Infrasound is sound lower than the extremity of human hearing, which is classified as anything below 20 Hz. While typical subwoofers are not capable of reaching infrasonic levels (and why should they be, you wouldn't hear it if they could), some specialty products can get down to 18 Hz. These infrasounds escape our conscious perception, but they can still have noticeable effects on the human body. In 2003, researchers in the United Kingdom held a concert where they played music "laced" with a 17 Hz tone. Concertgoers exposed to the tone reported feelings of uneasiness, sorrow, chills, or nervous feelings of revulsion or fear. Further, ghost sightings may have some component which is related to infrasound. NASA reports that the resonant frequency of the human eyeball is 18 Hz. A researcher in his laboratory working alone one night in a supposedly-haunted room reported feeling anxious and noticing a grey blob out of the corner of his eye. The next day, he noticed that his fencing foil was vibrating on its own while it was held in a vice. He investigated further and discovered that the ventilation fan in the lab was producing a frequency of 19 Hz, close to the resonant frequency for the eye. This led him to conclude that the "ghost" he saw had simply been his eyeball resonating, and may explain the feelings of fear, dread, and paranormal apparitions some people experience when they are in purportedly haunted locations.
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