You know that feeling you get when someone is watching you or coming up behind you, or something is about to happen?

What is that anyway?

I don't feel it nearly as often as I did when I was a kid. I knew every time someone was about to come through a door. I was confident of it too. I'd tell whomever was with me that someone was about to come in and, sure enough, in a couple seconds someone would open the door.

I sometimes also knew when the phone was about to ring, or when a certain song was playing on the radio even though there were no radios on or around. If I could get to a radio fast enough I could prove it, and I did a couple of times. But those instances aren't quite the same, I just thought I'd add them.

That feeling of imminence, however, was frequent and reliable before my teenage years. After that, I only occasionally had that feeling, and it was usually just a TV being turned on.

Yes, a TV. :) Or, rather, the low-frequency sound it makes.

It's called "infrasound," defined as "a wave phenomenon sharing the physical nature of sound but with a range of frequencies below that of human hearing" (lower than 20 hertz).

Infrasound is not heard, it is felt. It can cause you to experience strange sensations, primarily nervousness, alarm, and fear. Elephants use it for communication and defense.

From "The Sonic Weapon of Vladimir Gavreau" by Gerry Vassilatos:

Infrasound produces varied physiological sensations which begin as vague "irritations". At certain pitch, infrasound produces physical pressure. At specific low intensity, fear and disorientation. Nazi propaganda engineers methodically used infrasound to stir up the hostilities of crowds who were gathered to hear their madman. The results are historical nightmares.

Humans can produce this sound, and it occurs naturally (wind in the mountains, storms, ocean waves). But how are we doing it? If someone in a closed room can sense that I'm about to enter, what is it I'm doing to put out that sound that they detect? I don't consciously do it, so what's happening? Is it a type of communication on some level? Could we learn to control it or use it? Does it have any connection with how we can sometimes finish someone else's sentences or know what someone is thinking before they say it?

Just things I'd like to know.

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Ever get the feeling you're being... watched?