The many dimensions of Infrasound

The term “Infrasound” is a bit of an anthropocentric misnomer; it refers to sound frequencies  below human ability to auditorily distinguish. This doesn’t mean that humans can’t perceive infrasound, it’s just that distinct sounds below a certain frequency may be sensed as beats or flutters. Or if there is no pronounced leading edge to the signal – like a sinusoidal wave, we may just perceive it as pressure, or a “presence.”

One of the epic tales of this involves a “haunted” metal shop where people would sense a “presence” accompanied by seeing apparitions in the perimeters of their vision. The shop called in all sorts of exorcists and demon-chasers to no avail. Then exasperated, they called in an acoustician (cue heroic music here), who found that a large, out of balance centrifugal fan was oscillating the shop environment with really high energy 18 Hz “infrasound” – around the resonant frequency of the human eyeball. This was disrupting the light transmission through the cornea, and stimulating unstable images onto their macula.

High-energy, low frequency pressure fluctuations can also resonate other human cavities – the lungs, peritoneum, and gut – a perfect formula for anxiety. This is a known trick in horror movie soundtracks – cranking up the infrasonic ‘fear’ before the beast pops through the window, or the severed head falls out of the closet.

Tigers use a low frequency rumble to terrify their prey just before striking – freezing their prey in terror as the tiger moves in for the kill. But many other animals use infrasonic sounds to communicate over long distances – facilitated by the physics of sound.

Lower frequency sound is correlated with longer wavelengths, and is not attenuated by smaller objects in the environment – and thus can travel long distances across the horizon (even over the curvature of the earth in the ocean!).

Herd animals like horses and elephants use long wavelength sounds to communicate over many kilometers across the savannas and prairies, conveying information to their kin. And due to the even longer relative wavelengths of sound in the sea (due to 5x sound speed in water), whales can be heard thousands of kilometers away in the sea.

But not all infrasound is intentionally (or accidentally) generated; there are many low frequency geological pressure fluctuations. These would include earthquakes, waves crashing on beaches, fumaroles blowing in ocean trenches, wind blowing across mountain ridges like giant flutes, and at the lowest perceptible frequencies, fluctuations of local barometric pressures as weather systems move around the hemispheres.

Because these sounds are correlated with earth systems, predictably they are used as navigation cues by migrating animals. Evidence for this has been somewhat speculative, due to the difficulty reproducing repeatable results in a lab. But there is some published work on this involving homing pigeons – because their flight vectors are predictable. Increasingly though – due to some of the real-time tracking tools that ornithologist are currently using, other birds have been brought into the “global lab.”

The paper that inspired this newsletter was an examination of one of the more majestic birds in the ocean sky – the albatross. This is a bird that spends most of its life soaring across the oceans, using weather and terrestrial infrasonic queues to navigate across the seven seas. They land to breed and rise their young, but may travel 6000km across the sea to gather food for their chicks.

I bring this up because one of the biological acoustic niches we humans are beginning to invade is the band we call “infrasonic.” We can’t hear it, and thus we have not considered exposure regulations in this frequency band. But with the amped up development of offshore wind turbine farms we will be crowding out the navigation cues and communication channels of migrating birds and whales over and in the global ocean.