Are Assistive Listening Devices (ALDS)?
Assistive Listening Devices are amplification systems designed specifically
to help people hear better in a variety of difficult listening situations.
Even hearing aids can be
considered as assistive listening devices. Most of these devices can
be used with a personal hearing aid that has a telecoil (or t-switch)
or by themselves to:
· help overcome background noises
· minimize the negative impact and sound distortions of distance from
the sound source
· reduce room reverberations
The basic function of an ALD is to improve the "signal to noise ratio" for
the listener. This means that desired sounds (signals) are amplified,
and undesired sounds (noises) are minimized. The most common types
of ALDs used with children include Sound Field
Systems, FM Systems, Infrared
Systems and Loop Systems.
Field Systems <back to top>
Noises in and around the classroom are constantly competing with the teacher’s
voice. The shuffling of papers, the moving of a chair, the sound of cars whizzing
by from an open window, or other kids playing at recess all easily distract
the student’s focus from the teacher.
A sound field system is designed to give teachers the edge over poor
classroom acoustics and unwanted background noise. Through the use
of an FM transmitter and the portable speakers strategically positioned
in the classroom, the teacher’s voice is projected at a level
where students can hear comfortably without straining. This amplification
improves the signal-to-noise ratio so that the effects of echo, and
the distance between the teacher and students are reduced. Students
sitting in the very back of the class can hear and concentrate with
the same accuracy and clarity as those in the front row.
Unlike traditional FM systems, a sound field system doesn't require
listeners to wear receivers. Instead, speech is amplified 10dB to 12dB
above room noise through a single ceiling speaker or through speakers
placed around the room.
While students receive the academic advantages of an amplified classroom, teachers
can get every students attention day after day without ever having to raise
their voice. Students can listen and learn, and simultaneously, teachers can
reduce voice fatigue through amplification for a free field sound system. Studies
have shown that sound field systems not only help students with mild to moderate
hearing loss to learn better, but also those with auditory learning disabilities,
auditory attention deficits, and other auditory processing problems learn better
Systems <back to top>
FM systems transmit sound via radio waves. Basically,
the speaker uses a compact transmitter and microphone, while
the student uses either a portable receiver with headphone or
earplug. Those students whose hearing aids are equipped with
a telecoil can use a telecoil coupler such as a neckloop or an
individual ‘boot’. FM systems are ideal for classroom/meeting
use and work well both indoors and outdoors. If multiple FM systems
are used near each other, separate broadcast frequencies should
be used in order to prevent "spill over." FM signals
are not limited to line of sight and can penetrate walls and
ceilings. Personal FM systems are commonly utilized with children
who have bilateral hearing loss. Research has also supported
the use of personal FM systems with children who have unilateral
(or one-sided) hearing loss, fluctuating and/or conductive (middle
ear) hearing loss, and some types of learning disabilities (i.e.,
Attention Deficit Hyperactivity Disorder).
Large Area FM Systems are used for group listening in auditoriums,
theaters, places of worship, cinemas, etc. These systems have a transmitter
that connects to the existing sound system. The program is broadcast
throughout the listening area. Students use a personal receiver and
earphone or telecoil coupler to pick up the broadcast. This allows
the students to hear the program directly from the sound system and
bypasses background noise and distance from the sound source. These
products require specific installation and familiarity with sound system
equipment. A large area FM might be seen in a school auditorium.
Infrared Systems <back
Infrared systems transmit sounds by invisible light beams. To be effective,
the receiver must be within direct line of sight of the light beam from the
transmitter. There is added security in an infrared system because sound cannot "spill
over" to other rooms. Consequently, many multiplex theater facilities
are equipped with infrared systems. These systems cannot be used outside because
of interference from sunlight. Bright, incandescent light may also cause interference,
which can make classroom installation difficult. Infrared systems can be adapted
for use in auditoriums but require transmitters to cover the large area.
Loop Systems <back
Based on a principle of electronics called electromagnetics, these systems
are easily used by those having hearing aids equipped with a telecoil circuit.
The technology consists of a loop wire that is placed around a listening area.
The primary speaker uses a special amplifier and microphone. Speech signals
are amplified and circulated through the loop wire. Those wearing telecoil-equipped
hearing aids can pick up the resulting energy field and have it amplified by
the telecoil. Those who don't have telecoil-equipped hearing aids can use special
receivers with earphones to pick up the magnetic signal. It is easy to install,
and can be used in classrooms, small meeting rooms, and even in automobiles.