Infrasound, sometimes referred to as low-frequency sound, is sound that is lower in frequency than 20 Hz or cycles per second, the “normal” lower limit of human hearing.

Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher intensities it is possible to feel infrasound vibrations in various parts of the body.  It is known that elephants and whales use infrasound for communication, and that songbirds may use it to avoid storms and there is some evidence that pigeons may use it for navigation.

It possible to measure infrasound accurately with microbarometers and hydrophones, sensitive pressure sensors. A global network of infrasound sensors in the atmospheres and oceans is being installed as part of the International Monitoring System (IMS). The IMS is installed for the verification of the Comprehensive Nuclear Test-Ban-Treaty.

Infrasound is characterized by an ability to cover long distances and get around obstacles easily without much dissipation. In addition, as the atmosphere and ocean act as natural waveguides and duct sound wave energy over long distances.

Figure: Infrasound propagation is possible over thousands of kilometers, due the low absorption rates at these frequencies. The loss of acoustic energy over range is indicated by the colour bar [Assink et al., 2014].

Infrasound covers sound between 0.001 and 20 Hz. This frequency range is utilized for monitoring earthquakes, nuclear explosions, volcanoes and hurricanes. More recently, infrasound waves have been used for remote sensing of the deep ocean and the upper atmosphere.