eyes, but rather, by their ears. It may seem impossible to see with sound, but bats, naval officers, and doctors do it all the time. How is that possible? Jacques S. Abramowicz digs into the unique properties of ultrasound.

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