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.
- 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.
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
- 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