SCIENTIFIC AMERICAN: A Loopy Idea That Works: Using Telecoils to Turn Hearing Aids into Mini Loudspeakers

Standard hearing aids capture sound via a microphone and then send an amplified version to an earpiece. They work well in relatively quiet, intimate settings, but in public spaces filled with background noise, most users find them of little use. A simple technology that sidesteps the problem, long available in Europe, has finally begun entering the U.S. market. Advocates hope that with the success of pilot projects, the hearing impaired will be able to find public address announcements and other kinds of speech more intelligible.

The technology is an induction-loop system (known as a hearing loop), whereby electromagnetic waves produced by a microphone, public address system or telephone receiver induce an analogous current in the loop. The loop can broadcast the signals directly to a hearing aid equipped with an appropriate detector—specifically, a tiny copper telecoil wire, which picks up the signal (also via induction) and then sends it for amplification and transmission out of the earpiece. (Hearing loops can also broadcast signals to cochlear implants, which are surgically implanted devices that directly stimulate the auditory nerve.)

http://www.scientificamerican.com/article/sound-approach/