Guidelines for Buildings and Facilities (ADAAG)
Assembly Area. A room or space accommodating a group of individuals for
recreational, political, social, civic, or amusement purposes, or the
consumption of food and drink.
4.1.3. Accessible Buildings:
New Construction. Accessible buildings and facilities shall meet the
following minimum requirements [...]
(19)* Assembly areas
(b) This paragraph applies to assembly areas where audible communications
are integral to the use of the space (e.g., concert and lecture halls,
playhouses and movie theaters, meetings rooms, etc.). Such assembly areas,
if (1) they accommodate at least 50 persons, or if they have
audio-amplification systems, and (2) they have fixed seating, shall have a
permanently installed assistive listening system complying with 4.33.
For other assembly areas, a permanently installed assistive listening
system, or an adequate number of electrical outlets or other supplementary
wiring necessary to support a portable assistive listening system shall be
provided. The minimum number of receivers to be provided shall be equal to
4 percent of the total number of seats, but in no case less than two.
Signage complying with applicable provisions of 4.30 shall be installed to
notify patrons of the availability of a listening system.
4.30.7* Symbols of Accessibility.
(4) Assistive Listening Systems.
In assembly areas where permanently installed assistive listening systems
are required by 4.1.3(19)(b), the availability of such systems shall be
identified with signage that includes the international symbol of access
for hearing loss (see Figure 1).
Figure 1: International Symbol of Access for Hearing Loss
4.33 Assembly Areas
4.33.6* Placement of Listening Systems.
If the listening system provided serves individual fixed seats, then such
seats shall be located within a 50 ft (15 m) viewing distance of the stage
or playing area and shall have a complete view of the stage or playing
4.33.7* Types of Listening Systems.
Assistive listening systems (ALS) are intended to augment standard public
address and audio systems by providing signals which can be received
directly by persons with special receivers or their own hearing aids and
which eliminate or filter background noise. The type of assistive
listening system appropriate for a particular application depends on the
characteristics of the setting, the nature of the program, and the
intended audience. Magnetic induction loops, infra-red and radio frequency
systems are types of listening systems which are appropriate for various
Figure 2: An Induction Loop (IL) System
Figure 3: An FM System
Figure 4: An Infrared (IR) System
DOJ Title II rule
DEPARTMENT OF JUSTICE
28 CFR PART 35
Nondiscrimination on the Basis of Disability in State and Local
Subpart A -- General
Auxiliary aids and services includes—
(1) Qualified interpreters, notetakers, transcription services, written
materials, telephone handset amplifiers, assistive listening devices,
assistive listening systems, telephones compatible with hearing aids,
closed caption decoders, open and closed captioning, telecommunications
devices for deaf persons (TDD’s), videotext displays, or other effective
methods of making aurally delivered materials available to individuals
with hearing impairments.
Subpart E – Communications
(a) A public entity shall take appropriate steps to ensure that
communications with applicants, participants, and members of the public
with disabilities are as effective as communications with others.
(b)(1) A public entity shall furnish appropriate auxiliary aids and
services where necessary to afford an individual with a disability an
equal opportunity to participate in, and enjoy the benefits, of a service,
program, or activity conducted by a public entity.
(2) In determining what type of auxiliary aids and service is necessary, a
public entity shall give primary consideration to the requests of the
individual with disabilities.
AND TITLE III:
PART 36 NONDISCRIMINATION ON THE BASIS OF DISABILITY BY PUBLIC
ACCOMMODATIONS AND IN COMMERCIAL FACILITIES
36.303 Auxiliary aids and services.
(a)General. A public accommodation shall take those steps that may be
necessary to ensure that no individual with a disability is excluded,
denied services, segregated or otherwise treated differently than other
individuals because of the absence of auxiliary aids and services, unless
the public accommodations can demonstrate taking those steps would
fundamentally alter the nature of the goods, services, facilities,
privileges, advantages or accommodations being offered or would result in
an undue burden, i.e., significant difficulty or expense.
