Infra-noise leads to ghost reports

And then there is sound we can't hear — infrasonic. In 1998, a group of scientists working in a laboratory began to feel as if they were in a haunted house: They thought they saw blurry gray figures and were full of dread, a science journal reported. They finally traced the effect to an extractor fan, whose infrasonic noise happened to be "at just the right frequency to make eyeballs vibrate and so perhaps to generate visual illusions."

-- from a review in today's NYT by Katherine Bouton of
Discord: the Study of Noise, by Mike Goldsmith