Build a what?
Though they don't benefit everyone, hearing aids often help a person with some degree of hearing loss with hearing environmental sounds and speech. Unfortunately, in addition to helping amplify another persons voice on the phone for example, it also amplifies other sounds in the area. The TV, the hum of the furnace, other people talking, etc. This interfere's with the persons ability to hear the person on the phone.
A hearing aid which is equipped with a tele-coil (t-coil) switch allows the user to switch from the hearing aids built in microphone to a coil inside which picks up a magnetic field generated by a coil in the telephone. This transmits the sound signal directly from the phone handset to the hearing aid without picking up extra noise, producing a cleaner message for the user.
This is not limited to telephones however. It is possible to create a loop system almost anywhere. They can be found in classrooms and theaters among other applications. T-loop systems - also called Induction loop amplification systems - work by the process of induction, explained below:
"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."
(Source: http://www.harcmercantile.com/loop01.htm)
Here I will explain how to build an induction loop amplification system for your home where it might be used to watch television or listen to the radio. It not only provides the advantage of a clear, direct signal sent to the hearing aid, but also allows you to get the signal without having to turn the television up to the point where it might be disturbing to others.