Number 474 (Story #4), March 10, 2000 by Phillip F. Schewe and Ben Stein ATMOSPHERIC INFRASOUND. Humans hear sounds in the 20-20,000 Hz frequency range. Ultrasound, waves above 20,000 Hz, is used for things such as medical imaging and can be sensed by animals such as bats and dogs. But how about infrasound, sound at frequencies less than 20 Hz? Humans can sometimes feel (even if they can't quite hear) infrasound in the form audio systems with the bass turned way up. The monitoring of infrasound may contribute practical benefits in a number of areas: determining the location and nature of avalanches, tornadoes, meteor strikes, volcanoes, and nuclear weapons tests. (Physics Today, March /pnu/2000/.)