In A Nutshell
It’s called infrasound: This refers to a sound which has a frequency lower than 20 khz. This frequency is at the very bottom of the typical human range of hearing, and while it may not always be processed as sound, the body certainly registers it unconsciously. This can lead to some very strange reactions and may be linked to ghost sightings.
The Whole Bushel
Hearing is a very interesting facet of the human brain. After all, sound is just a wave of energy, which you can actually draw on paper in the form of a sine wave. It is rather remarkable to think, then, that your brain can register these waves and convert them into meaningful information. Most humans have a hearing range from approximately 20 Hz to approximately 20,000 Hz.
Things get weird when you approach these limits of human hearing. Hitting the high frequencies can be damaging to the eardrum, but these high frequencies are not very common in nature. The lower end of the scale, however, is quite common in the modern world we inhabit. The sounds of trains passing and wind blowing are often enough to create the ultra-low-frequency infrasounds. They were first studied properly in the ’60s by a French scientist. He discovered the effects of the sound while in his laboratory: He and many of his assistants reported pain in their ears and heightened feelings of anxiety.
When a human is exposed to these sounds, things start to happen psychologically. Experiment participants report feeling chills, a sense of anxiety often tied to the feeling of being watched, and inexplicable sadness. It’s as if these sounds impart a set of human emotions and fears into the minds of the listener that are not their own. In addition to this, many people report slight hallucinations in their peripheral vision, usually the form of a gray blob, causing them to be very wary of their surroundings. This leads to the theory that these infrasounds can be often linked with areas where ghost sightings are frequent, as the common effects of infrasound are similar to the alleged presence of a ghost. This association likely has something to do with the way we regard thunderstorms and other natural events as frightening (sort of a no-brainer, considering how dangerous they are). The low-frequency sounds they generate make us more alert and worried, which makes sense in terms of survival.
Measurements taken at such places have often showed the presence of such frequencies, but not enough experimentation has been done to say anything conclusively. Some of the best research done on this subject was performed by university lecturer Vic Tandy. He was one of the first people to propose a link between infrasound and the paranormal. He measured the presence of infrasound in various locations, including the reputedly haunted London Underground. He also concluded that the main frequency that produced these effects was 19 Hz, where many people may not consciously register that they are hearing sound.
The only thing that remains is to explain why this is the case. Predictably, explanations vary and are largely speculative. The main idea remains similar throughout: This is that humans evolved to be agitated and frightened by low sounds, which would be useful in avoiding danger such as avalanches or storms. Whatever the reason, it is undeniable that low-frequency sound causes strange feelings in a large amount of humans.