Infrasound is acoustic waves with a frequency below 20 hertz — below the threshold of human hearing. Physically it is the same sound as any other: pressure oscillations travelling through air, water or earth. The only difference is frequency — and that changes everything.
Where hearing ends
A healthy young ear perceives roughly 20 Hz to 20,000 Hz. The lower the frequency, the worse we hear it, and around 20 Hz sound becomes more of a sensation in the chest than a tone. Yet we never go completely "deaf" to infrasound: if it is loud enough, a person perceives it — the hearing threshold simply rises sharply as frequency drops.5 Below this boundary lies a whole world we do not hear but which is constantly with us: born of storms, waterfalls, machines, thunderstorms and distant volcanoes.2
Long wave — long range
Infrasound's main superpower is its wavelength. At 10 Hz the wave stretches about 34 metres; at 1 Hz — about 340 metres. Such waves barely notice obstacles the size of a house, tree or hill — they simply flow around them. And, more importantly, low frequencies barely attenuate in the atmosphere: high tones die out within hundreds of metres, while infrasound runs for hundreds and thousands of kilometres.1
High-pitched sound dies in the next room. Infrasound goes around the planet.
Why it circles the Earth
For infrasound the atmosphere is a waveguide. Temperature and wind at different altitudes form layers that reflect and steer the wave, keeping its energy near the surface. So a powerful enough source — a large eruption or explosion — produces a wave that instruments record all over the world, sometimes after several trips around the planet.1 It is this long reach that made infrasound the backbone of the international system for monitoring nuclear tests.3
- Infrasound = sound below 20 Hz; we do not hear it.
- Wavelength is tens to hundreds of metres, so it bends around obstacles.
- Attenuation is tiny — range up to thousands of kilometres.
- It travels through air, water and earth — and animals make use of this.4
Because infrasound from dangerous events travels far and barely weakens, it can be caught in advance — even with a simple network of pressure sensors. Our entire project rests on this.
Sources for this article
- review Bedard A.J., Georges T.M. (2000). Atmospheric Infrasound. Physics Today 53(3). physicstoday.aip.org
- review O'Connell-Rodwell C.E. (2007). Keeping an 'ear' to the ground: seismic communication in elephants. Physiology 22(4). physiology.org
- organization CTBTO. Infrasound monitoring (IMS). ctbto.org
- peer-reviewed Payne K.B., Langbauer W.R., Thomas E.M. (1986). Infrasonic calls of the Asian elephant. Behav. Ecol. Sociobiol. 18(4). springer.com
- peer-reviewedreview Møller H., Pedersen C.S. (2004). Hearing at low and infrasonic frequencies. Noise & Health 6(23). pubmed.ncbi.nlm.nih.gov