FM systems and sound-field systems are often used with deaf and hard-of-hearing children to enhance the sound they hear. One characteristic of sound is that the farther you are from the source of sound, the more the loudness of the sound decreases. For a deaf or hard-of-hearing child, this may mean that if they only use hearing aids, that once they are more than a few feet away from the sound, the level has dropped so much that they cannot get any benefit from their hearing aids. Background noise can also interfere with the speech signal a child is receiving. To counter these problems, FM systems and sound-field systems are used.
While FM systems are often used in classrooms, some educational programs for infants recommend that parents use FM systems with their infants and toddlers as well.
Another way of looking at why FM systems or sound-field systems are needed is to consider something called the signal to noise ratio, which is the relationship between the primary speech signal and background noise. A S/N ratio of about +20 dB is needed for children with a hearing loss. In a typical classroom, the S/N ratio averages about +4 dB. (1) FM and sound-field systems can boost this S/N ratio.
The American Speech-Language-Hearing Association (ASHA) makes the following recommendations on classroom acoustics:
FM systems and sound-field systems both require the speaker to wear a microphone. With a FM system, the child has an FM receiver either directly connected via a boot to their hearing aids or a stand-alone FM receiver having its own hearing-aid devices. With an FM system, the sound goes directly from the speaker to child's ear so that sound intensity is not lost.Sometimes FM systems are also used in conjunction with cochlear implants.
Sound-field systems are typically used in classrooms where a speaker or speakers are placed around the classroom. Personal, portable sound-field systems are also available.
FM systems can provide the an improvement in S/N ratio of +20-30 dB, while sound-field FM systems improve the S/N ratio by about +10 dB. (1)
A videotape that discussion the critical importance of proper classroom acoustics and the benefit of using sound-field systems for deaf children is "Enhancing Classrooms for Listening, Learning, and Language" by Dr. Carol Flexor. This 22 minute long tape is available for $23.95 from Info-Link Video Bulletin, 1206 North 150 East, Layton, Utah 84041.
References:
(1) "Individual and Sound-Field FM Systems: Rationale, Description, and Use", Carol Flexor, The Volta Review 99(3) Classroom Acoustics, Carl Crandell and Joseph Smaldino, editors
Some companies providing FM systems or sound-field systems include:
1-800-328-3102
800-227-0735
800-383-9362
800-236-4327