Infrasound
Sound too low-pitched for human ears, below about 20 hertz, which can travel thousands of kilometres through the air.
Infrasound is sound with a frequency below the range of human hearing, usually taken as below 20 hertz (20 vibrations a second). We cannot hear it as a note, although at very high intensity people can feel it as pressure on the eardrums.
Its long wavelengths are what make it special. The atmosphere absorbs very little of it, it slips around hills and buildings, and it can be channelled between the ground and the upper atmosphere, so it carries over enormous distances. Elephants use calls between 15 and 35 hertz to keep in touch across kilometres, and a worldwide network of stations listens for infrasound from volcanoes, meteors and nuclear tests.
The largest blasts shake the whole atmosphere. After Krakatoa erupted in 1883, its pressure wave was recorded on barometers around the world seven times over five days as it travelled round and round the globe at about 1,086 km/h. A similar wave raced round the planet after the Hunga Tonga eruption in 2022. These are called Lamb waves: pressure pulses that run along the whole depth of the atmosphere at close to the speed of sound.
A common misconception is that a sound you cannot hear must be weak. Some of the most energetic sounds on Earth are largely infrasonic. The boom of Krakatoa was heard about 4,800 km away, but its pressure wave went much further.
Coming up in future stories
- A Zoo Visitor Felt Elephants Talking Below Hearing May 1984 · Washington Park Zoo, Portland, Oregon
- The Bang Heard 4,800 Kilometres Away 1883 · Krakatoa, Sunda Strait, Indonesia
- The Ghost in the Lab Was a 19-Hertz Fan 1998 · a laboratory near Coventry, England
- The Volcano Boom That Lapped the Planet 2022 · Hunga Tonga–Hunga Ha'apai, Tonga