Humans car hear between about 20 hz and 20,000 hz. 20 hz is a very low sound that you can also "feel." Ever experience your car shaking when someone next to you at a stoplight has his music cranked? You can’t really hear the lyrics because they are in the higher frequencies, but you can feel the shake of the low frequencies. (By the way, anyone who listens to music that loud for a long time will develop hearing loss at a fairly young age.)

In truth, most people can't hear all the way up to 20,000 hz. One’s ability to hear high frequencies tapers off after adolescence. That is, 21-year-olds already have declining hearing (down in the 16,000 hz range--although this varies a lot with the individual).

Similarly, you may have noticed that your great grandmother can hear your father better than your mother. That's because women's voices are somewhat higher in frequency--in the ranges that your great grandmother has lost.