The first hearing aids were no more than simple amplifiers, and the development of hearing aids drew heavily on advances in communications technology. It is doubtful if anyone at that time could have predicted that hearing aids would one day have a significant impact on the development of an important area in communications technology—digital wireless telephones.
The first use of wireless technology in hearing aids followed the discovery that leakage of the electromagnetic field in early telephone handsets was sufficiently powerful for a small coil placed alongside the handset to pick up the audio signal. In 1937, Joseph Poliakoff of Great Britain obtained a patent for a magnetic induction loop communication system.1 The following year, an inductive coil (aka a telephone coil, telecoil, or T-coil) was incorporated in the British Multitone hearing aid (Model VPM). Sam Lybarger is credited with inventing the telecoil independently in the United States in 1947.2,3 The pas de deux between hearing aids and wireless technology had begun.
TELECOILS ENTER THE PICTURE
The use of a wireless link between a hearing aid and a telephone was found to provide a much clearer signal than that obtained with acoustic coupling. The use of a telecoil did not include distortions introduced by the telephone receiver in converting the electrical signal to an acoustic signal, or the distortions and noise introduced by the hearing aid microphone, or any background acoustic noise in the vicinity of the telephone.
There were, however, some difficulties in using a telecoil because its axis needed to be aligned appropriately relative to the direction of the magnetic field. That required the hearing aid wearer to position the telephone handset carefully in order to obtain a good connection. Finding the “sweet spot” was not always easy for the wearer.
In the 1970s, the Bell system introduced a more efficient handset without significant electromagnetic leakage. Telecoils, however, could not be used with these handsets, and so this engineering advance was denounced by hearing aid wearers who had grown accustomed to using telecoils with their superior sound quality. After an effective consumer lobbying campaign, the Bell system was required by law to maintain an electromagnetic field in its handsets so that telephones remained hearing aid compatible (HAC).4
It soon became evident that the telecoil in a hearing aid could also be used to pick up signals generated by devices other than a telephone. One valuable application of inductive coupling is to use a magnetic field to transmit audio signals to a hearing aid so as to avoid acoustic noise and room reverberation that would otherwise be picked up by the hearing aid microphone. The necessary magnetic field can be established quite easily by encircling the area to be served with a multi-turn wire loop and connecting the two ends of the loop to the output of an audio amplifier.5
Denmark led the way in implementing inductive loop systems in schools for the deaf.6 Since background noise and room reverberation present a major problem in school classrooms, these inductive loop systems proved to be of great value. Induction loops were later installed in many other locations, including museums, auditoriums, places of worship, transportation terminals, and other public places, for the benefit of hearing aid wearers. The use of induction loops is especially widespread in Europe. British trains and taxis, for example, now use induction loops for the benefit of their passengers with hearing aids.
A note on terminology
Before continuing this discussion, let's briefly discuss the terminology used for induction coils. The terms “telecoil” and “T-coil” (both abbreviations of telephone coil) are commonly used for the induction coil in hearing aids. This terminology was appropriate when the telephone was the only device that could be linked inductively to a hearing aid. Now many other devices can be linked to a hearing aid by means of this induction coil, yet it is still referred to as a “telecoil” or “T-coil.”
The electromagnetic (EM) field used for inductive coupling with a telecoil is commonly referred to simply as the “magnetic field.” Note, however, that an EM field has both an electrical and a magnetic component. At audio frequencies the wavelength of an EM field is enormous and the signals induced by the electrical component of an EM field at these wavelengths are negligible. The magnetic component of the EM field is thus the only component of interest (at audio frequencies); hence the use of the term “magnetic” rather than “electromagnetic.”
FM AND INFRARED SYSTEMS
Despite the many advantages of induction loops, there are nevertheless problems with their use. A major problem is that the magnetic field is not constrained to the intended area. The spread of the magnetic field to nearby areas, known as overspill, can be very troublesome at a school for the deaf since the magnetic field in one classroom can be picked up in a neighboring classroom as well.
In 1959, the Fredericia School in Denmark pioneered the use of wireless transmissions at radio frequencies to circumvent the spillover problem.6 The school obtained a six-channel AM radio transmitter in which each channel had a separate microphone input, one for each of six classrooms. The radio signals could be picked up anywhere in the school. The children were provided with AM receivers tuned in to the appropriate frequency channel for their class. The AM transmissions were later replaced with FM transmissions.
In the United States, the Electronics Futures Company of New Haven, CT, introduced an AM system for use in classrooms in 1963 and a portable FM system a few years later.7
Soon after, the Phonic Ear Company boldly broke the law and began marketing an FM system for classroom use that exceeded the power limits permitted by the Federal Communications Commission (FCC). This system was entirely portable and convenient for both teachers and children to use. Moreover, because of its higher transmission power, it was very effective in addressing the problems of interference from other FM transmissions (the system used a frequency band allocated for commercial transmissions).
