Assistive Listening Systems (ALS)
Assistive listening systems pick up sound at or close to its source and deliver it to the listener's ear. This more direct transmission improves sound quality by reducing the effects of background noise and reverberation and, as needed, increasing the volume. These devices serve people who are hard of hearing, including those who use hearing aids. Personal systems are available for use in a variety of situations, including individual conversations. ADAAG covers those systems used with public address systems in certain assembly occupancies.
How ALSs Operate
Like public address systems, ALSs convert live or recorded sound into an electromagnetic signal. A sound source (usually a microphone) sends the signal to a transmitter which relays it to receivers provided to listeners. Couplers, such as ear phones, decode the signal from the receiver and convert it back into sound for the user. Recommendation: While a "complete" system with the necessary components can be bought off-the-shelf, ALSs designed as "mix and match" systems can provide a range of components appropriate for individual users, events, and environments.
Types of Systems
Assistive listening systems are generally categorized by their mode of transmission. There are hard-wired systems and three types of wireless systems: induction loop, infrared, and Fh4 radio transmission. Each has different advantages and disadvantages that can help determine which system is best for a given application. For example, an FM system may be better than an infrared system in some open-air assemblies since infrared signals can be overpowered by the sun; on the other hand an infrared system is typically a better choice than an FM system where confidential transmission is important. Recommendations: In selecting an ALS, consultation with professionals such as sound system consultants or contractors is often a good idea. No matter which technology is used, consider too that the quality of the system often determines the quality of the sound and transmission.
Minimum Requirement
ADAAG requires permanently installed ALSs in those assembly areas where audible communication is integral to the use of a space (movie theaters, concert and lecture halls, playhouses, meeting rooms, etc.); where fixed seating is provided and where there may be an audio-amplification system. For other assembly areas, such as those without fixed seating, ADAAG requires either a permanently installed system or electrical outlets and supplementary wiring for a portable system; this requirement, however, does not necessarily require the addition of electrical outlets.
Assembly Areas
(where audible communication is integral to use of the space)
Public Address Systems and Microphones
ALSs can be integrated with public address systems or operated separately. Recommendations: Both systems can share sound sources (microphones, mixers, tape players, VCRs) but not all microphones of a public address system work well with ALSs. Also, it is highly recommended that the ALS signal processor and amplifier be separate from the public address system unit in order to maintain independent equalization and volume control and to prevent distortion and feedback. No matter what system is used, selection of microphones is critical. For achieving the best sound quality, microphones (including wireless and lavaliere) should:
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limit background noise (noise-suppressing directional types, such as those with cardioid or hyper-cardioid patterns are best)
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provide the highest gain signal (consider battery-powered or "phantom" powered condenser microphones)
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accommodate speech over all frequencies (use wide frequency response microphones and avoid foam wind screens since they can filter out high frequencies)
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be used with an automatic mixer (which activates microphones as they are spoken into) where multiple microphones are used
For best results, participants should be sure to use microphones and speak one at a time, talk close to microphones "across" the top (which improves transmission of consonant sounds), and use their regular volume level, allowing the system to do the work.
Receivers and Couplers
ADAAG requires that receivers be provided to serve at least 4% of the total number of seats (but in no case fewer than two). Couplers connect the ALS receiver to the listener's ear. Recommendations: While a system may come with only one type of receiver, a variety helps meet the needs and preferences of different users. People often choose to wear their hearing aids while using assistive listening systems. It is important that couplers or receivers be compatible with hearing aids. Neckloops and headsets that can be worn as neckloops will be compatible. Receivers that arc not compatible include earbuds, which may require removal of hearing aids, silhouette transducers, earphones, and headsets that must be worn over the ear, which can create disruptive interference in the transmission.
Signage
Maintenance
By far, the most frequent complaint about assistive listening systems is that they do not work properly and that no one at a facility knows how to operate or fix them. While this is not addressed by ADAAG, the Department of Justice's title III rule requires that accessible features and equipment be maintained in operable condition.
Audio Induction Loop
This system uses a wire loop to receive input from a public address system or microphone and transmits sound by creating a magnetic field within the loop. Listeners must be sitting within the loop and have either a receiver or a hearing aid with a telecoil. Receivers can be connected to a variety of coupler types and are compatible with all loop systems. Loop systems are readily integrated with conventional sound systems and relatively easy to maintain. The loop may surround all or part of a room and can be permanently installed in the ceiling, floor or walls of a room. Portable systems are also available.
Since the signal created is, in a sense, electromagnetic interference, multiple loop systems can easily interfere with each other and are susceptible to interference from electrical wiring, computer monitors, transformers, and unshielded fluorescent lighting. In addition, the loop can also cause hum in electrical devices not shielded from electromagnetic fields, like a nearby cassette tape player. Also, construction steel in a building may cause erratic coverage (steel can absorb the magnetic fields, causing fluctuating reception). There is a new technology known as a 3-D mat that can be used instead of the conventional induction loop. There is very little spillover with 3-D mats and they are less susceptible to electromagnetic interference.
Consistent and uniform coverage is sometimes difficult with wire loops (although not with 3-D mats), depending on the application and the relative position of the listener. A listener's seating position and shifting can influence reception, making it better or worse. Moving the receiver position just a few inches can sometimes correct the reception.
