Advances in technology offer promise of an expanding role for telecoils

One of the most underestimated and underused components available for the modern hearing aid is the telecoil. A telecoil is an inductive pick-up coil used to sense changing magnetic fields. The coil converts magnetic energy to electrical energy in much the same way that a microphone converts sounds waves to electrical energy. A hearing aid containing a telecoil significantly improves the patient's speech comprehension on the telephone. That's because the telecoil allows the hearing aid to reproduce sound directly from the telephone's electromagnetic field rather than through an open acoustic microphone, which is subject to background noise and feedback. Beyond their value with telephones, telecoils provide access to certain types of assistive listening systems, including neck-loop and room-based loop transmitters. AN EVOLVING TECHNOLOGY Perhaps the first use of a telecoil in a hearing aid occurred in 1947 when Samuel F. Lybarger incorporated a t-coil in a body aid under the Radioear trademark. The early Lybarger telecoil was a freestanding (bobbin-less) ring-shaped coil about 1–1/8 inches in diameter and approximately 1/16-inch thick. The subsequent introduction of a metal core or bobbin for telecoils allowed a significant reduction in size from the Lybarger design. This was an essential step in incorporating telecoils in head-worn hearing aids. Conventional telecoils continued to decrease in size from the 1970s through the early 1990s. This trend closely paralleled the miniaturization of hearing aids and their other components during the same period. Today, telecoils can fit in behind-the-ear (BTE) and in all but the smallest in-the-ear (ITE) hearing aids. Few in-the-canal (ITC) or completely-in-the-canal (CIC) aids have sufficient space to include a telecoil and a conventional microphone-telecoil (M-T) switch. However, the new touchless telecoil technology, discussed below, may alter the situation. CHALLENGES IN TELECOIL DESIGN The demand by hearing aid manufacturers for ever-smaller components presents a major challenge to telecoil designers. In response, telecoil manufacturers are exploring ways to develop further miniaturized and more sensitive devices—both passive and active. Passive telecoils consist essentially of many turns of ultrafine wire on a suitable core. An active telecoil, on the other hand, combines in one package a conventional telecoil with an integrated circuit pre-amplifier with filtering for attenuation of electromagnetic interference (EMI) generated by digital cell phones. In addition to increased sensitivity, the active telecoil has achieved a frequency response shape approximating that of a typical hearing aid microphone. Another characteristic of significance for telecoils, particularly active ones, is the noise level or signal-to-noise ratio. Active telecoils have an inherent advantage because their packaging allows the telecoil output to be placed very close to the pre-amplifier input. Using low-noise telecoil and pre-amplifier designs further improves speech comprehension. A closer look at sensitivity Other parameters being equal (shape, wire size), sensitivity is roughly related to the square of the telecoil's size (volume). Therefore, decreasing the size by a factor of two reduces sensitivity by about a factor of four. To increase sensitivity without increasing size requires a change in telecoil design or fabrication technique. The doubling in sensitivity for a given size between 1976 and 1984, for instance, resulted primarily from the development of techniques for winding finer and finer wire, permitting twice as many turns for the same size coil. The majority of high-sensitivity coils currently produced by Tibbetts Industries, a major telecoil maker since 1974, are wound with AWG54 magnet wire. This ultrafine wire has a diameter of approximately 0.0007 inch. To understand how fine that is, consider that a 1-pound spool holds over 160 miles of wire, and several strands of AWG54 wire can be threaded through a hole in a human hair. Improving performance Current research and development in both coil design and electronic techniques are addressing past shortcomings and bringing new functionality to the table. For example, progress has been made in reducing sensitivity to stray magnetic fields for telephone users. Efforts are also under way to provide, where advantageous, less directional telecoil performance. In addition, the future will bring greater use of the telecoil as an input device for the hearing aid. This may include enabling programming by programmable aids and allowing a user to remotely control functions such as volume adjustment or switching between microphone and telecoil inputs. In fact, “touchless” technology for telecoils is now available through a partnership between Micro-Tech (a Starkey Laboratories company) and Tibbetts. Using this technology, systems sense the static magnetic field of the telephone and automatically switch between microphone and telecoil input and back again. This feature simplifies patient access to the benefits of telecoils and makes it easier for dispensers to teach clients how to use the telecoil. Surface-mount technology Another key development in telecoil evolution is more durable packaging through the introduction of surface-mount technology manufacturing processes. Surface mount is a type of electronic component package. Most electronic components can be divided into two categories—through hole and surface mount. Through-hole components are designed to be loaded on one side of a printed circuit board (PCB) and soldered on the other. Surface-mount components are loaded and soldered on the same side of the PCB. Whereas the standard passive telecoil has lead wires connecting it to the hearing aid circuitry, a surface-mount telecoil (SMT) has two conductive pads for direct attachment to the PCB. The basic construction of an SMT involves mounting a telecoil between two L-shaped metal tabs. The telecoil end wires are connected to the inner surface of each tab and the outer tab surfaces provide the connection points to the PCB. The device is completed by totally encapsulating the telecoil and internal connections in a durable and heat-resistant black epoxy to create a monolithic, rectangular block with surface-mount terminations at each end. Though SMTs are all passive devices at present, soon amplified telecoils with their inherent advantages will also be available in surface-mount packaging. Because a surface-mount telecoil is completely encapsulated, it is not subject to mechanical handling damage during the hearing aid manufacturing process. Also, their standard form facilitates the integration of SMTs into automated manufacturing processes. If it becomes easier for manufacturers to include the telecoil option in their hearing aids, the cost of telecoil-equipped hearing aids may decrease. Lower cost, coupled with greater dispenser and consumer awareness of the benefits of telecoils, has the potential to increase the number of hearing aid wearers who take advantage of telecoils.