infrasound

Imagine the deep rumble of a distant earthquake or the slow groan of a moving glacier. These are not sounds you hear in the usual way. They are sound waves with frequencies too low for the human ear to detect, usually below 20 Hz. Even when we do not hear them clearly, we may still feel them as vibrations or pressure.

Scientists study this kind of sound to learn about volcanic eruptions, storms, and animal communication. Elephants, whales, and perhaps rhinoceroses use it to send signals over long distances. Human-made sources such as large machines, rockets, and explosions can also produce it. It is a hidden part of the world around us: easy to miss, but often very powerful.

Infrasound is sound with frequencies below the lower limit of human hearing, generally taken to be 20 hertz (cycles per second). Think of it this way: ordinary sounds reach your ear and are heard as speech, music, or noise. These waves are much lower and longer, so people usually do not hear them as clear sounds. Instead, they may notice vibrations, pressure, or a faint sense that something is moving through the air.

The term "infra" means "below," which fits its place below the normal hearing range. Humans usually cannot hear it, but strong low-frequency waves can still affect the body. Some research has linked certain kinds of exposure to discomfort, unease, or nausea, though the effects depend on the strength and the situation.

In nature, it can be produced by volcanic eruptions, earthquakes, avalanches, meteors entering the atmosphere, waterfalls, and even ocean waves. Some animals use it because it travels far. Elephants can send calls across long distances, whales use very low sounds in the sea, and rhinoceroses may use such signals during social and mating behavior.

Scientists use special sensors to detect these low-frequency waves. That helps them monitor natural disasters, study animal movement, and detect events such as large explosions, including nuclear tests. It reveals a hidden layer of sound in the world around us, one that is often felt more than heard.