(b) Examples. The term “auxiliary aids and services” includes – (1)
Qualified interpreters, notetakers, computer-aided transcription services,
written materials, telephone handset amplifiers, assistive listening
devices, assistive listening systems, telephones compatible with hearing
aids, closed caption decoders, open and closed captioning,
telecommunications devices for deaf persons (TDD’s), videotext displays,
or other effective methods of making aurally delivered materials available
to individuals with hearing impairments;
(c) Effective communication. A public accommodation shall furnish
appropriate auxiliary aids and services where necessary to ensure
effective communication with individuals with disabilities.
This technical assistance
is intended solely as informal guidance; it is not a determination of the
legal rights or responsibilities of entities subject to the ADA.
landmark Americans with Disabilities Act (ADA), enacted on July 26, 1990,
provides comprehensive civil rights protections to individuals with
disabilities in the areas of employment (title I), State and local
government services (title II), public accommodations and commercial
facilities (title III), and telecommunications (title IV). Both the
Department of Justice and the Department of Transportation, in adopting
standards for new construction and alterations of places of public
accommodation and commercial facilities covered by title III and public
transportation facilities covered by title II of the ADA, have issued
implementing rules that incorporate the Americans with Disabilities Act
Accessibility Guidelines (ADAAG), developed by the Access Board.
U N I T E D S T A T E S A C C E S S B O A R D
A FEDERAL AGENCY COMMITTED TO ACCESSIBLE DESIGN
ASSISTIVE LISTENING SYSTEMS
BULLETIN 9B: FOR INSTALLERS
How do assistive listening systems (ALS) interface with public address
systems are, in fact, assistive listening systems. They are designed to
help people hear across a distance. In a PA system, the sound is picked
up at a microphone and delivered through speakers that bring the amplified
sound closer to listeners in an audience. For many people and in many
circumstances, a loudspeaker may provide enough gain in volume for
adequate speech perception. But no matter how well-placed the
loudspeaker(s), perception of air-borne speech signals will be difficult
for people with hearing loss unless background noise and reverberation can
be carefully controlled. The same microphone pick-up used in the PA
system can also serve an assistive listening system (ALS or ALD). By
bridging the acoustical space between the source and the listener, an ALS
circumvents the effects of distance (drop in volume), background noise
(competing sound), and reverberation (reflecting sound that blurs the
sound signals delivered by the ALS do not travel through acoustical space
before arriving at listeners’ ears. Thus, they are not weakened by
distance or degraded by noise and reverberation during the transmission
process. Instead, signals are transmitted via electromagnetic, radio, or
light waves to specialized receivers used by listeners. An ALS eliminates
the last acoustical leg of the signal transmission path, providing
listeners with hearing impairments with a parallel transmission path that
short-cuts the usual delivery process.
What are the benefits of bridging acoustical space?
necessary here to emphasize the distinction between audibility and
comprehension. Certainly, the signals delivered by loudspeakers can ensure
audibility for almost everyone. For a hearing-impaired person, however,
loudness is just one part of the listening equation. Of course, the signal
must be audible to listeners with and without hearing aids in order to
understand it. But, for most people with hearing loss, the comprehension
of verbal messages takes more than audibility. Comprehension also depends
upon the nature of the hearing loss and on the acoustical properties of
the space. In the most common type of problem, particularly affecting
older persons, hearing acuity is poorer at the higher frequencies than the
lower. However, the acoustical characteristics of speech that allow
listeners to distinguish between speech sounds occur largely in the higher
frequencies. Thus, the common complaint of people with hearing loss, “I
can hear but I don’t understand”: they can “hear” the low frequency
components of speech signals, and thus know someone is talking, but cannot
“understand” because the higher frequencies that carry the sounds
necessary for differentiation between letters and sounds are being
filtered out by their hearing loss. Increasing loudness, by itself, will
not measurably improve this situation.
addition to the filtering impact of the hearing loss, the nature of many
hearing problems is that the analytic powers of the cochlea are also
compromised. Thus, people may have difficulty resolving the separate
components of complex acoustic signals (as in a piano chord) or
discriminating fine temporal differences within speech sounds. For
example, the distinction between such voiced and voiceless sounds as /p/
and /b/ or /t/ and /d/ depend as much on detecting timing differences as
it does upon hearing the voiced components. Beyond a pure sensitivity
loss, then, and depending upon the specific site and type of damage to the
cochlea and the higher auditory pathways, other psychoacoustic
abnormalities may co-exist with diminished hearing thresholds and
interfere with speech perception.
