A typical set up of an induction loop system is shown in Fig.1 above. The loop amplifier is mounted on a wall closest to the sound source - for example, close to the hall stage or near the TV in the case of a home loop.
The induction loop wire (white) is fed around the room and clipped to the skirting board or hidden under the carpet edge if present. A microphone can then be plugged into the induction loop amplifier. Audible sound reaching the microphone is converted into a small electrical signal (small orange wave) and fed into the amplifier. The amplifier then amplifies this signal (large orange wave) and feeds this around the room - through the loop wire as an alternating current (a.c.) signal. The bigger the room, the bigger the amplifier needed for the job.
As the alternating current is passed through the loop wire, a magnetic field is generated around it (see Fig.2). If a second wire is then brought within this magnetic field, a corresponding alternating current is created within the second wire. In technical jargon, it is said that the alternating current is 'induced' in the second wire - hence the term 'induction loop'. This magnetic principle known as 'Flemings right hand rule' is the basis on which all transformers, electrical motors, dynamos and generators operate.
Inside a conventional hearing aid is a very small coil of wire - this acts as a pick-up if the wearer switches the hearing aid to the 'T' position and stands inside the induction loop area. A direct copy of the audio signal reaching the microphone is now 'induced' in this coil and the hearing aid converts this electrical signal back to audible sound for the wearer to hear. In this way, someone sitting at the back of the hall can clearly hear the speaker at the front - simply by being inside the loop area.
If an induction loop, infra-red or radio system is not present people who are hearing impaired have to use the hearing aid set to the 'M' position. The microphone in the hearing aid picks up sound from all directions; close sound is amplified more than a distant sound. If a noise is made i.e. a chair is moved or perhaps someone close to them speaks they will be unable to hear the main speaker.
Hearing aid M and T recording in a church
Click the enlarged image to switch between O (off), T (telecoil) and M (microphone) position to hear the difference an induction loop system makes