Over-the-counter hearing aids: electroacoustic characteristics and possible target client groups.

Over-the-counter hearing aids (OTCs) are those directly purchased from retail outlets, without the benefit of prior professional hearing health care. They are particularly common in developing countries. This study examined the amplification characteristics of a selected sample of OTCs to determine if any target client group or groups were suitable for the OTCs. The electroacoustical performance of 10 OTCs was measured. The measurements included saturated sound pressure level curve, high-frequency average full-on gain, frequency response, total harmonic distortion, equivalent input noise level, and input-output curve. The full-on gain curve of each hearing aid was used to estimate the hypothetical hearing loss of target clients for each aid as it would be calculated by four hearing aid prescription formulae. Real-ear probe tube measurements were also performed on 10 adult subjects to determine the amplification that could be achieved by the OTCs before audible feedback occurred. The OTC hearing aids were not able to meet the prescription gain requirements of the majority of elderly clients who usually purchased them.