Although the individual hearing aids from a single day's production may be of a uniform appearance, their amplifying capacities may very well differ. As physicians we are accustomed to trusting our co-workers, e. g., the pharmacists, expecting specific types of tablets to contain identical quantities of a certain drug. But a similar standard cannot be applied in the case of hearing aids. The individual hearing aid is composed of numerous components, all of which may deviate in some respect, thus causing final discrepancies.
In many countries the official authorization of a type of hearing aid is based on some few selected specimens, a procedure which, according to our experience, seems rather insufficient, since it is not very difficult for any manufacturer to select five almost uniform hearing aids to be submitted for authorization. Since October, 1951, the Copenhagen Hearing Center has practiced a current control, by means of an "artificial
EWERTSEN HW. Hearing-Aid Evaluation. AMA Arch Otolaryngol. 1956;64(6):520–525. doi:10.1001/archotol.1956.03830180070010
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