Body-Baffle and Real-Ear Effects in the Selection of Hearing Aids for Deaf Children

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Abstract
A difference between expected and obtained results in hearing-aid selection often can be explained on a physical basis alone. For example, the frequency response of a hearing aid typically is obtained by suspending the instrument in a free sound field and measuring the acoustic output of the receiver in a standard 2-cc coupler. However, when a body-type hearing aid is used by a deaf child, it normally is worn on his chest, and its receiver is coupled to an earmold in his ear canal. Under these conditions, the acoustic input to the hearing-aid microphone and the acoustic output of the receiver will differ from that measured in the laboratory. Because the dynamic range of sensitivity of profoundly deaf children typically is small, a neglected 5–10 dB of amplification can be critical in the selection of hearing aids. A clinical procedure is proposed by which the output of a hearing aid can be matched more accurately to the sensory capacities of an impaired ear. This method defines both thresholds and hearing-aid output in terms of sound pressures generated in a 2-cc coupler and considers only the specific body-baffle and real-ear effects that are produced by the patient himself.

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