Impact of background noise on methods for categorical loudness scaling

The initial crucial step in hearing rehabilitation is a reliable diagnosis of hearing impairment. Pure tone threshold measurement in a quiet clinical setting is the current gold standard for this purpose. The reliability of pure tone threshold measurement depends on a quiet environment, especially when the threshold of the test subjects are near normal hearing. As a relatively novel procedure for characterizing individual hearing characteristics, ISO 16832 defines a reference method for categorical loudness scaling, where most of the presented stimuli are above the threshold by design. Here the test subject rates the subjective loudness of a set of narrowband auditory stimuli adapted during the test to cover the full dynamic range of the individual. The response is a subjective rating using an 11-point scale from not heard to extremely loud. In the outlined reference method, most of the presented stimuli are above the threshold by design, ideally with only one or two presentations characterized as not heard. The research behind the reference method suggests that the threshold can be reliably predicted by fitting an appropriate model on the collected set of responses, even when the presented set of stimuli are mainly above the threshold. The present study investigates the effects of performing categorical loudness scaling in a noisy environment. Simulations of added background noise to an existing dataset from a clinical setting illustrate the difficulties in estimating auditory thresholds and loudness growth. For test subjects with elevated thresholds, the increase in stimulus level may often be sufficiently above the environmental background noise such that little or no influence is expected. At low audiometric frequencies, however, the predicted hearing threshold is likely to deviate in excess of 5dB for a significant number of patients.