Additionally, these auditory pathologies interact with external acoustic
conditions in such a way as to produce a disproportionate effect upon
speech perception. In an optimal acoustical situation, that is, in quiet,
a normally hearing person can achieve speech perception scores of 96%
while a listener with a hearing impairment can obtain no more than an 84%
score. If a moderate degree of noise and/or reverberation is introduced
into the room, scores will drop: to 88% – a slight decline – for the
listener with normal hearing but to 40% – a precipitate and overwhelming
change – for the person with hearing loss.
Increasing the loudness of the signal will not remedy this situation
because greater signal volume also produces higher levels of background
noise and reverberation. What needs to be increased to improve speech
perception is the signal-to-noise (S/N) ratio. Studies show that
increasing signal volume relative to background noise and reverberation
can compensate to some degree for the disproportionate effect of noise and
reverberation on speech perception by people with hearing loss. By
delivering an amplified signal directly to the ear, signal volumes can be
increased even though noise levels in the room remain the same. In
effect, what we’re doing with an ALS is attempting to replicate a perfect
listening situation for the person with a hearing impairment, the one in
which (in the example above) a score of 84% was achieved. While we can’t
always provide the perfect signal, we can, through an ALS, significantly
improve its quality over that which would be received via the loudspeaker
ALSs do for people with hearing loss, then, is to permit them to function
to the limits of their residual hearing capacities. They do more than
this, however. Often people with hearing impairments are able, with a
great deal of effort, and by expending a great deal of energy, to
understand speech signals in large-area listening venues. They can get the
message, but in doing so they have to focus so intently on receiving the
message that they have difficulty attending to what is being said. Unlike
people with normal hearing, they can’t really relax and enjoy the
listening experience. ALSs can optimize the listening experience and
minimize the stress of concentration.
How many people in our society can benefit from an ALS?
regarding the number of people with hearing loss in our society vary
depending upon the source and the criterion used to define hearing loss.
Most sources give this number as between 24 and 28 million people, or
about 10 percent of the population. Hearing-impairment increases with age;
it is estimated that the majority of people over the age of 65 have some
degree of hearing loss. Additionally, noise-induced hearing loss is
increasing steadily. Due to the increased longevity and the aging of our
population, the total number (and the percentage) of people with hearing
loss is likely to be substantial in the future. Most of these people would
be able to benefit from an ALS. Some will benefit more than others, but
everyone can obtain some advantages from using an ALS, in comprehension
and in the effort they have to make in order to comprehend.
What are the statutory requirements for ALS for specific venues?
with Disabilities Act of 1990 (ADA) requires that buildings and facilities
be accessible to and usable by people with disabilities. This includes
communications access for people with hearing loss.
ADA Accessibility Guidelines (ADAAG), adopted as the ADA standards for
accessible design by the Department of Justice (DOJ) in 1991, require that
certain newly constructed and altered assembly facilities where audible
communications are integral to the use of the space, be designed and
constructed to include assistive listening systems (see sidebar). In
addition, DOJ regulations implementing title II (covering the public
sector) and title III (covering the private sector) of the ADA include
requirements for effective communication with people with disabilities
that may require the installation of fixed or portable ALSs in existing
assembly facilities (see sidebar).
ADA does not cover private clubs and entities that are operated and
controlled by religious organizations. However, many houses of worship
make ALSs available to their congregants, not as a matter of law but as a
service, and club facilities used by other organizations must support ALSs
required for meetings and performances.
What types of systems are available?
three types of large area ALSs:
Induction Loop (Figure 2)
In the first
type, the induction loop (IL) system, a loop of wire encircles the
listening area or is embedded in a mat placed under a rug. This loop of
wire is connected to the amplifier output of a public address (PA) system
instead of, or in addition to, the usual loudspeaker (input is through the
microphone serving the PA system). The IL system produces an
electromagnetic field around the wire that can be picked up by a telecoil
in a hearing aid. About 30% of hearing aids include T- coils, which also
facilitate telephone communication. When the electromagnetic field
emanating from the wire loop intersects these coils, it “induces” an
alternating electrical current in the coil. This electrical current is
then processed by the hearing aid in the same way a microphone processes
acoustical signals. The major advantage of IL systems is that listeners
whose hearing aids include T- coils always have an ALS receiver with them.
