Have you ever experienced unexplained phenomena that gave you the impression of a ghost? A sudden cold chill, a feeling of dread, unexpected sounds in the night? Folklore and horror films give us an easy explanation- your house is haunted by a dead person who never left. There might be a more scientific reason for that feeling, like infrasound.
Infrasound is sound at levels so low humans can’t hear it (though other animals, like elephants, can). Low frequency vibrations can cause distinct physiological discomfort. Scientists studying the effects of wind turbines and traffic noise near residences have found that low-frequency noise can cause disorientation [PDF], feelings of panic, changes in heart rate and blood pressure, and other effects that could easily be associated with being visited by a ghost [PDF]. For instance, in a 1998 paper on natural causes of hauntings [PDF], engineer Vic Tandy describes working for a medical equipment manufacturer, whose labs included a reportedly haunted room. Whenever Tandy worked in this particular lab, he felt depressed and uncomfortable, often hearing and seeing odd things—including an apparition that definitely looked like a ghost. Eventually, he discovered that the room was home to a 19 Hz standing wave coming from a fan, which was sending out the inaudible vibrations that caused the disorienting effects. Further studies also show links between infrasound and bizarre sensations like getting chills down the spine or feeling uneasy.
That's only one of six plausible explanations for hauntings you can read about at Mental Floss. I was surprised that "cat" wasn't one of them. I always blame strange sounds, moved objects, and the feeling of someone in my bed on my three cats.