(The following is excerpted from the "Report on Assistive Listening Devices" by Ruth Warick, Catherine Clark, Jesse Dancer, and Stephen Sinclair, published by the National Task Force on Quality of Services in the Postsecondary Education of Deaf and Hard of Hearing Students, 1997).
For the most hard of hearing students, and for some who are deaf, hearing aids and related sound amplification devices are of great benefit in their communication and learning. We are all familiar with hearing aids. If we don't wear a hearing aid ourselves, almost certainly we know others who do.
Hearing aids, the predecessor to ALDs, have been in existence for over a hundred years in one form or another, and have become increasingly sophisticated over the years. Unlike ALDs, hearing aids amplify sound through a single unit leading into the ear. There are different types of hearing aids such as behind-the-ear-aids, in-the-ear aids and body aids.
Basically, a hearing aid system consists of a tiny microphone that picks up sound waves from the air and converts them into electrical signals, a battery that provides electrical energy to operate the hearing aid, and a tiny loudspeaker called a receiver that converts the amplified signal back into sound waves and directs them into the ear through a specially fitted mold.
Most behind-the-ear and body aids have a "t-switch" that controls a telecoil which in turn picks up electromagnetic signals from a telephone or another listening device. This is reconverted into sound, magnified, and sent to the listener through his/her earmold. This offers enhanced sound quality and avoids picking up extraneous sounds. One disadvantage is that, besides blocking out erroneous sounds, it also blocks out the wearer's own voice. This problem can be solved by using a combined microphone/telecoil mode. If the student uses two hearing aids, he/she may choose to wear one in the customary microphone mode and the other in the telecoil mode. Alternately, the student may be using only one hearing aid.
While some hard of hearing persons rely solely on their hearing aids, other find them inadequate in some environments, and choose to use an assistive listening device.
ALD's differ from each other in numerous respects, but they also have common features not shared by hearing aids. Like hearing aids, ALD's have transmitting and receiving components. However, whereas hearing aids package both these components in a single unit for wearing on the user's body, ALD's place the microphone/transmitter unit at or near the source of the sound, e.g., speaker, musical performance. The transmitter sends the signal through the air by cable to the receiver being worn by the user. This separation of the two components enables an ALD to:
(a) amplify sound over a considerable distance,
(b) provide clear sound over distances by eliminating echoes and reducing surrounding noises,
(c) overcome poor sound quality when a microphone/public address system is in use,
(d) amplify sound from several vantage points
It is a matter of student choice whether or not to use an ALD. While it may appear that a particular student would benefit from the use of a system, the individual student must choose whether to wear it; if he/she does not want to use ALD, this choice must be respected, with the recognition that each hard of hearing person differs in his/her response to such devices.
Many hard of hearing students function well with a hearing aid in one-to-one interactions, but they may not be able to hear in large classrooms. A distinct acoustic advantage of ALDs compared to personal hearing aids is the position of the input microphone at a location close to the talker's mouth. While the noise and reverberation characteristics of a room generally are constant at all places, the intensity of the primary speech signal decreases as distance increases from the talker (called the "speech-to-noise ratio"). By placing the mircophone of the auditory amplification system close to the talker's mouth, the most advantageous ratio is obtained between the intensity of speech and the level of background noise.
ALDs maintain this favorable speech-to-noise ratio due to the fact that the signal is transmitted by electronic, magnetic, infrared, or radio media rather than acoustically (as in the case of the hearing aid) to each listener's ear.