In the U.S.A. and other parts of the world the mains frequency is 60 Hz. Therefore the signal is most positive after 5 milliseconds, back to zero after another 5 milliseconds, down to its most negative after the next 5 milliseconds and finally back to zero after a final 5 milliseconds. This whole cycle occupies 20 milliseconds or 20 mS and repeats 50 times a second. A.C. at audio frequencies extends from 20 Hz to about 20,000 Hz or 20 Khz. Depending upon your age you will not actually hear it beyond 15 Khz and older people are unable to hear much beyond 10 Khz.
Animals can hear much higher frequencies.
Largely becoming superseded by satellite transmissions. Beyond 3 Ghz is virtually satellite transmissions. It is interesting to note by way of numerical comparison that firstly, each band is 10 times the previous band. Secondly the L.F. band spanning 30 to 300 Khz could be duplicated 10,000 times over in the space occupied by the U.H.F. band. Also at the bottom end of 30 Khz the signal cycle repeats 30,000 times a second.