Electromagnetic (EM) fields can be a serious source of interference for hearing aids being operated in the telecoil (T-coil) setting to use telephones and assistive listening devices (ALDs). Interference can be experienced as a hum, buzz, crackle, or feedback whine through a hearing aid, competing with and often entirely masking the sound source one is trying to hear. In extreme cases, the noise may be loud enough to damage hearing.
Telecoils work by picking up the changes in an electromagnetic field generated by a hearing-aid compatible (HAC) telephone or an inductive loop in an assistive listening system and transforming it into an acoustic signal that is then amplified by the hearing aid. By their nature and function, telecoils are thus sensitive to interference from other electromagnetic fields.
EM fields are extremely common in our modern world. They are used to transmit radio, television, emergency, and cellular telephone signals, including global positioning systems. Electromagnetic fields are also the by-product of many electrical devices and systems, such as fluorescent lighting, electric motors, and electric power lines. Many modern digital devices, such as computers, electronic game systems, and microprocessor-controlled equipment, have switching rates whose frequencies can produce interference (EMI). Digital wireless telephones, wireless local area networks and other data transmission devices that operate at high frequencies can also cause interference problems for listeners whose hearing aids contain telecoils. There are also naturally occurring sources of electromagnetic fields, such as those created by lightning and, perhaps most commonly, electrostatic discharges.
Electromagnetic fields are but one component of the environment in which hearing aids and assistive listening devices will be used. Just as equipment must be designed to operate correctly over a reasonable range of temperature and humidity, it must also be designed to operate over a reasonable range of electromagnetic field environments. Increasingly, manufacturers are engineering hearing aids for higher levels of EMI immunity. In addition, individuals and advocates for people with hearing loss have been encouraging manufacturers of electrical devices to alter circuitry to minimize the inadvertent transmission of EM fields. Interference from digital cellular telephones–a particular problem–has recently been addressed in a new American National Standards Institute (ANSI) protocol for assessing hearing aid and telephone compatibility.
It is not possible to eliminate all EM interference in hearing aids or ALDs, but it is possible to reduce the interference to a level that is either not audible or barely audible so that it does not interfere with the use of the hearing aid or ALD. Experience has shown that the elements in the hearing aid most sensitive to interference are those at the front end of the instrument. For microphone mode, this is the microphone and input to the first amplifier. For telecoil mode, it is the telecoil and the input to the first amplifier. These elements are particularly important because interference received and demodulated in the front end components will be amplified by the entire amplification of the hearing aid or ALD.
Ultrasonic sound is being used increasingly for control signals, such as TV controls, security systems, and similar devices. Although the ultrasonic frequencies are not themselves audible, demodulated ultrasonic signals can be audible if picked up by an audio amplifier in the hearing aid containing one or more non-linear circuit components. Two ultrasonic frequencies are in common use at present, 25 kHz and 40 kHz. These signals can be measured using a wideband sound level meter (C-scale). To avoid interference with hearing aids, the level of the 25 kHz signal should not exceed 90 dB SPL and the level of the 40 kHz signal should not exceed 100 dB SPL.
In addition, interference on FM systems used as personal and public assistive listening systems has been reported from paging and some emergency communications systems. FM systems that operate in the 72-76 mHz bandwidth have been particularly vulnerable. A change in channel tuning can usually eliminate interference, but ALDs used in schoolroom settings may need all of the available channels to serve the needs of several children who may change classrooms during a school day. The Federal Communications Commission has designated an additional bandwidth for ALD use at 216-217 mHz.
Some interference comes from elements of the building environment, particularly from lighting and from the electrical distribution system and equipment connected to it. If interference is reported by telecoil users, an induction loop field strength meter can be used to locate room EMI sources. Ballasts in fluorescent lighting are frequent offenders, particularly newer energy-conserving types or others close to the end of their useful life. Replacing them will often reduce interference to marginal levels. If a television and/or VCR proves the problem, relocation of the equipment will often provide marked improvement. Computers and their monitors may also be a troublesome source; different equipment may provide a correction. Flat-screen liquid crystal displays (LCDs) should produce less EMI than cathode ray tube (CRT) monitors. Interference of this sort usually waxes and wanes relative to the orientation of the telecoil or antenna in the hearing aid and so will respond to equipment relocation. It may be easier to relocate an office or pay telephone to an area less subject to EMI than to reduce the EMI. Small reductions from a variety of sources can have a markedly beneficial effect.
