The National Acoustic Laboratories' procedure for selecting the saturation sound pressure level of hearing aids: experimental validation

The primary aim of this study is to evaluate the accuracy of a new procedure for selecting the saturation sound pressure level (SSPL) of hearing aids. Secondary aims are to investigate what limits the minimum SSPL that is acceptable to clients and whether the type of limiting (peak clipping or compression limiting) affects the SSPL required.

The study comprised two experiments. In the first, subjects increased the SSPL of a laboratory master hearing aid until they experienced loudness discomfort and decreased it until the sound became less acceptable in some way. In the second study, subjects wore multi-memory programmable hearing aids in their own environments and reported which of the two programs, differing only in SSPL setting, provided the more acceptable sound quality and comfort.

The theoretical procedure being investigated prescribed SSPLs that were within the acceptable range for 86% of the subjects in the laboratory study and for 63% of the subjects in the field experiment. On average, the theoretical predictions were neither too high nor too low. Incorporating individual measurements of loudness discomfort level into the prescription formula increased accuracy by such a small amount that it was not considered worthwhile. For a compression limiting hearing aid, the first thing that subjects noticed as SSPL was reduced was inadequate loudness. For the peak clipping hearing aid, however, both inadequate loudness and perception of distortion limited the acceptable SSPL range.

The theoretical procedure provides a good initial prescription of three frequency average SSPL, but it is still essential to evaluate the fitting and, if necessary, fine tune the individual's hearing aid. Compression limiting hearing aids can have slightly lower SSPL settings than peak clipping hearing aids for the same acceptability.