Induction Loop Systems

Induction loop technology has been around the longest for use with the hearing impaired.  Originally designed to provide assistance in the classrooms at schools for the deaf this technology has advanced to other uses today.

Induction loop amplification operates on the basic principle of electronics called magnetic induction.  When an electrical current is amplified and passes through a loop of wire, a magnetic field is generated around the wire.  The field varies in direct proportion to the strength and frequency of the signal being transmitted.  When another wire is placed close to the field of the first wire, an identical electrical current is induced within it.  The second current can be amplified and converted into an exact duplicate of the original sound signal.

The listener can receive an audio signal without being physically or electrically connected to the loop.  The listener receives the sound in one of two ways; 1) through the use of the T-coil or telephone switch on their hearing instrument or 2) through an induction receiver that can work with earbuds or headphones or as an ear insert device that is simply a T-coil.

The system consists of 1) a microphone or signal source, 2) an amplifier to boost the signal, 3) a loop or wire around the room or listening area and 4) the receiver in the ear.  It is also possible to combine other amplification systems to be used with an induction loop.  For example, an F.M. system can use a neckloop attached to the F.M. receiver.  The F.M. receiver sends the message through the neckloop via electromagnetic energy, and the T-coil in the ear picks up the sound.  The neckloop allows the hearing impaired individual to couple other ALD's directly through the hearing instrument that was fit specifically for their ear.

Induction loop amplification does provide some advantages over other systems.  First of all, it is easy to use.  Once installed, the person only needs to make sure the power is on and that their T-coil is working.  People appreciate the fact that there is not as much equipment to turn off at the end of the session.

The system can be installed with relatively little knowledge.  Some groups use portable loop systems that can be moved from one location to the next.  Maintenance frequently is minimized, because there are fewer components involved than other systems.

The induction loop system does present some limitations. For example, the strength of the telecoil in the hearing instrument may not be very powerful, thereby making the signal weak.  Or the hearing instrument may not have a telecoil in it at all. 

The signal strength within the loop area varies.  The closer the listener is to the electromagnetic field, the stronger the signal will be.  There is also the possibility of "dead spaces" within the looped area where sound will not be received.

Only one loop can be used in a specific listening area because of "spill over," which means that sound that exists within the loop also will exist outside the loop.  Therefore, loops cannot be placed too close to each other.  A recently developed loop system that works with a mat reportedly has eliminated "spill over."

If a building has a high metal content, the sound coverage may be erratic.  When installing a loop system care should be taken not to install it over large metal objects, such as heaters or around metal baseboard moldings.  The metal acts as a shield and weakens the signal.

Positioning within the loop is important.  For example, one will not hear sound when standing directly over the loop wire because the magnetic field changes direction from the vertical to the horizontal.  The person has changed their orientation to the receiving antenna.  Another time positioning is important is with neckloops.  The head position will determine how the signal is received.