Assistive Listening Devices (ALDs) are amplification instruments that are designed to be helpful in specific, but not all listening situations.  Some examples of ALDs are types that can amplify the TV, the telephone, in theaters, places of worship and public speakers.  Hearing aids, which also amplify sound, are instruments that can be used in all listening situations.  But because a hearing aid is an all purpose instrument, it may not be as successful in each and every application.  Think of an ALD as a pair of binoculars for the hearing aid.
The construction of an ALD is different from a hearing aid in one main aspect.  All of the components of a hearing aid - the microphone, amplifier, and the loudspeaker - are located within the body of the instrument that is placed in the user's ear.  Therefore the microphone of a hearing aid picks up both signal and noise.  This background noise is one of the main reasons hearing aid users get frustrated and end up putting their new hearing aids in the drawer.  In contrast, the microphone of an ALD is separated from the body of the instrument and is placed at the sound source so it is most apt to pick up the desired signal without background noise.   This is the biggest advantage of ALDs.

In ALDs there are four primary methods used to send the signal from microphone (transmitter) to receiver:
Hard Wired:  The microphone is attached by a thin cable to the receiver.·
 FM:  The signal is transmitted by an FM radio signal.  Often used in classrooms for the hearing impaired.
Infrared:  The signal is transmitted by an infrared light signal.  This system is often used in theaters.
Loop:  The transmitter is attached to a loop of wire that is placed at the perimeter of the room and generates an electromagnetic field which is picked up by the receiver.  This system is also used in classrooms or while watching TV at home.

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