Infrasonics:
(Arts-informed Inquiry)
"Infrasonics is the study of sound below the range of human hearing.
These low-frequency sounds are produced by a variety of geophysical processes including earthquakes, severe weather, volcanic activity, geomagnetic activity, ocean waves, avalanches, turbulence aloft, and meteors and by some man-made sources such as aircraft and explosions."
http://www.etl.noaa.gov/et1/infrasound/
"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."
http://geology.about.com/library/weekly/aa121497.htm
bravenet.com