Everybody knows about ultrasound, right? Sound that's too high-pitched to hear unless you're a dog or a bat. But what about infrasound? There is sound that's too low to hear, and the Earth scientists paying attention to it have things to show us.
Human ears can register sound down to about 20 hertz, the deepest bass note. Infrasonics researchers are interested in sounds of 10 Hz and below, all the way down to 0.001 Hz. Lower than that is meteorology! In fact, this frequency range is the same one that seismographs use for monitoring earthquakes. Some peculiar things go on down there.
What Makes Infrasound
Earthquakes, as you might expect, shake the air along with the ground, creating seismic infrasound that can travel far from the epicenter. Volcanoes make some pretty impressive infrasound. Big storms at sea generate waves in the air, called microbaroms, from the water waves beneath. (Examples of all of these are shown on the authoritative Infrasonics Home Page at the Geophysical Institute in Fairbanks, Alaska.)
The atmosphere itself makes infrasound as bodies of air move over mountain ranges. This is like the sound you make by blowing over your teeth, only about 10,000 times lower in pitch. Tornadoes and turbulence in the upper air produce infrasound. And so might sprites, those mysterious upside-down lightning strikes high above large thunderstorms.
Who Listens to Infrasound
Infrasonics researchers have compelling practical reasons for doing this grab-bag sort of science, because of one particular human source of infrasound—not rocket launches, supersonic jets, or mining blasts, although all these are recorded around the world. It's nuclear bomb testing.
Before its satellites took over the job in the 1960s, the U.S. Air Force used infrasound monitors to detect other nations' testing, and with the recently signed Comprehensive Test Ban Treaty a new worldwide network of infrasound stations is being built (along with other networks described in this article). The scientists working on the network are thinking about the problem of false alarms, the most spectacular source of these being meteor falls.
So infrasonics research is due for its own sort of boom as money increases and new specialists are trained. And assuming that nuclear weapons are never again exploded, the folks at those monitoring stations will be very contented scientists. If you think this sort of research is cool, you can do it yourself—check out the Public Seismic Network, a group of amateur seismologists that dabbles in homemade infrasonics, and also sign up for The Bell Jar, the amateur infrasounder's zine.
PS: Some people are into making rather than listening to infrasound. Some are audiophiles playing with their top-end stereos, of course, but another group is Chinese health practitioners. Apparently infrasound treatment does wonders for your qi.