FM stands for Frequency Modulation and is a type of radio transmission.  These ALD's pick up the speaker's voice though a microphone, then send the signal to a receiver using an FM radio wave.  The signal is then heard using one of the connection methods described earlier.

Because the signal can bend around walls and has a wide range, FM ALD's are appropriate for educational, social and home situations when the users will be moving and my not be within line of sight.