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Assistive Listening Devices
Many students who use hearing aids effectively in quiet environments have a difficult time following information presented in large college classrooms. In the classroom, the instructor’s voice is competing with background noise, room echo, and distance. Therefore, the intelligibility of the instructor’s voice is degraded by the poor room acoustics as well as the hearing loss. Most Assistive Listening Device systems (ALDs) use a microphone /transmitter positioned close to the instructor’s mouth to send the instructor’s voice through the air or by cable to the receiver worn by the student. By placing the microphone close to the instructor’s mouth, ALDs can provide clear sound over distances, eliminate echoes, and reduce surrounding noises. Assistive Listening Devices have proven to be an effective teaching tool for students with hearing loss. Providing a good listening environment can have a major impact on an individual’s academic performance.
ALDs utilize different technologies. Typically, they are wireless or wired. Wireless ALDs make use of radio frequencies, light rays, or magnetic inductive energy to transmit sound. Hardwired ones use direct electrical connection to transmit the auditory signal. Each system has special features, capabilities, advantages, and disadvantages. Three ALD systems–FM, Soundfield Amplification, and Induction Loop Systems–will be discussed.
An FM system is a wireless, portable battery-operated device that uses radio transmission to send auditory signals, i.e. speech, from a transmitter to a receiver. With most FM systems, the instructor wears a lavelier microphone connected to a body-worn transmitter. The student wears the FM receiver unit clipped to his/her clothing. The FM receiver can also be connected to the student’s hearing aid via an induction neckloop system or direct audio input cables. Special FM cables are also available for cochlear implant users. When the instructor speaks, the speech signal is broadcast by radio signals to the FM receiver linked to the student’s hearing aid. The ranges of FM systems extend from 30 ft. to more than 200 ft., depending on the power and antenna. FM systems can transmit through walls and buildings .Therefore, multiple frequencies are required for adjacent room usage.
Recently, the FM receiver units have been significantly miniaturized. In FM/BTEs (behind-the-ear hearing aids), the FM receiver is built into the same casing as the hearing aid. Hearing aid manufacturers have also introduced wireless FM boot receivers that attach to the bottom of a hearing aid. An audiologist can assist with the selection and fitting of an appropriate FM system.
Soundfield amplification systems amplify and broadcast the instructor’s voice through wall or ceiling-mounted loudspeakers. The system consists of a microphone/FM transmitter, amplifier, and one or more loudspeakers. A loudspeaker can also be placed next to the student. The soundfield speakers should be strategically placed in order for the student to achieve the most benefit from the system. The system should be installed under the guidance of an audiologist or someone who understands room acoustics.
Induction loop systems use electromagnetic waves for transmission. Sounds are picked up by the instructor’s microphone, amplified, and sent through the wire/loop, creating an invisible electromagnetic field. The telecoil (T-switch) in the student’s hearing aid serves as a receiver for the signal. The loop can encircle the entire room or be small and hidden under a chair or table. When using large loop systems, care should be taken not to loop adjacent classrooms, as the electromagnetic energy will spill over, causing interference. Reportedly, newer three-dimensional loops have eliminated the problem of spillover.
A distinct acoustic advantage of ALDs compared to personal hearing aids is the position of the input microphone at a location close to the instructor’s mouth. The micro-phone location allows the level of the instructor’s voice to stay constant to the student regardless of the distance between the instructor and the student. The instructor’s voice is also heard clearly over room noises such as chairs moving, fan motors running, and students talking.
Assistive listening devices will provide maximum benefit when used appropriately. Here are helpful tips for using assistive listening devices.
There are a variety of Assistive Listening Devices which can be utilized effectively in the classroom. No single technology is without limitations or can be expected to fulfill all the essential auditory needs of all users. Consult with an audiologist and the student to determine the most appropriate assistive listening device. ALDs can maintain a clear presentation of the speech signal in the presence of poor room acoustics. Therefore, the student with a hearing loss has better access to classroom information.
If you have any further questions on this topic, about working with an individual who is deaf or hard of hearing, or would like more information, contact:
Deaf and Hard of Hearing Services
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