Facilities that provide an IL system must also provide telecoil receivers
for people who do not use hearing aids or who wear hearing aids that do
not have telecoils. These special receivers come in various shapes and
sizes, but all contain a wire coil to detect the electromagnetic field and
an amplifier to increase the signal level.
Disadvantages of IL systems can include spill-over of the magnetic field
into adjacent areas (both horizontally and vertically), susceptibility to
stray electromagnetic fields, variations in the electromagnetic field
within the loop, and issues related to the quality and physical
orientation of the telecoils. With a proper installation and appropriate
hearing aids, these problems can be minimized and often eliminated.
FM (Figure 3)
type is the FM system. An FM assistive listening system is simply a
variation on the commercial FM radio. The signals are broadcast by FM
transmitters and picked up by listeners using a receiver tuned to the
transmitting frequency. FM receivers must be made available by the
facilities that use FM-based ALSs. The FCC has reserved the non-commercial
72MHz to 75MHz and the 216 MHz to 217 MHz bands for auditory assistance
devices. The lower band is a non-exclusive band, which means that
interference from other users in the same frequencies may occur (such as
from emergency vehicles of various kinds). The effective range of the
lower FM band is a radius of about 300 to 500 feet, given the power limits
set by the FCC (80 millivolts per meter at 3 meters). The effective
transmitting range of the 216-217 MHz band is approximately twice that of
the lower band.
There are several disadvantages of FM systems. The first is that privacy
is not possible. The FM signals do not stay contained within the four
walls of the enclosure. If privacy is a consideration, then an FM system
is not appropriate for that facility. Different rooms can broadcast at
different frequencies to receivers tuned to those frequencies, making FM
systems useful in school classrooms and multiplex cinemas. But they
should not be used in courtrooms where confidentiality is an issue. The
second potential problem is the flip-side of the first: radio signals
originating outside of the facility can enter the facility and interfere
with reception. One cannot prevent occasional interference, as when some
emergency vehicle in the area transmits on the same frequency used in the
venue. However, persistent interference can usually be overcome by
selecting alternate frequencies within the permitted bands. On the up
side, it is relatively easy with an FM system to ensure adequate signal
strength at all seat locations, even in the largest venues.
Infrared (Figure 4)
The third type
of ALS is the infrared (IR) light system. In an IR system, audio signals
from any source are conveyed to listeners via infrared light waves (using
light-emitting diodes) invisible to the human eye. The light waves are
picked up by a photo detector diode contained within the optical bubble
found on every IR receiver. The receiver then extracts the audio
information from the IR signal and delivers an amplified version to the
ears of a listener. Ordinarily, strict line-of-sight is necessary between
an IR emitter and the transparent lens on the receiver, but this can be
modified in rooms with light-colored surfaces (the IR waves are reflected
off them) or by adding additional emitters. Since IR systems are light
waves, they exhibit the advantages and disadvantages of light waves. The
IR signals are contained within a room, thus ensuring privacy, and
adjacent rooms in a facility can use IR systems without fear of inter-room
interference. They are also not as subject to radio or electromagnetic
interference as are FM systems. However, outdoor use is problematic
because of the effect of sunlight (which contains a great deal of infrared
energy) and it is more difficult to cover the largest venues with IR
systems than with an FM system.
IR systems require a radio-frequency (RF) sub-carrier as an intervening
step between the audio and the light waves. That is, the audio signals
first modulate the RF sub-carrier, which in turn modulates the IR light
signals. Until now, 95 kHz has been the unofficial RF sub-carrier,
permitting a person to use the same IR receiver in different venues.
Compatibility between venues has always been a major advantage of IR
systems. The situation may now be changing because of the electromagnetic
interference at this frequency produced by newer, more energy efficient,
fluorescent lights. Because of this, some facilities are or may be
switching to different sub-carrier frequencies (250 kHz, 2.3 MHz) with
their IR systems. This will not be a problem for consumers as long as the
facility provides them with compatible IR receivers. However, switching
sub-carrier frequencies may affect those consumers who have, or desire to
purchase a personal IR receiver, since no commercially available IR
receivers are able to detect all the possible sub-carrier frequencies.
