Sometimes, you get the feeling that something is crossing on your sight. Could these kinds of ghosts be sound waves, just below the human hearing range?

The sound is basically mechanical energy in the form of a pressure wave with crests and troughs. Vibrations create a disturbance in the surrounding air and ripple outward, like tossing a pebble in a pond. Frequency measures how many crests occur within one second in a wave.

The unit of measurement is called a Hertz (Hz), and 1 Hz is equivalent to 1 vibration per second.

The typical range for human hearing runs from 20 Hz to 20,000 Hz (20 kHz). Everything above this range of hearing is called ultrasound and anything below is called Infrasound.

Experts call the infrasound in the 18.9 Hz range the “fear frequency” because such sounds may induce feelings of anxiety. Especially at higher intensities.

Scientific researchers of NASA have found that the resonant frequency of the human eye is around 18 Hz. This could explain why sounds in this range might produce hallucinations.

So, low-frequency sounds can cause people to have unusual experiences and make them see “things” that some refer to it as “ghosts”.

A 2008 experiment led by British psychologist Christopher French proved even more intriguing. With colleagues from University of London College, he built a “haunted” room rigged up with infrasonic generators (as well as sources of electromagnetic pulses); 79 brave Londoners volunteered to spend some time inside.

“Most people reported at least some slightly odd sensation, such as a presence or feeling dizzy, and some reported terror, which we hadn’t expected,” French told Scientific American in 2008. But he stopped short of claiming that low-frequency electromagnetic fields or infrasound were the direct cause of such feelings; suggestibility seems to also play a role.

However, animal hearing range varies. Bats and dolphins, for example, can hear frequencies up to 100kHz. Elephants can hear sounds at 14-16 Hz.

Here is a logarithmic chart of the hearing ranges of some animals: