Audio induction systems operate on the electronics principle of electro magnetics. When an electrical current is amplified and passed through a loop of wire, a magnetic field is generated around the wire that varies in direct proportion to the strength and frequency of the signal. If another wire (or coil of wire) is placed in proximity to this field, an identical current will be passed - induced - to it which can then be amplified.
A typical induction system consists of a microphone or other signal source, a power amplifier to boost the signal, and a loop, or induction "antenna". It can also be the earpiece of a telephone receiver. The loop can be as small as a personal neckloop - the size of a long necklace - worn by an individual, or can be as large as a wire which encircles a room, auditorium or other listening area....DIAGRAM
The receiving portion of the system is most often a personal hearing aid with a built in telephone coil. Not all hearing aids are equipped with a T-coil, but today, almost all hearing aids can be built with a T-coil, including custom in the ear aids and most smaller canal sizes.
Listeners who do not have hearing aids equipped with T-coils, or who do not use hearing aids at all, can use special induction receivers with earphones.
When using a telephone and a hearing aid with T-coil, it takes some experimenting to find the best position for the phone receiver to pick up the strongest induction signal. Usually best results come from placing the receiver slightly behind the ear.
If the placement of the T-coil is such that the signal seems weak, or if the phone is not hearing aid compatible, then an induction phone amplifier is suggested. This amplifier compensates for T-coil placement and makes a strong induction field for better coupling.
