Pressure and Field Response of the Ear in Hearing Aid Performance Determination

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Abstract
Inasmuch as hearing aid acoustical output is measured and specified in terms of the action of an artificial ear, this output does not represent the free field action of the human ear (which is the norm). In order to make the hearing aid output correspond to the true action of the unobstructed hearing ear, measures based on the artificial ear must be corrected for the discrepancy in frequency response between the artificial ear and that of the hearing ear. This correction amts. to the difference in sound pressure operation of the artificial ear and the free field operation of the hearing ear. In order to make the hearing aid output correspond to the reaction of the hearing ear, an additional correction of hearing aid output must be made because of the baffle effect of the human body on the transmitter of the hearing aid. The magnitude of the first correction has been measured and computed in terms of both objective and subjective procedures, and the computations have been utilized along with an estimated correction for the body-baffle effect in order to secure an over-all correction for certain hearing aid instrument characteristics. The magnitude of the correction needed is too great to be neglected in the evaluation of hearing aids and individual hearing aid requirements.

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