Music Isn't Just Acoustic Listening

Your mastoid bone (behind your ear) is built to resonate, like the wood in a Stradivarius violin. The bone is spongy and contains specialized receptors to detect vibration through skull conduction. When you sing, hum, or speak, you're experiencing sound through both air conduction (acoustic hearing) and bone conduction (vibration) simultaneously. Below about 20 Hz, you don't hear frequencies at all; you feel them as pure vibration propagating through fascial networks and bone. This is why deep bass sounds move you, why deaf (and award-winning) classical percussionist Evelyn Glennie takes her shoes off in order to perform, why you close your eyes when the music gets good. Vibration is part of how blind people are able to mountain bike.

When an alligator's voice drops really low, you can't hear it acoustically any more; but you can feel it thrum in your bones.