The chills down your spine…the heaviness in your chest…the feeling of deep and abiding dread. Sound like a horror movie? Maybe an encounter with a wayward spirit?
It could be something far more subtle: infrasound.
Infrasound is the name used to describe very low frequency sounds below the range of human hearing. Sounds in these frequencies can be produced naturally by events such as ocean waves, avalanches, earthquakes, volcanic activity and meteors. They can also be produced as a by-product of man-made events such as chemical and nuclear explosions, as well as being intentionally created for research purposes.
Nature puts infrasound to good use. Migratory birds use it as a navigational aid, and whales use it to communicate over long distances. The animal kingdom may indeed also rely on infrasound to hone survival instincts. Ever heard of animals “predicting” events such as earthquakes? They may be picking up on infrasound waves.
Infrasound has also made an appearance in urban legend with the “brown note“–the sound that supposedly forces humans to evacuate their bowels involuntarily. Rumors have long circulated that military agencies have been experimenting with just such a noise, but the accounts remain anecdotal. So far, evidence does not support the existence of such a bodily reaction; instead, infrasound more frequently produces psychic symptoms ranging from a sense of uneasiness to acute fear and dread, sometimes accompanied by visual hallucinations.
Because of this, scientists are now investigating whether or not infrasound could be related to (if not directly responsible for) paranormal phenomena such as ghost sightings. A team of British scientists played infrasound notes over pieces of contemporary music for listeners at a concert hall to test the effects on human subjects. Listeners were not told which of the pieces contained infrasound notes and which did not. 22% of the listeners reported feelings of unease, fear, and revulsion during the infrasound pieces, leading researcher Richard Wiseman to conclude: “Some scientists have suggested that this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost–our findings support these ideas.”
Another British academic, Vic Tandy, played part-time ghostbuster as well. A business school lecturer, Tandy was intrigued by rumors of hauntings at his workshop. His research led him to conclude that a fan in the workshop was generating infrasound and was responsible for strange phenomenon such as vibrating a piece of metal in a vise. A fix to the fan and the phenomenon disappeared.
(“Hearing” Ghosts–National Academy of Engineering)
Apart from paranormal researchers, the military and the academic set, artists seem to be the most avid exploiters of the phenomenon. Movie producers have used it to evoke a sense of dread in audiences, as have music collectives. Perhaps this is the logical extension of art, itself a discipline which uses illusion to evoke ideas and mood. But there’s something creepy about artists working outside the range of human perception to influence experience. So the next time you feel those chills running up and down your spine and suspect you’ve had a brush with the beyond, perhaps it isn’t that old black magic, but just might be infrasound at work.