What principles govern the selection of ALSs for specific venues?
It is always a
good idea for the installer (or the equipment distributor) to consult with
the provider prior to the selection and installation of an ALS. Whenever
possible, a preliminary visit to the facility is advisable. At the least,
the installer should obtain a detailed description of the facility, its
operation, and its unique needs prior to decision-making. Below are just
some of the considerations that should be jointly considered:
privacy a major consideration? Is it necessary that the events taking
place within a facility not be accessible to people outside the
enclosure? If so, then an IR system must be employed. There are really
no other alternatives for facilities such as courtrooms, confidential
meeting venues, and even musical performance facilities where
“bootlegging” a recording may be attempted.
an outdoor facility (an amphitheater with lawn seating or a racetrack,
for example)? In this case, an FM system would be the appropriate
Is this a
large indoor facility, such as a concert hall or auditorium with
balconies, overhangs, and other nooks and crannies? While a skilled
person can satisfactorily install an IR system in such locations (and it
should not be ruled out), it is easier to ensure adequate signal
strength at all seat locations with an FM system. However, this choice
will be influenced by the possible presence of electromagnetic
interference from outside sources, requiring shielding, or the
possibility of interfering with nearby external users of radio systems,
such as hospitals and public safety organizations.
large number of simultaneous events going to be taking place in
adjoining facilities? While there are a sufficient number of potential
FM carrier frequencies available to ensure non-interference between
rooms, and thus an FM system is a possibility, it may then be necessary
to provide FM receivers that can be tuned to all the possible
frequencies or to match supplies of receivers to particular rooms. How
will the audience respond to the necessity to change frequencies? Will
it cause difficulties if someone in one room can “tune in” to events in
a different room? If these possibilities may turn into future problems,
then an IR system is advisable. IR systems can be installed in every
room in a facility with no interference between rooms. Furthermore, the
same IR receiver can be used in every room.
going to be necessary to use the same system alternately in a number of
different rooms (such as in a community center, switching from one
activity room to another)? Ordinarily, FM systems are somewhat more
flexible and can be used both indoors and out (as in a tour group).
However, some IR systems are also relatively easy to deploy, and
portable units will work well in the smaller activity rooms, though they
will not operate as effectively outdoors.
a possibility of radio interference within the auditory assistance FM
frequencies (72-75 MHz and 216-217 MHz bands)? This can be determined by
using a frequency scanner to determine the possibility of interference
prior to the installation. If the interference is likely to persist, and
this is not amenable to a change in carrier frequency (which most
problems would be) or shielding, then an IR system would be the best
Except for a few specialized locations (like schools for the deaf), IL
systems are rarely used in large listening venues. This is ironic, since
of all the ALSs, they are probably -- from the viewpoint of the facility
-- the simplest system to provide. The IL receiver is simply the telecoil
in the person’s own hearing aid, thus relieving the facility of the
necessity to supply and care for a large number of receivers. Specialized
IL receivers with telecoils are available for people who wear hearing aids
without telecoils (and for those with mild hearing loss who wear no
hearing aids at all).
What do I need to know about ALS performance?
recommended electroacoustic performance standards reflect the results of a
research project completed at the Lexington Center’s Rehabilitation
Engineering Research Center (RERC) in 1998 and 1999. In conducting this
study, the RERC researchers opted to focus on the last stages of the
transmission process, i.e., the signals actually being delivered to the
earphones through ALSs. By comparing the input to the output, all the
factors that ALSs can impose upon an audio signal would be subsumed. The
complete project, as well as the background state of the art paper can be
accessed on the Lexington Center website at
primary metric used to define the quality of the output signal was the
Speech Transmission Index (STI). Basically, the STI compares the integrity
of an audio signal at two different points in the transmission process,
e.g., at a talker’s lips or loudspeaker compared to the same signal being
delivered through the earphones. It does this by measuring the fill
between adjacent peaks in a simulated or actual speech envelope. This fill
represents the addition of noise and reverberation to the primary signal.
The more the fill, the lower the STI. An STI of 1.0 represents the source
signal; anything less reflects the amount of noise and reverberation added
to the primary signal.
