T-Coils: Getting The Most Out Of Your Hearing Aid

Introduction

Consumers report they want hearing aids to provide more benefit in noisy situations, have better sound quality and produce less feedback (Kochkin, 2002). Many of these problems occur due to limitations associated with hearing aid microphones. The microphone for ear-level hearing aids has a relatively small "effective area" within which it works maximally. The effective area is dictated by microphone size and location and can be thought of as the ideal area between the talker and listener. When the hearing aid user is listening to talkers located beyond this effective area, the strength of the signal of interest (the talker's voice) diminishes rapidly as the distance increases, making it difficult to hear. At the same time, interfering sounds (such as other talkers' voices or environmental sounds) that may be present within the effective area will be heard more dominantly than the signal of interest. As the signal of interest decreases and originates beyond the effective area, and unwanted sounds increase within the effective area, the signal-to-noise ratio (the loudness of the talkers voice versus the loudness of interfering noises) is reduced, making it difficult for the hearing aid wearer to hear clearly.

Hearing aid microphones can also cause difficulties while listening on the telephone. Feedback - the annoying whistling sound that hearing aids emit when a phone is located too close to the hearing aid - often occurs when people try to use the telephone while wearing hearing aids.

Fortunately, there is an inexpensive, useful, and often underused hearing aid component which overcomes these microphone problems. This option is the Telecoil (T-coil). T-coils allow different sound sources to be directly connected to the hearing aid, improving sound quality and allowing the hearing aid wearer to easily perceive the signal of interest in almost any environment, and regardless of background noise.

What is a telecoil and how does it work?

T-coils were first made available in hearing aids in the late 1940's (Lybarger, 1982). T-coils are comprised of a metal core (or rod) around which ultra-fine wire is coiled. T-coils are also called induction coils because when the coil is placed in an electromagnetic (EM) field, an alternating electrical current is induced in the wire (Ross, 2002b; Ross, 2004). The T-coil detects EM energy and transduces (or converts) it to electrical energy. T-coils can also be used to pick up magnetic signals, just as a microphone picks up an acoustic signal; the T-coil then sends the signal to the hearing aid circuit or processor for amplification.

T-coils may be passive, which is the conventional type of T-coil, or active. Passive T-coils are primarily a rod with a thin wire wrapped around it. Active t-coils are essentially passive T-coils with a pre-amplifier which may be useful to filter the electromagnetic interference generated by digital cell phones (Marshall, 2002).

T-coils can usually be built into behind-the-ear (BTE) and in-the-ear (ITE) hearing aids, and, can occasionally be built into in-the-canal (ITC) and completely-in-the-canal (CIC) hearing aids, provided there is sufficient room in the hearing aid shell to accommodate the telecoil. Telecoils can be found in analog, digitally programmable analog (DPA) and digital signal processing (DSP) hearing aids. T-coils are included in some cochlear implant processors.