What is a Telecoil (T-Coil)?

How is a T-Coil Used in a Hearing Aid?

Telecoil (T-Coil) Description

A Telecoil or T-Coil is a small device that is rather simple. It consists of a metal rod with copper wound around it, similar to a spool of thread as shown in the image, below, except that the T-coil is much smaller, and the post around which the copper is coiled is about the thickness of a small nail or tack:

One of the basics of physics is that if you have an electric current running though a wire, then place a telecoil 90° to the position of the wire, depending on how many coils there are on the telecoil,  the telecoil will oscillate to the frequencies running along the wire. This phenomenon is called induction coil oscillation.

The process of inducing an electrical current in one wire as a result of current flowing in a nearby wire is called induction—hence the term induction loop system—or just "loop system" for short.

How does a Telecoil (T-Coil) Work in a Hearing Aid?

When an electrical current passes through a wire such as a neck induction loop or a room induction system the resulting oscillations create an ever-changing magnetic field. If a hearing aid is equipped with a a T-Coil, and is oriented properly, the T-Coil vibrates to the magnetic oscillation as in the source wire. Generally, the strength of the inductive capacity of the T-coil is determined by the number of turns of the copper wire around the metal axis rod. Larger rods permit more turns and more powerful (and correspondingly larger) T-coils. The strength of the electrical current "induced" in the telecoil by the electromagnetic field is directly proportional to both the energy in the magnetic field and to the relative positions of the induction coil in the hearing aid to the magnetic field (in a telephone or wire loop). The greater the orientation toward a 90° relative location, the stronger the signal will be.

Telecoils can be oriented differently in a hearing aid to take advantage of the orientation of the transmitting device. For example, if the wearer wants the hearing aid with a telecoil optimized for picking up sounds from a room loop i.e. a loop that has been especially installed around a room, then the telecoil would be installed so that is vertically oriented. The vertical orientation will maximize the capability of the telecoil to pick up the oscillations from the room loop that is horizontal i.e. they are both in a 90° orientation to each other.

When the hearing aid wearer has a neck loop (details below) that has a telecoil incorporated or the hearing aid wearer wants to take advantage of the telecoil for listening on a telephone,  then the best orientation of the telecoil in the hearing aid should be horizontal as most wearable loops and telephones have their loops in a vertical orientation, again, optimizing for the 90° orientation respective to each other.

When a hearing aid is switched to the T-Coil position, the telecoil is set to detect only an electromagnetic field, although some hearing aids have a "mixed" mode where both the T-Coil and the microphone in the hearing aid are turned on. The hearing aid connects the T-Coil to the input of the hearing aid's amplifier instead of its microphone. The T-Coil is sensitive to nearby magnetic fields such as the one produced by the loop system or a room loop and they both induces a corresponding electrical signal into the telecoil. The hearing aid amplifier amplifies this signal and a faithful reproduction of the original speech signal is heard.

A hearing aid compatible telephone has a telecoil is inserted in the phone. The telecoil detects, or is compatible with, a similar telecoil in the hearing aid, rotated 90°. This allows the hearing aid to "couple" with the telephone through an electromagnetic field and pick up signals directly from the telephone.

The Loop System and the Hearing Aid

Personal loop systems are used as an amplifying intermediary between the wearer of hearing aids with T-Coils and the sounds in the immediate area. They are often called neck loops and consist of a microphone, an amplifier and a T-coil. Audio signals are picked up by the microphone in the loop or directly (electrically) or from a sound source like your TV or stereo (some TV's have loops so a hearing aid can pick up the sounds oscillated through a T-Coil, bypassing the need for a neck loop). If sounds are amplified in a neck loop, they then travel through a loop of wire that surrounds the listening area which then produces a magnetic field in the looped area that mirrors the frequency and intensity characteristics of the original sound signal.

What are the advantages of a telecoil (T-Coil)?

One of the big advantages of a telecoil in a hearing aid is that there is no interference from the outside world.  All you hear are the frequencies picked up through the magnetic oscillations.

What are the disadvantages of a telecoil (T-Coil)?

Telecoils have a tendency to pick up extraneous electrical interference that may be generated in the immediate area of the wearer; They may pick up hum or static from electrical equipment and wiring.

The smaller the hearing aid, the less room there is for a telecoil, and thus in tiny aids telecoils are either weaker or not available.

If two people are wearing neck loops, and they sit next to each other, the interaction of their telecoils may produce interference (high pitched squeals).

We offer telecoils as options in our Melody A2 and Melody A3 BTE open fit hearing aids.