This is the roll-off frequency of a mechanical high pass filter on the ir air pressure signal coming into the sensor (1.0 Hz and 0.05 Hz). You need some kind of limit to the lower frequency end (or changes in the barometer will “pin” your pressure sensor high or low as the weather changes). There is a matching setting in the program you have select. I believe the purpose of that is to achieve a relatively flat response down to (and below) the frequency in question
My observation is that using the 20-second filter will result in some really big readings when the wind around here starts to exceed 20 MPH. It has not been a problem in SWARM because I can always do extra filtering there. It does make a mess of the helicorder if that bothers you …
Perhaps others will chime in with what sorts of things make sound below 1 Hz. I have seen that severe thunderstorms will do that (not lightning but rather pressure changes due to downdrafts and whatnot).