An Induction Loop or Audio Frequency Induction Loop System (AFILS) is a system commonly used by hearing aid users to clearly hear the sound in a theatre, through the bandit-proof glass screens in a bank or just a telephone call.
It can also be used to provide wireless talkback or IEM to crew or presenters without using up valuable radio frequency Spectrum.
An Induction Loop is essentially very simple. All that is required is a moderately powerful audio amplifier and a wire running around the perimeter of the area in which you want people to be able to hear. The wire is normally run around the area a number of times creating a coil of a few turns. This can be achieved by using a cable with several cores and sequentially connecting the ends of each core. In engineering terms this forms the primary winding of a simple, if very large, transformer. Receivers are fitted with small internal coils and these function as secondary windings.
Specialist Induction Loop amplifiers are available as the signal to the loop needs to be processed to reduce the dynamic range and the amplifier itself needs to drive a current of a few amps around the coil. Ordinary loudspeaker audio amplifiers may become unstable or object in other ways to feeding a loop, they don’t have the signal processing either.
A disadvantage of induction loops is that the giant transformer is indiscriminate and will induce audio signals into any coil in the vicinity. These may include dynamic microphones and (particularly) unbalanced audio cabling.
This Guide to Audio Frequency Induction Loop Systems is a rather more detailed description of Induction Loops, including some of the governing legislation and standards.