Parents, educators, audiologists, and others concerned with the needs of deaf children were extremely impressed with the system and successfully lobbied the FCC to make allowances for radio devices that served people with hearing loss. In 1971, the FCC reserved the 72 to 76 MHz radio spectrum for these devices and later added the 216 to 217 MHz band, which, because of its shorter wavelength, offered additional advantages.7 Unlike loop systems, FM systems are not restricted to a single channel; different channels can operate on different carrier frequencies.
In the 1970s another form of wireless transmission using infrared light was introduced for use in hearing aids and assistive listening devices.8 Infrared systems operate in much the same way as FM systems, but have the advantage that infrared light does not travel through walls and hence cannot interfere with transmissions in neighboring rooms. Infrared assistive listening devices are used primarily in theaters, auditoria, houses of worship, and similar locations. They do not work well outdoors because of interference from sunlight.
WIRELESS PROTOCOLS EMERGE
The most recent additions to the many forms of wireless communication are two new protocols for wireless signal transmissions, Bluetooth9 and Wi-Fi.10 Both protocols operate at the same radio frequencies (2.45 GHz range). Bluetooth is designed for short-distance communication between digital devices in close proximity and is most commonly used with digital telephones and handheld computing devices. Wi-Fi was designed originally for wireless local area networks (LANs) and is both more powerful and covers a wider range than Bluetooth.
Wi-Fi has also found a range of other applications that overlap those of Bluetooth, such as connecting to the Internet using the Voice over Internet Protocol (VoIP), which can provide broadband telephone links, as well as interconnecting a wide range of consumer electronics (e.g., television, audio and video players, and electronic games). The use of both Bluetooth and Wi-Fi is expanding rapidly and their protocols are continuously being updated. Wi-Fi, at present, has a broader bandwidth than Bluetooth, but this is likely to change if there is a demand for Bluetooth connections with a larger bandwidth.
Bluetooth and Wi-Fi are supported by major companies with substantial sales and large-scale mass production. As a result, inexpensive chips are being produced for implementing these protocols. Although Wi-Fi has a number of possible applications relevant to acoustic amplification, such as the use of LANs at transportation terminals for transmitting announcements to hearing aid wearers, Bluetooth is more widely used for hearing aid applications, such as linking hearing aids to wireless telephones.
ADVANCES AND CHALLENGES
Every curtain has its seam. While rapid advances in digital wireless systems and their ubiquitous use have opened up advantageous new ways of linking hearing aids to other communication systems, this technology has also introduced a new problem for people with hearing loss. The methods for encoding speech in digital wireless transmissions were not designed with the needs of hearing-impaired people in mind. The received speech signals are imperfect replicas of the speech signal prior to digital encoding. The distortions introduced in the digital encoding and decoding of speech in wireless telephones reduce speech intelligibility.
We have grown accustomed to some reduction in intelligibility when we use the telephone. Incorrect recognition of names or words out of context is a common problem with telephone speech, even for people with normal hearing. However, the reduction in intelligibility is much greater for people with hearing loss, especially when they use digital wireless telephones, which are subject to additional distortions, including fading and momentary interruptions of the received signal.
Another problem is electromagnetic pollution. Digital wireless telephones generate relatively strong electromagnetic fields that can produce audible interference in hearing aids.11-13 The transmission frequencies used by wireless telephones are in the gigahertz range. At these frequencies even a very short piece of metal acts as an antenna. The wiring and other metal components in a hearing aid pick up these signals, which are then demodulated by non-linear components in the hearing aid.
In addition, the signal modulations in digital wireless transmissions are large and are in the audio frequency range, which means that the demodulated signals can produce substantial audible interference in hearing aids. It is often assumed, mistakenly, that the electromagnetically induced interference is picked up by only the telecoil. In fact, electromagnetic interference can be picked up by any metal component anywhere in the hearing aid. Note also that the electromagnetic fields generated by Bluetooth transmissions are not a serious problem since these signals are low in level and of insufficient strength to generate audible interference.
In the 1990s, as wireless communication systems switched to digital transmissions, large numbers of hearing aid wearers found they could not use digital wireless telephones because of the high levels of interference. Much progress has since been made in improving the immunity of hearing aids to this form of interference and reducing the interference generated by wireless telephones.
Standardized methods for measuring and controlling the interference have been developed,14 and the FCC has established limits regarding acceptable levels of interference generated by wireless telephones.15 The problem, however, has not been completely resolved, and new issues will arise as new applications and methods are developed for digital wireless communications.
The telephone serves an important societal need. The hearing aid also serves an important societal need, as does the linking together of telephones and hearing aids. It is thus not surprising that as these two technologies developed, each technology would influence the other's development. This interdependence could hardly have been foreseen when the first hearing aids were developed as stepchildren of the burgeoning telephone industry.
Preparation of this paper was supported by Grant H133G050228 from the National Institute on Disability and Rehabilitation Research (NIDRR). The opinions expressed in this paper are those of the author and do not necessarily reflect those of the Department of Education.
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