Reporting live from San Diego, where the 2008 Joint Mathematics Meetings are being held! The Meetings officially start today, so Saturday our family went to the Reuben H. Fleet Science Center. One of the cool exhibits there was a pair of Whisper Parabolas.
Whisper Parabolas are two parabolic dishes, set far apart and facing each other in such a way that if one person speaks quietly in front of one, a second person standing in front of the other can hear the first person clearly. How does this work? If the first person is standing at the focus of the first parabolic dish, all of the sound waves bounce off the parabola in parallel lines (much the same way that the light from a flashlight bulb bounces off the parabolic mirror in the flashlight and forms a bright narrow beam instead of shining in all directions). These sound waves then bounce off the second parabola directly to its focus, where the second person is standing.
I’ve seen this before (there’s a pair outside of the Rochester Museum and Science Center) but it amazes me every time how someone a room away can whisper and be heard clearly. According to the Cassini Fun Facts Archive, the Cassini spacecraft visiting Saturn also exploits this property of parabolas in its high-gain antennae in order to send information back to earth (unless the spacecraft is facing away from earth; then it needs the low-gain antennae which send signals in all directions).