Measuring the STI
developed a simplified software version of the basic STI calculation.
This software, available on-line from the Lexington RERC, can be used to
measure the STI with input from either a live microphone or an audio
track. With live microphones, a test loudspeaker is placed at the location
of the talker. The output signal from the STI measurement system is
broadcast from the test loudspeaker, picked up by the microphone, and
passed through the ALS for measurement. When an audio track is the source
(such as in a movie), the line output of the STI measurement system
replaces the audio track. In either case, the output of the ALS is
monitored either by means of a line output from the ALS, or via earphones
through a coupler of some kind (e.g., Zwislocki coupler). The output from
the ALS is connected to the STI measurement system through the line input
port of the computer’s sound card. The person testing the system can then
run the software and in about three minutes the STI measurement is
complete. It is important to note that the recommended STI is applicable
in any type of listening situation. It can be employed in the absence of a
sound system simply by comparing the signal at a talker’s mouth (the
input) to that picked up at any point in an enclosure (the output).
Obtaining the software
information about the Speech Transmission Index Software, including system
requirements and equipment setup, may be found at the following website:
www.hearingresearch.org/STIinfo.htm. The software itself may be
hearingresearch.org/stidownload.htm or ordered on CD-ROM from
apply in all seating locations in every venue employing an ALS.
STI of .84 is recommended with any ALS in any large-area listening
situation. All facilities should strive to exceed this minimal figure,
in this and in all of the other electroacoustic recommendations given
signal-to-noise ratio of 18 dB is recommended. This result is also based
on the results obtained in the listening project conducted with
hearing-impaired listeners. When the system is being used with
hearing-impaired children, it is recommended that the S/N be increased
to 25 dB, to reflect the fact that children in the process of learning
auditory language require greater signal saliency than adults who use
the sound to recognize a previously learned language system.
system must be capable of providing at least 110 SPL output, but not
exceed an output of 118 dB SPL. The assumption here is that people who
require greater signal levels would be employing some kind of external
coupling to personal hearing aids (acoustical, inductive, or direct
Are all the possible transmission variables subsumed under these
measuring the signal at the last stage in the transmission process, i.e.,
the headphones, all the variables and factors that can reduce signal
quality would be included. For signals originating from microphones, this
would include the microphone position relative to the talker, the type of
microphone, and room acoustics as well as any signal degradation occurring
between the source and the headphones. When any of the electroacoustic
targets are not met, the installer must troubleshoot the system, locate
the source of the problem, and correct the situation. It is impossible to
predict and anticipate all the factors that may affect signal quality, and
there is no desire to dictate the details of an ALS installation to the
professionals who install them. By focusing only upon the electroacoustic
targets, installers would then be able to utilize their own skills and
creativity in ensuring that these criteria are being met. In some
instances, because of a clearly inadequate system, it may be impossible to
achieve the desired targets. In such a situation, it would be incumbent
upon the installer to recommend a higher quality ALS, one with which the
electroacoustic targets can be met.
What types of pre-processing strategies are desirable?
This is a very
difficult question to answer specifically. A number of the participants in
the RERC’s focus groups suggested that the transmitted signal be as
transparent as possible, with any signal processing strategies
accomplished at the personal receiver level. However, other participants
pointed out that certain pre-processing strategies may be inevitable,
given the need to compensate for wide input signal variations and to
provide additional high frequency emphasis when transmitting speech
signals. This issue was left unresolved. The danger of being too specific
in recommending certain pre-processing strategies is that it may inhibit
future product development by manufacturers who trying to improve their
products. As long as the electroacoustic targets are met (or exceeded),
installers can exercise a wide range of pre-processing strategies to
obtain the desired targets.
