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.
Sound Field Systems
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 as well.
FM Systems
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
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
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.
