From my experience (hah) i've seen that they try to aim for 20hz-20,000hz..

(oh man, remember that early 90s "bass" sample? bass! i wish i could embed a wav into this.)
so the low end being the bass, at that point, doesnt it kind of lean more towards vibration than audible tones? i'm no audio engineer.. just speculating.

I don't see many subwoofers that go below 22-24hz.. but then again, i haven't been looking either.

just a thought