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Winter 2003 Table of Contents
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Assistive Listening Devices

By Jim Durkel, Coordinator of Statewide Staff Development, TSBVI Outreach

Noise and distance make listening hard for anyone. The more background noise there is, the harder it is to hear. The farther away a speaker is, the harder it is to hear. For people with auditory impairments, noise and distance are additional problems on top of the impaired hearing.

Hearing aids don't amplify just speech, they amplify everything. This includes the background noise. Ideally, what you want to hear (called the signal) should be louder than background noise. This relationship is called the signal-to-noise ratio. A good signal-to-noise ratio is 20 dB. That is, the signal is 20 decibels louder than any noise. A more usual listening situation is a signal-to-noise ratio of 10 or 0. That is, the signal may be only 10 decibels louder than the noise or the signal and noise may be of equal loudness. In many classrooms, the signal-to-noise ratio can be 10 or 0. Rustling papers, children moving in their seats, the air conditioner running _ all of these normal background noises can overwhelm a teacher's voice.

In many classrooms, the teacher does not stay within 3 feet of the child. Sometimes the teacher may turn his or her back to the child (as when talking while writing on the blackboard). These actions decrease the intensity of the signal, making it harder for the child to understand what is being said.

A group of devices, called assistive listening devices (ALD), were designed to help a person with a hearing impairment better cope with the problems of noise and distance. An ALD works by having the speaker wear a microphone connected to a receiver, worn by the listener. The speaker's voice is then sent directly to the listener's ear. No matter where the speaker is in the room, his or her voice is as far from the listener as the microphone is from his or her lips. Background noise is not picked up by the microphone and so its effects are decreased.

ALD's vary in how the microphone and receiver are connected to each other. In schools, the most common type of ALD is the FM unit. The microphone broadcasts the speaker's voice on an FM radio channel. The listener's receiver is tuned to that special frequency and picks up the radio signal. Many movie theaters, large churches, and public auditoriums use an ALD that sends the speaker's voice to the receiver by way of infrared light (kind of like your TV remote control). Some very inexpensive units connect the microphone to the receiver with a wire (these are called "hardwired"). This type of device is fine when the speaker and listener are not going to be very far apart, such as when they are sitting at a table together. If you have used headphones to listen to a stereo, you have used a hardwired ALD. There are infrared and hardwired devices for use at home for watching TV or listening to music without driving everyone in the house nuts with fights over the volume control!

ALD's also vary in how the receiver is hooked up to the listener's ear. The receiver may be connected to a set of headphones. There may be a wire running from the receiver to a "button" that snaps into a special earmold. The receiver may be connected by a special wire plugged into the listener's hearing aid. This is called a "boot". Finally, the listener may use a "loop". In this set up, the hearing aid on/off switch is set to "T". "T" stands for telecoil. This is a setting for the hearing aid to use electromagnetic energy instead of sound energy coming through the air. The listener has a special loop of wire, connected to the ALD receiver, that he or she wears around his or her neck. The receiver transforms the sound energy into electromagnetic impulses that the hearing aid picks up. Sometimes, auditoriums have giant loops running around their walls. Anything a speaker says through the microphone on stage can then be picked up through a hearing aid without the listener needing to wear any other type of receiver. There are new behind-the-ear hearing aids with small FM receivers built into the hearing aid. With this device, the speaker wears a microphone and the signal is sent directly to the hearing aid. This is the wave of the future and may soon become the most common ALD. When an ALD can be connected to a listener's hearing aid, there is less fuss, more consistent hearing aid use, and the sound will be amplified in a way that is most appropriate for that listener.

There is research showing that all children, with and without auditory impairments, benefit from good signal-to-noise ratios. As a result, there is another type of ALD starting to be used in some classrooms. With this device, the teacher wears a microphone. However, his or her voice is sent through speakers located around the classroom. This works like a small PA system. Now, the teacher's voice is louder than the classroom noise and his or her voice stays constantly loud, regardless of how far away the teacher is from the child.

Children with central auditory processing disorders and children with hearing loss only in one ear may have more troubles with noise and distance than listeners with no impairments. These people may not wear hearing aids or need amplification but may benefit from the use of an ALD.

There are some problems with ALD's. If they are set so that all background noise is blocked and only sound from the microphone is received, comments from fellow classmates may not be heard by the ALD user. The teacher needs to be sensitive to this and either repeat all comments made by classmates, or pass the microphone to the classmate before allowing him or her to speak. This can be cumbersome. ALD's can be set to allow some background sound in. This allows for hearing comments from speakers without a microphone, but decreases the effectiveness of the ALD in providing the best signal-to-noise ratio. Many adolescents do not like to wear ALD's because they make them stand out from their peers. Some teachers feel uncomfortable with wearing a microphone. Some ALD's require a belt to hook the microphone on, limiting a teacher's wardrobe. Some teachers complain that the cords get in the way of movement. The microphone may fall in a water fountain and cords get tangled around hands while trying to cook. Wearing a microphone takes some getting used to and once you forget you are wearing it is right when you leave it turned on while taking a restroom break! ALD's need to be recharged every night and checked at the beginning of every day. There needs to be a process for handling repairs and maintenance in a timely fashion. Finally there is the issue of who will pay. An FM unit typically used in the schools can easily cost $1,000.

If you feel your child may benefit from an ALD, contact your district's teacher for students with auditory impairments, your district's assistive technology team, or visit an audiologist. If you can show that your child is in situations where his or her ability to listen is adversely affected by noise and distance, you would have a good case for the use of an ALD. The ALD may not be needed all day. For example, if reading instruction occurs in a very small group in the library, an ALD may not be needed then. However, for your child to hear instructions from the coach during PE in a large, noisy gym, an ALD may be quite helpful. Visit the various settings your child is in during the day. Think about how much background noise was present. How far away from the student is the teacher? Are there times when an ALD would improve the student's safety? An ALD can keep a child in touch with the teacher's voice out in the community and on job sites. This may increase the likelihood that the child would hear the teacher's voice giving an alarm. However, a student wearing an ALD set to block background noise may not hear traffic sounds. You can see there is no simple answer to whether and when a child should wear an ALD. This why a team approach usually yields the best results. If possible, include the child on that team to get a commitment to care for and use the ALD. Once everyone understands the benefit of using these devices, any problems will seem worth the effort.


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Last Revision: August 25, 2003