You're correct that they do hear airborne sounds and they hear best in low frequencies. However, they can hear the human voice, as well.
http://www.anapsid.org/reptilehearing.html is an interesting article about herp hearing, though not limited to snakes. It makes the point that snakes' hearing is superior to cats' hearing at the 200-300 Hz frequency spectrum.
Humans have a wide vocal spectrum. Any human speech produces multiple frequencies simultaneously. That said, the vocal range of the speaking voice of most adult male humans has a fundamental frequency range of 85-180 Hz. The average adult female speech runs from 165-255 Hz. Quoting the above cited article, "most snakes can hear a person speaking in a normal tone of voice in a quiet room at a distance of about 10 feet (3 m). So, if you think your snakes recognize their names, you are probably right."
Pretty cool, huh?