Amplitude Modulation

So now we can generate a radio frequency wave of a single frequency (top picture) but anyone listening at the other end will hear nothing because there is nothing to hear - radio frequency waves are way, way above the highest frequency that humans can hear.

The bottom picture shows a radio frequency wave but, this time, it has a lower, audio frequency superimposed upon it. The amplitude (strength) of the radio wave is varied in time with the audio wave. This is called Amplitude Modulation.

Because the strength of the radio wave is constantly changing and because most types of radio interference are AM, this type of modulation is losing favour to other methods. Another common method is called Frequency Modulation.