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Hearing Assistance |
Primer: Assistive Listening Systems
While
assistive listening systems (ALS) have become more common in recent years, they are hardly
a new concept. Certainly the technology has been greatly improved, but the basic idea of
providing electronic assistance to those with a hearing impairment is as old as the audio
industry itself. The success of a particular facility in meeting the needs of the hearing
impaired will, in large part, depend on selecting the correct assistive listening system
for the physical environment and intended usage.
The systems commonly in use today can be divided into two categories: hard-wired and wireless. The wireless variety can further be broken down into four types: Induction Loop, AM, FM and Infrared. Each type of system has its own advantages and disadvantages. The choice of a particular type of system must take into consideration these pros and cons as well as the performance and price requirements of the purchaser.
Hard-wired systems
The most basic, and one of the oldest, of the various systems used today
is the hard-wired type. These systems consist of headset jacks, placed throughout the
listening area, which are hard-wired back to an amplifier being fed from the main sound
system. A user then plugs in a headset at one of the jack locations. Volume controls are
usually provided at each jack so the user can adjust the level to suit his or her needs.
These systems have the advantage of requiring very little investment in equipment. The sound quality, while dependent on the headset used, is generally quite good and is immune to any outside interference.
On the down side, hard-wired systems typically require a great amount of labor time to install due to the need to have wiring run to each listening location. In some installations a separate amplifier may be needed to provide the necessary output power. But perhaps the biggest disadvantage is the fact that many people reject using this type of system because it forces them to listen from specific, predetermined locations. This often causes groups of people to be split up due to limited space in the "hard of hearing" areas.
Induction Loop Systems
These are the most basic of the wireless assistive listening systems. A
loop of wire is placed around the perimeter of the listening area. The loop is connected
to an amplifier which is fed from the sound system. The audio signal from the amplifier
creates a magnetic field around the loop which can be picked up by a special loop receiver
or a hearing aid with a telephone pick-up coil, or "T-Switch". A variety of
earpieces are available for use with the loop receivers.
Because it is one of the oldest, this type of system is well known and accepted by hearing impaired persons. A listener who has a hearing aid equipped with a T-switch can receive the loop signal without any additional equipment.
However, a relatively small percentage of the hearing impaired population have hearing aids which contain pick-up coils, and the number has been decreasing with the trend toward smaller, "hidden" hearing aids. In addition, the systems are costly to install due to the wiring requirements, are prone to interference from electrical sources such as motors and fluorescent lights, and may have limited range in large rooms.
AM Systems
AM systems were the next step in the development of wireless assistive
listening systems. In these systems the signal from the sound system is fed to an AM radio
transmitter. The transmitter is then connected to an antenna within the listening area.
The transmitters operate in the AM broadcast band which allows the use of inexpensive AM
pocket radios to receive the broadcast. For listeners with T-switch equipped hearing aids,
a neckloop cable can be used with the receiver. The neckloop allows the hearing aid to
pick up the signal in the same manner as with an induction loop system.
These systems provide the same kind of mobility as induction loop systems with the added benefit of improved sound quality. But the performance of AM systems are very dependent on building construction and interference sources. They are susceptible to noise from motors, fluorescent lighting, dimmers and electrical storms. Installation difficulty, and therefore cost, varies with antenna design and construction.
FM Systems
Since 1982 the FCC has allowed assistive listening system operation in what is
known as the FM auditory assistance band (72 to 76 MHz). The techniques are similar to
those used in the AM systems with one exception being the receivers used. Because of the
dedicated frequencies used, a special fixed-frequency receiver must be utilized.
Due to the many improvements in performance with FM systems, AM systems are now seldom used. In fact, most manufacturers have ceased production of AM models. The FM signal is not affected by typical noise sources and the special frequencies used reduce the possibility of radio interference. The audio quality is extremely good and the transmission range allows better coverage of large auditoriums. Many installations can use the antennas which are built-in to most transmitters, resulting in reduced labor costs.
The primary disadvantage is that any radio based system is open to the possibility of outside interference. In many areas this is limited to another FM ALS operating nearby due to the restricted frequencies used. In urban areas however, other radio services are allowed to use some frequencies within the auditory assistance band, and this can result in interference. Careful selection of the specific frequency used in a given area can minimize these problems.
Infrared Systems
The infrared systems are the newest assistive listening technology. These
systems use modulated infrared light to transmit the audio signal to the listeners. An
array of infrared light-emitting diodes (LED's), contained within a panel, produce the
transmission. Special receivers with infrared detectors pick up the transmission. A
modulator unit, either in a separate package or contained within the emitter panel,
converts the audio signal from the sound system into the frequency and amplitude modulated
voltages necessary to drive the emitter panel(s).
These systems perform well when adverse RF conditions exist. And because the signal is blocked by opaque objects, infrared systems are the best choice for secure areas or when large numbers of adjacent rooms must have independent systems (such as classrooms and multiplex theatres). But because the emitter panels are relatively inefficient, several may be required to cover the listening area. Each of these panels must be interconnected with cables. Also, if the signal needs to be available in an adjacent room, emitter panels must be installed there as well. These factors can result in higher installation costs.
Another problem with infrared systems is interference from newer, high efficiency fluorescent lighting systems (typically, those using electronic ballasts). These lighting systems produce a harmonic modulation which interferes with the infrared receiver's ability to "see" the desired signal. There are now special models of infrared systems which use a higher modulation frequency to avoid this problem. You should make sure you request one of these newer systems if your facility utilizes high efficiency fluorescent lighting.
The wide range of assistive listening systems available insures that practically every facility can afford to install one for the benefit of visitors. When deciding which system to install you must take into consideration architectural, environmental, performance and economic considerations. The above information is intended to provide a starting point for addressing these points. To make sure you choose the system that's right for your application, contact one of our professionals for assistance and guidance. We're happy to help.
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