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INDUCTION LOOP PRIMER
Conventional induction loop (IL) systems consist of a sound source, loop amplifier and wire placed around the circumference of the "listening area."  This area may be confined to one person when a neck loop is used, or it may be as large as an auditorium or theater.  Audio signals are amplified and sent through the loop wire, resulting in an electromagnetic field.  Hard of hearing individuals access this field with hearing aids, cochlear implants or tactile devices that are equipped with a "T" (telephone/telecoil) switch, or with battery powered induction receivers.  When a hearing aid is switched to "T", the unit's internal microphone is replaced by a tiny coil of wire.  In effect, switching to "T" transforms the hearing aid from a sound sensing device into an electromagnetism receiver.  IL systems range in power and features from portable small area loop systems that accept two microphones to basic large area systems designed to interface with existing public address systems, to large area stand-alone  systems that accommodate many direct inputs.

The objective of all hearing assistance systems is to amplify the original audio signal without the intelligibility-degrading effects of room noise and reverberation (echoes).  Among the most desirable aspects of the IL system is that a telecoil-equipped hearing aid can access the signal without the need for special receivers.  In other words, the user of a telecoil-equipped hearing aid simply switches to "T" to receive a direct audio transmission from the podium, stage, teacher, film or any audio input.  The potential for universal accessibility, cost-effectiveness, low maintenance, and unobtrusiveness of the IL system make it especially attractive  for use in homes, schools, meeting rooms, public transportation centers, vehicles, houses of worship and many other public areas.  As with all wireless systems, a "pre-installation listening check" should be performed to make sure there are no local sources of potential interference.

HISTORY
1998 marks the 60th anniversary of IL systems and hearing aid telecoils.  The first portable hearing aid to incorporate a telecoil was the Multitone Model VPM in 1938, following an early patent for an IL hearing assistance system by Joseph Poliakoff of Great Britain in 1937.  Interestingly, IL systems were used in Europe long before becoming available in the USA, making its first appearance as an imported product after World War II. In the USA, interest in IL technology peaked in the late 1960's and early 1970's. In particular, many programs and schools for deaf and hard of hearing children were equipped with this technology. 

During the height of IL systems' popularity, no clear standards existed for manufacturing and setting up the systems;  as a result,  performance varied considerably.  With the development of other wireless technologies (FM, IR) in the 1970's and 1980's, IL technologies seemed to vanish...from the USA!  In other parts of the world such as Great Britain and Scandinavia, IL systems continued (and continue) to be used extensively.   A major reason for the ongoing popularity of IL technologies is the telecoil.  Hearing aids distributed through the National Health Service of these countries always include the telecoil as a standard feature in behind-the-ear hearing aids.  In contrast, the trend in the USA continues to be "smaller is better", and the telecoil began to disappear from the increasingly diminutive hearing aid.  As we will discuss in part II, in countries less affluent than the USA the IL system is often preferred because of its cost-effectiveness and minimal maintenance requirements.

In 1984, technical standards were adopted internationally that made a big difference in the performance of IL systems.    In the USA, the venerable telecoil has staged a comeback thanks to the ADA, miniaturization of circuits and the efforts of hard of hearing consumer activists who have long fought for hearing aid compatible telephones.  No longer an afterthought in hearing aid design, new telecoils have dedicated preamplifiers and yield high quality sound from telephones as well as IL systems.  Cochlear implant manufacturers are following the lead and plan to build telecoils into their processors.

In recent years researchers have studied the effects on human health of electromagnetism generated by power lines, radio transmissions, wireless phones, electric blankets, computer monitors, etc.  While it can be shown that all living things respond to electromagnetic fields on a cellular level, recent studies have not demonstrated a consistent positive link between exposure to electromagnetism and disease.  In over sixty years of world wide use, IL systems have never been implicated in having a negative effect on health.  One operational aspect that distinguishes IL systems from the types of electromagnetism cited in studies to date is the nature of the signal itself.   All the sources studied generate a constant signal  suggesting a lasting change and/or adaptation in living tissue.   In contrast, the electromagnetic field from IL systems is always changing.  Unlike FM radio transmissions, IL fields are not "riding" on a constant carrier and are continually changing in frequency and intensity, mirroring the input speech signal.  In fact, IL electromagnetic fields are only "on" when there is an audio input signal.

ADVANCEMENTS
Although IL systems have been in use for many years in Europe and are seeing a rebirth of popularity in the USA and other parts of the world, technical limitations have prevented the use of this equipment in certain situations.  Signal spillover and inconsistent field uniformity are two conditions that had always plagued IL technology.  The term "spillover" refers to the tendency for electromagnetic fields to flow out of the immediate looped area, causing interference with nearby telephone use and other looped areas.  No practical, commercially viable method for shielding or blocking the signal has ever been implemented.  Inconsistent signal uniformity refers to the variations in signal strength that occur with conventional IL systems.  The strength of the IL signal received will depend on where the listener is seated within the loop and the orientation of the telecoil relative to the IL field.  Over the years loop manufacturers devised ingenious schemes for addressing these problems, requiring elaborate loop wire installations that turned out to be impractical as some of their names imply, such as  "four square", "clover leaf" and "figure 8" loops.

In 1988, the U.S. Department of Education's National Institute on Disability and Rehabilitation Research supported the feasibility study, development and commercialization of a new type of IL system, the "3-D".  The 3-D is a radical departure from the traditional single loop IL system.  Rather than one loop wire,  the 3-D uses four,  prefabricated in a special geometric pattern and sandwiched between two molded carpet mats.   Powered by newly developed digital signal processing electronics, the 3-D became the first commercially available IL system that could be installed in adjacent rooms with minimal spillover interference.  The other IL shortcoming of inconsistent signal uniformity is also minimized by the 3-D.  When positioned over the 3-D "loop mat" a hearing aid can be rotated 360 degrees without a noticeable loss in signal. 

In the 90's, other IL system innovations have resulted from further research supported by the U.S. Department of Education.  Custom designed 3-D systems have been installed at airports.  And, the MobiLoop is the first commercially available IL system in the USA for use in public transportation vehicles.   Unfortunately, the communication, navigation and propulsion systems on board commercial airplanes generate so much electrical "noise", IL technologies would be useless in this setting.  Next time you are in an airplane, turn down the volume control of your hearing aid, switch to "T" and have a listen!

In Part II we will look at some interesting and unusual IL applications, and discuss how you can maximize the performance of your hearing aid telecoil.

Please contact the authors for detailed references on information cited in this article. Paula Hendricks, M.A., is Educational Director and Norman Lederman, M.S., is Director of Research & Development at Oval Window Audio, 33 Wildflower Court, Nederland, CO  80466,  phone/fax/TDD: 303-447-3607.