What Do AM & FM Stand for on the Radio?

Most radios have both AM and FM channels, which allow you to listen to music, sports and talk shows. The way the music or talk reaches your ears differs depending on what channel you're listening to and whether it's broadcast on an AM or and FM channel. An explanation of what AM and FM stand for will help you understand the differences between the two.

AM stands for amplitude modulation. This refers to the fact that AM radio waves change in height. The radio waves associated with AM radio stations travel parallel to Earth and then move up toward the sky. The AM waves then bounce off the ionosphere and travel back down to Earth, according to Darlene R. Stille, author of "Waves: Energy on the Move." FM stands for frequency modulation, which refers to the changes in frequency, or distance between the radio waves, responsible or the sounds heard on FM radio stations. Like AM radio waves, FM radio waves also travel parallel to Earth, but instead of stopping at the ionsphere, FM waves travel through the atmosphere and out into space.

You can remember the difference between AM and FM radio waves by comparing them to a flashlight, suggest Robert M. Hazen and James Trefil, authors of "Science Matters: Achieving Scientific Literacy." AM radio waves are comparable to alternately dimming and brightening a flashlight while FM waves are comparable to changing the colors emitted by the flashlight. The alternate dimming and brightening represent the changes in height of the AM radio waves, and the changing colors represent the changes in frequency of the FM radio waves.

FM radio waves are also transmitted at a higher frequency than AM radio waves. The sounds that come from FM radio stations don't travel long distances, so they can't be heard as well when a person is located far from the broadcast location, and the waves tend to fade about 50 or 60 miles away. AM radio waves are about 1,000 feet in wavelength, and AM radio stations can be heard for hundreds of miles from where they're broadcast. There is more crackling heard on AM radio stations, however, because of electricity in the atmosphere, Stille notes.

The sounds that come from AM and FM radio stations are courtesy of electrons that move back and forth to create electromagnetic waves, according to Hazen and Trefil. The exact frequencies of those electromagnetic waves coincide with the radio station number on the dial. These radio waves also carry codes that determine how the music and talk sound, Stille notes.