Infrasound is sound below 20 Hz, lower than humans can perceive. But just because we don’t consciously hear it, that doesn’t mean we don’t respond to it; in certain individuals, low-frequency sound can induce feelings of fear or dread or even depression.

They may even be responsible for some ghost sightings, but we’ll get to that in a second.

What causes infrasound? Anything from earthquakes to meteors to ocean waves to fans to old, vibrating pipes in your attic. The occassional nuclear explosion, too (we actually use infrasound to detect them). Even certain animals produce infrasound to communicate with one another, like whales and elephants.

“Elephants, in particular, produce infrasound waves that travel through solid ground and are sensed by other herds using their feet, although they may be separated by hundreds of kilometres.”

Animals also react to infrasound. This may be one of the reasons they’re so quick to react when a natural disaster is looming, such as an earthquake.

In humans, infrasound can cause a number of strange, seemingly inexplicable effects: headaches, nausea, night terrors and sleep disorders. But low-frequency sound can do even more than that.

Infrasound and Ghosts

If infrasound hits at just the right strength and frequency, it can resonate with human eyes, causing them to vibrate. This can lead to distorted vision and the possibility of “ghost” sightings. Or, at least, what some would call ghost sightings. Infrasound may also cause a person to “feel” that there’s an entity in the room with him or her, accompanied by that aforementioned sense of dread.

This bizarre phenomenon has been documented on multiple occasions. For example, one night while working at a “haunted” laboratory, Vic Tandy of Coventry University experienced feelings of anxiety, and even witnessed a dark “blob” out of the corner of his eye. The hairs on the back of his neck stood up. But when he turned to face the strange figure, he found nothing there.

The next day, Tandy saw the dark figure again, and he also noticed that the fencing foil he was working with — clamped to a vice — was inexplicably vibrating. So he decided to investigate.

As it turned out, there was a silent fan in the laboratory. The fan was giving off low-frequency sound waves at 18.98 Hz, right around the resonant frequency of the human eye. It had also created a standing wave in one area of the room, which is what caused the foil to vibrate.

According to Tandy, “When we finally switched it off, it was as if a huge weight was lifted.”

The strange vibrations, optical illusions, and depressed feelings were due to infrasound, and had given the laboratory the reputation of being haunted. But it was all because of a vibrating fan.

Anyway, is infrasound an explanation for all ghosts sightings and supernatural activity? Of course not. The infrasound connection is ultimately just a theory. But if you ask me, the fact that low-frequency sound can affect humans in various ways is extraordinary in and of itself.