Infrared Systems
These systems use invisible (infrared) light beams to carry information from a transmitter connected to a public address system or microphone to special portable receivers worn by the listener. The receiver is connected by wire to any of a variety of couplers or directly to a hearing aid equipped with direct audio input. A receiver and a coupler must be supplied to each listener, including those using hearing aids.
Infrared light, like visible light, does not penetrate most construction materials but can reflect off many materials and is broad beamed. An uninterrupted line of sight is required between the transmitter and the receiver. The configuration of rooms and architectural elements such as columns or decorative pillars are important factors in selecting and designing an infrared system. The overall strength of the incident light on the receiver will determine whether or not a usable signal can be received. Emitters must be installed high up on the walls and more emitters may be added and located as necessary to insure proper signal strength. Since the signal is overpowered by the sun, infrared systems may not be suitable in some outdoor areas that are unshaded, uncovered, or exposed to much sunlight. Incandescent and fluorescent lighting can also produce interference. One reported problem, not well documented, is possible interference from other devices using signals, such as audiovisual controllers for slide or VCR presentations.
An infrared system is the only wireless system that provides confidential transmission since the signal cannot go through non-transparent materials. There is no spillover to nearby systems and all transmitters and receivers in a facility can be tuned alike, an important consideration where multiple systems are provided in a facility. A person can walk from room to room, as in a conference, and not need a new receiver, or need to adjust a receiver for each room. Multiple channel systems are available and can be used to support descriptive video for people who are blind or simultaneous translation in multiple languages. Two channels are typically available by modulating the wavelength. (ALSs broadcast an infrared signal at a wavelength of 950 nm.) Infrared systems are reported to have somewhat better frequency response than FM systems.
FM Systems
FM systems use a short-range radio transmitter which converts sound to a radio signal sent to small FM radios tuned to the same channel usually worn around the neck. The radio receiver can be connected to a choice of couplers. FM systems are highly portable and flexible, subject to requirements for frequency control, and provide an alternative for facilities with multiple venues which can provide the requisite support. An FM system is simple to install and easily integrated with existing sound systems. There are few constraints on transmitter location since FM broadcasts are not constrained to the line of sight like infrared systems. Signal range is usually determined by the size of the antenna and the power of the transmitter but in most cases only one transmitter is needed (typical ALS range is roughly 300 feet). This, plus the fact that sunlight does not interfere with the signal, make it the best choice for large outdoor assemblies such as open-air arenas.
Each listener using the system must have an FM receiver but can move around more freely than with other wireless systems (loop, infrared). A choice must be made between providing fixed channel receivers (usable only with a compatible transmitter) and tunable receivers (more flexible, but also more expensive). Some people with hearing impairments have difficulty tuning a receiver, especially when multiple programs are being conducted.
The fact that FM signals can spillover into adjacent rooms or buildings can present problems. If confidentiality is required then an FM system is likely not the best choice. Use of an FM broadcast might also raise copyright issues for certain performances since anyone with a properly tuned FM receiver can intercept the broadcast. Some FM systems may experience signal drift (depending on the quality of the system), radio wave interference (depending on the environment), and interference from other FM devices using adjacent channels, such as pagers. Frequency management is an important consideration. However, most institutions that support multiple broadcasts are likely to have an audiovisual support staff which can assume responsibilities for this important task. The use of tunable receivers simplifies the problem of frequency control, but puts the responsibility on the listener. Implementation plans should include instructions and tuning assistance for users available where receivers and couplers are issued.
Multiple frequencies are possible and can support various programs such as audio description for people who are blind and simultaneous translation in multiple languages. However, a separate transmitter is needed when simultaneous programs are conducted within a facility, or even in nearby facilities.
Three formats are available within the frequency range (72 - 76 mHz) for ALS applications:
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40 channels for the educational band (used in educational settings, this allows many simultaneous classes or events)
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10 channels for wide band transmissions (popular in single channel applications)
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17 channels for narrow band transmissions (useful where multiple FM systems are used in close proximity, such as a theater complex)
Hardwired Systems
Hardwired systems incorporate a direct wire link between the sound source and the coupler. Although the quality of the transmitted sound may be good, the fact that individual seats must be connected is so limiting that hard-wired systems are rarely used any more.
| INDUCTION LOOP | INFRARED | FM | |
| Sources of Signal Interference | wiring, computers, transformers, florescent lighting (less with 3-D mat); construction steel can cause erratic coverage | sunlight, incandescent, and florescent light, other IR devices | other FM transmitters |
| Spillover | yes (wire loop) or very little (3-D mat) | none through non-transparent materials | yes |
| Confidentiality of Transmission | low | high | low |
| Sound Quality | can be uneven (better with 3-D mats) | good | good but signal drift possible |
| User Convenience | listeners restricted to loop (or mat); may need to adjust seating position to improve reception (less with mat) | line of sight required between emitter and receivers | listeners have flexibility to move around; tunable receivers require adjustment by listener |
| Receivers | people with telecoil hearing aids need no receivers; receivers are compatible with all loop systems | can be tuned to the same channel or frequency for an entire building; compatible with most infrared emitters; broadest frequency response of wireless systems | each event must use a frequency pre-tuned or adjusted by the user |
| Maintenance | low | low | high |
| Integration with Public Address System | possible | easy | easy |
| Installation | can be complex in large areas (installation of wire loop) | can be complex in large areas (installation of emitters) | simple |
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