What types of receivers and coupling arrangements are available?
frequency FM receivers are about the size of a pack of cigarettes and feed
either headsets or earbuds. All include on/off switches and volume
controls. The receivers may be worn hung around the neck, clipped to a
belt or placed in a pocket. People who use hearing aids may prefer to use
a neckloop instead of headphones or earbuds. A neckloop operates on the
same principle as the large-area IL system; it fits around the neck rather
than around a room. It is plugged into the receiver earphone jack and
transmits an electromagnetic field to a hearing aid telecoil. For the
individuals involved, inductive coupling is a convenient way to use an ALS
receiver, since it enables them to continue to use their personal hearing
aids. However, only about 30% of modern hearing aids include telecoils,
mainly because of size restrictions (they won’t fit into the smallest
hearing aids). Other hearing-aid (behind-the-ear type) or cochlear implant
users may prefer to directly connect the ALS receiver to their personal
listening devices through a wire cord. In these instances, users would
ordinarily supply their own patch cords.
body pack receivers are similar to FM receivers and employ the same
coupling arrangements (headphones, earbuds, neckloops, patchcords). The
major difference is that every IR receiver has an optical bubble which
collects the IR light wave for processing by a photo-optical circuit. IR
receivers are also available in forms not available with FM receivers,
such as under-the-chin stethoscope units and self-contained headphones.
Stethoscope units place the electronics, volume control and optical bubble
in a single unit that dangles from a user’s ears. Some of these units also
include an output jack for insertion of a neckloop.
Is it possible or desirable to mix and match transmitters from one company
to the receivers of another?
Yes, it is
possible, as long as both transmitter and receiver function at the same
frequencies (FM or IR). Generally, however, it is not desirable. Even
though the same frequencies are being used, there are issues of receiver
sensitivity and bandwidth that can affect the quality of the received
signal. It is advisable, therefore, to use receivers and transmitters from
the same company to preclude possible problems. On the other hand, many
consumers have purchased IR receivers that use the 95 kHz sub-carrier and
find themselves able to use the same receiver in a number of different
venues that employ this frequency (which up to now has been a kind of de
facto universal standard).
How many receivers should be made available?
The number of
receivers should be equal to at least four percent of the total number of
seats available, with a minimum of two in any facility with fixed seating
of 50 or more.
What possible problems should I be aware of?
There are two
types of problems to be considered, one type at the time of installation
and the other that may occur later. The presence of interfering radio
signals, in the event of an FM installation, or interference from
fluorescent ballasts with an IR installation, are examples of the first
type of problem apparent at the time the ALS is being installed. In some
locations, it may be desirable to monitor the presence of potential radio
interference for some period of time, and to do this throughout the
facility, before making an FM installation. A frequency scanner would be a
useful device to employ. These problems can be managed before the system
is put into operation. When extraneous radio signals of particular
frequencies are found to occur often, the installer has the option of
shifting to another radio frequency.
second type of problem is one that often bedevils installers (as well as
providers and consumers). Some period after making what they believed to
be an excellent installation, they may get calls from an irate provider
who reports that their patrons are complaining of poor or non-existent
reception. After visiting the facility and troubleshooting the problem,
the installer may find:
emitters moved from their previous location because some maintenance
person or stage designer felt that they were intrusive in a particular
curtains, or some other fixture placed between the audience and an
emitter and thus “shadowing” some of the IR light waves.
Transmitter system settings modified from the original ones (e.g., VU
meter readings below optimum level).
electromagnetic interference from an adjacent facility, or from within
the facility, not present during original installation.
Maintenance problems with the receivers (dead batteries, broken cords,
poor connectors, etc.).
change in microphone usage (i.e., number, location, type) which severely
affects the quality of the signals reaching the microphones.
solutions in these examples are obvious. Facility personnel must
understand that the ALS installation is a permanent addition to the venue,
not to be tampered with for any reason, and how it works with the basic
sound system. The quality of an ALS installation can be compromised at any
point in the transmission path by what may appear to be a simple
adjustment or change of some kind. When problems occur subsequent to the
original installation and the problems rectified, the technician should
retest the system to ensure that the electroacoustic targets are once
again being met.
Rehabilitation Engineering Research Center on Hearing Enhancement, website
has a great deal of useful information on assistive listening systems.
Other resources include the technical assistance center at Gallaudet
and the Access Board,
The Access Board also
provides a toll-free technical assistance number at (800) 872-2253 (voice)
or (800) 993-2822 (TTY).
N I T E D S T A T E S A C C E S S B O A R D
1331 F Street, N.W. Suite 1000 Washington, DC 20004-1111
800 872-2253 (v)
800 993-2822 (TTY)
fax: 202 272-0081