What a possibility can be : As humans, we can also hear with our skull. There is very weak directional information and the frequency curve we can hear with our skull is limited (Of course we hear with the cochlea but the sound is past on through the skull).
Perhaps that is a clue what spiders do... Spiders have sensitive hairs that sense minute pressure differences in the air.
Spiders are very good to also detect vibrations in the material they stand on. We humans have that too. Low infrasonic sounds or vibrations often creates a fear response (alert , watch out !) in people through the pressure senses inside the body (perhaps fluid dynamics, standing wave) and below the skin.
Perhaps this sense we humans have is the reason why people despise wind turbines when in close vicinity to home (People sleeping).