Acoustograms are pressure
time-series records, often measured in pascals (Pa) above and
below ambient pressure. For reference,
the ambient pressure at sea level is 100,000 Pa and typical ‘loud’
recorded volcanic infrasound is usually less than +/- 100 Pa. Resolution on good quality infrasonic
microphones is about 10 mPa.
Because microphones record
pressure in the fluid atmosphere, there is no sense of partical
motion directionality and infrasonic microphones are one channel only. Because of this infrasonic microphones
are also omni-directional, i.e., they record the same thing
regardless of their orientation (e.g., pointed toward or away from the
source).
By definition infrasonic
microphones are sensitive to frequencies below 20 Hz. However, various types of
infrasonic microphones have lower and/or higher response and/or
better response calibration. Various
types of infrasonic microphones have been used for volcano studies
including absolute pressure transducers, electret condenser elements,
and microbarometers (see illustrations on next panel).