An examination of several characteristics that affect the prediction of OSPL90 in hearing aids

Investigators at the National Acoustic Laboratories have provided a theoretical derivation and experimental validation of a formula for setting the maximum output of hearing aids (Dillon & Storey, 1998; Storey, Dillon, Yeend, & Wigney, 1998). Given that measurement of discomfort levels for setting maximum output can be both time-consuming and of questionable reliability, the use of a prescriptive formula warrants consideration. In this article, an extensive data base was considered and issues of normal hearing, clinical protocol, age and gender were investigated in an effort to further determine optimal maximum output settings.

Data were gathered from five previous investigations, for a total of 433 subjects (total ears = 710). Threshold of discomfort (TD) measures were obtained using one of two adaptations of the Ascending Method of Limits, one with category anchors and one without.

Subjects with normal hearing had significantly lower TDs than subjects with hearing loss. A different regression line for measured TDs as a function of hearing level was noted for subjects whose hearing threshold levels fell between 20 and 60 dB HL and those with thresholds above 60 dB HL. When all effects (hearing level, method, age and gender) were considered in a single predictive model for the two threshold groups, only method and threshold were significant predictors of TD. However, for the subjects with thresholds between 20 and 60 dB HL, less than 4% of the variance in TD measures could be accounted for by those factors. For subjects with threshold above 60 dB HL, 22% of the variance in TD measures could be accounted for by those variables.

For both groups of subjects (20 to 60 dB HL and above 60 dB HL) methodology and hearing thresholds are significant predictors of discomfort levels. Age and gender are not. Given the small variance accounted for by any of these factors, measures of discomfort using standardized methodologies seem warranted.