Tuesday, June 13, 2006

Infrasound

I've just been rereading an item in the Skeptic's Dictionary about Infrasound and how it produces some of the effects that people traditionally associate with haunted houses; it has an interesting example from a 'haunted' laboratory, where we have the effect:
Several years earlier, Tandy was working late in the "haunted" Warwick laboratory when he saw a gray thing coming for him. "I felt the hairs rise on the back of my neck," he said. "It seemed to be between me and the door, so the only thing I could do was turn and face it."
the cause:
The explanation, he discovered, was that infrasound was coming from an extractor fan.
and the reason:
When he measured the infrasound in the laboratory, the showing was 18.98 hertz--the exact frequency at which a human eyeball starts resonating. The sound waves made his eyeballs resonate and produced an optical illusion: He saw a figure that didn't exist.

There is a link to a story in the Guardian about a haunted cellar where the source of the infrasound seems to be a newly built corridor to the cellar, where something is resonating at the right frequency to spook tourists going into the cellar. Wouldn't that be a great thing to add into a haunted house 'ride' in a theme park or fun fair?

Also, right at the bottom it says:
Elephants have the ability to emit infrasound that can be detected at a distance of 2 km.

2 comments:

JP said...

Cool, I never heard of infrasound...how can I make some?

HistoryElephant said...

There's an entry in Wikipedia about the so-called 'brown note' which describes various experimental tests and the problems with generating the required sound. http://en.wikipedia.org/wiki/Brown_note