Infrasound
is acoustic (sound) waves lower than 20 Hz (the approximate limit of human audibility). We focus on this portion of the acoustic
spectrum produced by volcanoes and radiated into the
atmosphere because:
1)Volcanoes
appear to radiate most “loudly” (or energetically) in the infrasonic bandwidth
(and specifically the spectral peak at many volcanoes is between 0.5 and 5
Hz).
2)Infrasound
recordings are not significantly affected by local site response (i.e., whether
the sensor is located behind a rock, in a forest, or inside an unsealed
plastic enclosure)
3)Infrasonic
bandwidths are amenable to traditional seismic recording devices and sample
rates. In fact, the target bandwidths
of typical seismic and infrasonic records is often the same… a Nyquist
frequency of 50 Hz is overkill for most volcano acoustic studies.