Earth's Longest-Reaching Sounds: Infrasound

In the realm of sound, there exists a frequency beyond the threshold of human hearing, a realm known as infrasound. With frequencies below 20 Hz, these low-frequency waves have unique properties that make them durable and able to travel long distances, potentially stirring feelings of fear or even hallucinations.

Infrasound, though inaudible to the human ear, can be produced by a variety of sources. From the roar of wind turbines and the thunderous crash of waterfalls, to the rumble of diesel engines, lightning, aurorae, and the "voice of the sea," infrasound is a common yet often unnoticed phenomenon. Even certain musical instruments, such as the octobass or hyperbass flute, can produce infrasonic notes.

The exact reason why infrasound can cause feelings of fear or hallucinations is not clear. However, an acoustic engineer named Vic Tandy experienced such sensations due to infrasound created by a lab fan in the early 1980s. This incident, among others, has led some to believe that infrasound has the potential to scare people.

However, modern parapsychologists do not believe that infrasound can explain ghost sightings or hauntings on its own. The idea that certain infrasonic notes can cause uncontrollable bowel movements is likely a myth, possibly originating from a satirical article in 1974. The mythical "brown note" does not exist.

In the animal kingdom, infrasound plays a crucial role in communication over vast distances. Elephants, whales, and some species of alligators use infrasound for communication and navigation. Whales, in particular, can project their infrasonic songs over thousands of kilometers. One human, Tim Storms, is also capable of producing infrasound.

Interestingly, the 1883 eruption of Krakatoa was the first infrasound measurement ever made, and it was picked up by weather stations in London, more than 10,000 kilometers (6,200 miles) away.

In warfare, infrasound has also played a role. During the First World War, shots from cannons could be heard near the batteries, not at a distance, due to the infrasound waves.

Today, horror movie directors use infrasound to add extra freak-out triggers to their audiences, leveraging the uneasy sensation that infrasound can evoke.

In conclusion, while the exact mechanisms behind infrasound's ability to cause feelings of fear or hallucinations remain unclear, it is a fascinating and powerful force in the world of sound, with applications ranging from communication in the animal kingdom to horror movie production.