Assistive Listening Systems 154542.1 Nature of the Problem
There are millions of people in the world (20 million in
America alone) with hearing impairments for whom the
acoustical and electronic systems described elsewhere
in this book are inadequate. It is surprising that the
special hearing needs of so large a group have been
largely ignored for so long, especially when each of us
faces the very real probability of joining that group
through disease, trauma, or just by growing old.
According to the National Association of the Deaf,
NAD, assistive listening systems (ALSs), sometimes
called assistive listening devices (ALDs), are amplifiers
that bring sound directly into the ear. They improve the
speech-to-noise ratio by separating the sounds, particu-
larly speech, that a person wants to hear from back-
ground noise.
Research indicates that people who are hard of
hearing require a signal-to-noise ratio increase of about
15–25 dB in order to achieve the same level of under-
standing as people with normal hearing. An ALS allows
them to achieve this gain for themselves without
making it too loud for everyone else.
ALSs are used by people with various degrees of
hearing loss, from mild to profound, including hearing
aid users and those with cochlear implants, as well as
those who use neither. ALSs are sometimes described as
“binoculars for the ears” because they stretch hearing
aids and cochlear implants, thus extending their reach
and increasing their effectiveness.
ALSs address listening challenges by minimizing
background noise, reducing the effect of distance
between the sound source and person with hearing loss,
and overriding poor acoustics such as echo. ALSs are
used in places of entertainment, employment, education,
and home/personal use.
The hearing impaired are not just the people who
wear aids. In fact, only about 20% of the hearing
impaired wear aids. Many people with hearing losses
are able to function in close or face-to-face situations
but are lost in noisy or reverberant settings. Even people
who wear hearing aids have problems in reverberant
rooms or where there is a high background noise level.
Our standards for speech intelligibility are based on
listening tests with normal-hearing subjects and are not
directly applicable to the hearing impaired, see Chapters
2 and 40. Noise and reverberation degrade intelligibility
far more rapidly for the hearing-impaired individuals
whether they are fitted with hearing aids or not. Often
the very highly prized acoustical qualities of theaters
and concert halls operate against the needs of the
hearing impaired, and the acoustical design of most
classrooms and lecture halls are inadequate for the
hearing-impaired student.
For years the only special assistance offered to the
hearing impaired was headphones in a couple of pews
in the front of the church sanctuary. In recent years new
wireless technologies have been developed or adapted
to meet the special needs of the hearing impaired in
public assembly spaces. No longer is the user restricted
to a specially wired seat; now every seat is available; no
special ticketing is required. The user is free to sit with
family and friends.
Wide area ALSs are covered under Title III of the
ADA (Americans with Disabilities Act of 1990). This
title stipulates that ALSs be provided in public places
unless a provider can prove that it is an undue burden.
Examples of such venues include movie cinemas, live
performance theaters, and public classes. The ADA
specifies that ALS receivers be provided at no cost and
specifies the number of receivers that must be provided
depending on the number of seats (4% rule). Revised
ADA Guidelines to be released in the future are
expected to increase standards for performance of ALS
and address related issues.
ALSs may also be indicated under ADA Title I
(employment accommodations) as well as Title II
(accommodations provided by state and local govern-
ments). Other public policies that may require use of
ALSs include Section 504 of the Rehabilitation Act
(affecting federally funded agencies) and Individuals
with Disabilities Education Act.42.2 Types of Assistive Listening SystemsThere are four basic types of wireless systems: magnetic
induction, FM broadcast, AM broadcast, and infrared
light. Each type has its own set of advantages, prob-
lems, and limitations. There is no single best system for
every application; each system is simple to operate and
to install.
The system, no matter what type, must pick up the
program sound. In a fully mic’ed event, this pickup
could be a feed from the reinforcement control console.
Where the event is not mic’ed, there must be a special
microphone or microphones to feed the hearing-
impaired system. It is very important that the feed to the
system be of the highest quality possible with a
minimum of reverberation pickup and extraneous
noises. A pressure zone-type microphone, see Chapter
16, on the forestage floor or mounted on an acoustical
reflector panel over the forestage would be good for
many shows. An even better system, which would
reduce room effects, would be to individually or close-