Microbaroms: the ocean's constant hum near 0.2 Hz

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If you switch on an infrasound receiver anywhere on Earth, it will almost always hear a steady hum near 0.2 hertz — a period of about 5 seconds. These are microbaroms: the continuous "background noise of the planet" created by the ocean itself.

How waves create sound

When two groups of waves travel towards each other — for example at the centre of a storm, or where swell reflects off a shore — they combine into a standing wave. Such a wave presses on the body of water not up and down, but rhythmically "rocks" the pressure, and this oscillation goes up into the atmosphere and down into the seabed. In the atmosphere it becomes microbaroms; in the earth, the related microseisms. The theory of this mechanism was worked out back in 1950 by Michael Longuet-Higgins.1

The frequency of microbaroms is exactly half the frequency of the sea waves that created them.

Modern work has refined exactly how the ocean surface radiates these waves and how their intensity depends on storms in the North Atlantic and the Southern Ocean.2 The first complete global model — which reproduces this hum across the whole planet from maps of ocean waves — was built by Ardhuin's team.4 The finite-depth theory of microbarom generation by ocean waves (De Carlo, Ardhuin & Le Pichon, 2020) refined how we model the "voice of the sea".5 The seasonal pattern of microbaroms effectively draws a map of the planet's storm activity.

Curious facts

Microseisms (the earth's "version" of this hum) are the main source of constant noise on the world's seismographs.
Microbaroms allow ocean storms to be tracked remotely.
You can "hear" hurricanes via microbaroms: their infrasound depends on the ocean wave field under the storm (Hetzer et al., 2008), and the same method tracks Mediterranean "medicanes".6
They are both nuisance and benchmark: the steady background helps calibrate infrasound stations.

Why this matters for HERD

Microbaroms are the main constant background against which we will have to search for "useful" events. Knowing their frequency and seasonality helps us tune out the sea and avoid mistaking a storm for a real alarm.

Sources for this article

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

peer-reviewedhistory Longuet-Higgins M.S. (1950). A theory of the origin of microseisms. Phil. Trans. R. Soc. A 243. royalsocietypublishing.org
peer-reviewed Waxler R., Gilbert K.E. (2006). The radiation of atmospheric microbaroms by ocean waves. JASA 119(5). pubs.aip.org
organization CTBTO. Infrasound monitoring (IMS). ctbto.org
peer-reviewed Ardhuin F., Stutzmann E., Schimmel M., Mangeney A. (2011). Ocean wave sources of seismic noise. J. Geophys. Res. Oceans 116. doi.org
peer-reviewed De Carlo M., Ardhuin F., Le Pichon A. (2020). Atmospheric infrasound generation by ocean waves in finite depth. Geophys. J. Int. 221. doi.org
peer-reviewed Hetzer C.H., Gilbert K.E., Waxler R., Talmadge C.L. (2008). Infrasound from hurricanes: dependence on the ocean surface wave field. GRL 35. doi.org
history Posmentier E.S. (1967). A Theory of Microbaroms. Geophysical Journal of the Royal Astronomical Society 13(5). doi.org
peer-reviewed Donn W.L., Naini B. (1973). Sea wave origin of microbaroms and microseisms. Journal of Geophysical Research 78(21). doi.org
peer-reviewed Landes M., Ceranna L., Le Pichon A., Matoza R.S. (2012). Localization of microbarom sources using the IMS infrasound network. Journal of Geophysical Research: Atmospheres 117(D6). doi.org
peer-reviewed Smets P.S.M., Evers L.G. (2014). The life cycle of a sudden stratospheric warming from infrasonic ambient noise observations. Journal of Geophysical Research: Atmospheres 119(21). doi.org
peer-reviewed De Carlo M., Le Pichon A., Ardhuin F. (2021). Global Microbarom Patterns: A First Confirmation of the Theory for Source and Propagation. Geophysical Research Letters 48(3). doi.org
peer-reviewed Vorobeva E. et al. (2021). Benchmarking microbarom radiation and propagation model against infrasound recordings: a vespagram-based approach. Annales Geophysicae 39(3). doi.org
peer-reviewed Bowman J.R., Baker G.E., Bahavar M. (2005). Ambient infrasound noise. Geophysical Research Letters 32(9). doi.org