More significant problems need to be investigated by engineers with electro-acoustic expertise who can isolate and diagnose the most troublesome frequencies, which may arise from interior or exterior sources, and recommend corrective strategies. IEEE Standard 473 – 1985 describes the recommended practice for electromagnetic site surveys. Because the level of the EM interference can vary over time, measuring instruments that record peak levels over extended periods of time should be used.
In new construction, intrusive EMI can be avoided by careful design and construction to isolate or shield areas where telephones and assistive listening devices will be used–offices, pay telephone banks, auditoria and meeting rooms, classrooms and testing facilities. Incoming electrical service, electrical rooms, and transformers should also be isolated and shielded and pipe entries grounded. Looped piping systems designed for multi-building sites and campuses may require non-conductive isolators to avoid inadvertent current transmission.
The degree of interference produced in a hearing aid, assistive listening device, or other electronic device depends on a range of independent factors. These are:
1. THE INTENSITY OF THE EM FIELD.
2. THE FREQUENCY OF THE EM FIELD. Two frequency ranges are of particular concern for hearing-aid interference, the audio frequencies, between 20 Hz – 20 kHz (often referred to as baseband frequencies), and the ultra high frequencies commonly used for transmission in the megahertz (MHz) and gigahertz (GHz) region. EM fields at these very high frequencies can generate significant voltages in even very small electrical components, such as those used in a hearing aid. The higher the frequency, the smaller the wavelength of the transmission.
3. THE MODULATION OF THE EM FIELD. Hearing aids and ALDs are designed to process signals in the audio range. Therefore, currents and voltages induced by audio band EM fields add directly to the intended signal, interfering with them.
4. THE DISTANCE FROM THE SOURCE OF THE EM SIGNAL. The EM interference in an audio amplifier will decrease as the amplifier is moved away from the EM source.
A telephone that is hearing-aid compatible will generate a baseband magnetic field that can be picked up efficiently by the telecoil of a hearing aid or ALD. This is the desired magnetic field. The telecoil is susceptible to unwanted interference in two frequency bands: direct interference in its baseband frequency and interference demodulated from higher frequencies. Only those hearing aids or ALDs used with a telecoil will be susceptible to both forms of interference. Demodulated high-frequency EM signals, however, can be picked up either by the telecoil or the hearing aid itself.
Modern hearing aids with a telecoil input are designed to produce an audio signal equivalent to 60 dB SPL at the input when placed in a magnetic field of 31.6 ma/meter (IEC standard 118-4). The interference picked up by the telecoil should be at least 30 dB below this audio signal. The interfering baseband magnetic field should thus be below 1 ma/meter. The interference produced by the demodulation of a high frequency EM signal is generated by both the electrical and magnetic components of this signal. The level of interference is dependent on both the strength of the EM field and the relative immunity of the hearing aid to EM interference.
It should be noted that the signal picked up by a telecoil is critically dependent on the position of the telecoil relative to the direction of the magnetic field. Inappropriate placement or orientation of the telecoil relative to this field will result in the desired audio signal being weaker while the background noise produced by an interfering magnetic field may be stronger.
Digital wireless/cellular technology introduced in the mid-1990s has introduced a new barrier to telephone use by people who wear hearing aids. These telephones have a high-frequency radio transmitter in the handset that generates powerful near-field interference when held against the ear. The buzzing sound of the interference sounds through the hearing aid, making speech understanding difficult or impossible. The ANSI C63.19 standard describes how to measure the EM field generated by a wireless communications device coupled to a hearing aid and also provides a set of tests and categories to use in evaluating hearing aid immunity to EMI so that cellphones and hearing aids may be appropriately matched. These categories were developed in order to specify the performance a person wearing a hearing aid may expect when using a wireless digital telephone.
Guidance on the application of various interference reduction techniques is readily available from a number of books and other sources. Measurement of hearing aids from a number of manufacturers has demonstrated that the levels of immunity recommended by ANSI C63.19 are quite achievable and in fact are available in a number of commercially available products today.