What is infrasound? Sound below the threshold of hearing

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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

20,000 Hzupper limit of human hearing

~2,000 Hzthe ear's most sensitive range

20 Hzlower limit of hearing — infrasound begins below it

5–14 Hzelephant rumbles

~0.2 Hzmicrobaroms — the ocean's constant "hum"

0.002 Hzwaves from the Tonga eruption that circled the Earth

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

Key takeaways

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

Why this matters for HERD

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

These sources are part of the full HERD library — 272 vetted sources, with meaning search and topic filters.

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
review Hedlin M.A.H., Walker K., Drob D.P., de Groot-Hedlin C.D. (2012). Infrasound: Connecting the Solid Earth, Oceans, and Atmosphere. Annual Review of Earth and Planetary Sciences 40. doi.org
peer-reviewed Drob D.P., Picone J.M., Garces M. (2003). Global morphology of infrasound propagation. Journal of Geophysical Research: Atmospheres 108(D21). doi.org
peer-reviewed Sutherland L.C., Bass H.E. (2004). Atmospheric absorption in the atmosphere up to 160 km. The Journal of the Acoustical Society of America 115(3). doi.org
review Evers L.G., Haak H.W. (2010). The Characteristics of Infrasound, its Propagation and Some Early History. In: Infrasound Monitoring for Atmospheric Studies (Springer). doi.org
review Waxler R., Assink J. (2019). Propagation Modeling Through Realistic Atmosphere and Benchmarking. In: Infrasound Monitoring for Atmospheric Studies, 2nd ed. (Springer). doi.org
peer-reviewed Waxler R., Evers L.G., Assink J., Blom P. (2015). The stratospheric arrival pair in infrasound propagation. The Journal of the Acoustical Society of America 137(4). doi.org
peer-reviewed Negraru P.T., Golden P., Herrin E.T. (2010). Infrasound Propagation in the "Zone of Silence". Seismological Research Letters 81(4). doi.org
peer-reviewed Assink J.D., Waxler R., Smets P., Evers L.G. (2014). Bidirectional infrasonic ducts associated with sudden stratospheric warming events. Journal of Geophysical Research: Atmospheres 119(3). doi.org