Infrasound
is not audible to humans, but its waveform pressure amplitude can be compared
to sound pressure amplitudes we are familiar with. Humans can hear from 20 to
20,000 Hz and for in-band sounds (centered around several thousand Hz) the pressure
audibility threshold is lowest at 2x10-5 Pa around ambient pressure (see Fletcher-Munsen
curves on next page). Sound pressure
level (SPL) is a scale many are familiar with and is related to
excess pressure by this minimum audibility level:
Using
this scale, infrasound can be converted to an equivalent SPL if it were in the
audible bandwidth. The
next page highlights recorded infrasound overpressures from
their literature and their effective conversion to SPL at an equivalent radius
of one kilometer using an assumed 1/r pressure decay that is
appropriate for a homogeneous atmosphere. It should be noted that many of these cited
volcanic studies provided little information about precise
acoustic bandwidths and/or sensor calibration specifications.
Volcano Infrasound Characteristics (2) – ‘Volume’