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Centrum Sound Quick Referenceand Application Guide for Assistive Listening Systems |
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The Purpose of Assistive Listening Systems Background noise and reverberation, or simply distance to the sound source, degrade intelligibility much faster for hard of hearing people, whether they wear hearing aids or not. Thus, hard of hearing people are prevented from participating on equal terms with hearing people in larger assembly areas which are not equipped with assistive listening systems. Even the best in digital sound systems technology, combined with the best in digital hearing aid technology, cannot solve the intelligibility problems the hard of hearing people faced with. In recognition of this, requirements for making assistive listening systems and devices available in places of public accommodation were included in the Americans With Disabilities Act (ADA) signed into law in July of 1990. |
 The International symbol for hearing loss as adopted by the ADA |
| The purpose of an assistive listening system is to transmit sound as directly as possible to a transducer in the hard of hearing person's ear. The simplest way to achieve this is to use a small personal amplifier with a microphone and an extension cord. The microphone is simply placed near the sound source. The user wears a headset or neckloop and can adjust the loudness on the amplifiers volume control. However, hardwired systems like this are not very practical in places of public accommodations. Apart from personal body-worn systems, they practically all have been replaced by modern wireless systems. |
Wireless Technologies, How They Work, Advantages and Disadvantages. Today there are three basic wireless technologies available which provide different methods of transmission: Induction loop technology (teleloop), FM broadcast technology, and Infrared (IR) light technology (IR). No single technology is best for all applications. Each type has its own advantages, problems and limitations. All three types of assistive listening technologies can easily and successfully be used for personal and commercial applications alike, as long as their individual limitations are kept in mind. A SUMMARY OF ASSISTIVE LISTENING DEVICES AND SYSTEMS can also be found in the ADA Accessibility Guidelines (ADAAG) A.4.33.7 Types of Listening Systems - Table A2. Please note that the ADA chart was reprinted and modified from an earlier Centrum Sound publication. However, the basic content does not differ from the following.TELELOOP TECHNOLOGY (Induction loop). This technology is based on electromagnetic transmission and has the unique advantage that the signal is received directly by the user's hearing aid when it is equipped with a Tele-coil (telemagnetic pickup coil). It is most often referred to as a T-coil. A teleloop system consists of an amplifier and a discrete wire (the loop) that runs along the perimeter of the room. When the loop amplifier is hooked up to an audio source such as a TV or PA system, the sound is received wireless by the user's hearing aid without the need for an additional receiver as is required by all other technologies. Induction receivers are available for hearing impaired people without hearing aids or without the telecoil feature.All Oticon/Logia teleloop amplifiers distributed by Centrum Sound are of the constant-current type design. This technology requires only a single turn loop for most applications as it features automatic impedance matching. In addition all amplifiers feature a powerful automatic gain control (AGC) to insure independence of the speaker's voice level and or distance to the microphone.
FM BROADCAST TECHNOLOGY. In principle, FM system designed for hearing assistance application work just like commercial FM broadcast systems operating in the 88 to 108 MHz range. However, in the United States, the FM systems operate at FCC-designated frequency band in the rage from 72 to 76 MHz and from 216 to 217 MHz. The 72 to 76 MHz band is used for both personal and commercial applications. The 216 to 217 MHz band is intended for auditory assistance applications only. Since each system may use its own broadcast frequency, several systems can operate simultaneously at one location without interfering with one another. However, unlike the loop system, the FM system requires a special receiver for each person, whether s/he has a hearing aid or not. There are several listening options available for FM receivers. The most convenient for public facilities are Under-the-chin type stetoclip headsets or Walkman style headsets and neck loops (personal teleloop) for T-coil users.The 72 to 76 MHz band used for commercial applications can be split into 40 narrow-band channels or 10 wide-band channels. Narrow-band systems are less susceptible radio interference than wide-band systems but receivers are generally more expensive. Centrum Sound carry both narrow-band and wide-band systems. To alleviate potential interference problems with wide- band systems, all systems carried by Centrum Sound feature field tuning to any wide-band channel.
INFRARED LIGHT TECHNOLOGY (IR). Infrared light can be used for signal transmission in same fashion as with FM transmission. An infrared system consists of three basic components: a transmitter (base station), an emitter, and a receiver. The audio signal is conveyed onto a sub-carrier in the base station which in turn is converted into infrared light by the emitter. There are four sub-carrier frequencies used for hearing assistance systems: 95 kHz, 250 kHz, 2.3 MHz and 2.8 MHz. The receiver detects the IR signal and converts it back into the original audio signal. However, unlike FM transmission and induction loop technology, infrared light can not pass through walls. Therefore, infrared light transmission is ideal for facilities operating several systems simultaneously in different rooms in that all receivers can be identical with no need for frequency coordination.As with FM technology, each person must use a receiver, whether or not s/he has a hearing aid. Receiver types include lightweight under-the-chin style, over-the-head receivers for 360 degree reception and lapel style. The same listening options are available for the lapel style receivers as for the FM receivers.
APPLICATION GUIDELINES The cost effectiveness of an assistive listening system depends largely on the technology applied. Therefore, thoughtful considerations must be given to the daily operating requirements, simplicity, flexibility, and of course, acquisition costs. Long term issues such as maintenance, operating costs, and expandability are also factors in a decision. For the individual user, the degree of hearing loss and discrimination score is also important The greater the loss, the more the need for a device and a T-coil hearing aid. The following should be used only as a general guideline as to which technology may be most suitable (highlighted) or cost effective for a given application.
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