A sound wave, an AM wave, and an FM wave compared

Amplitude Modulation, usually shortened to AM, is a simple way to send a radio signal. The signal can travel long distances, and appear in faraway places, because of the earth's ionosphere.

Amplitude is the height of a wave. A carrier wave is modulated (changed) by adding audio. The combined signal is transmitted over a frequency. AM signals can be received by a crystal-powered radio set, that needs no batteries or electrical power source. Many early radios were homemade, and used earphones.