A Phoneme Perception Test Method for High-Frequency Hearing Aid Fitting.

Background: Outcomes with hearing aids (HAs) can be assessed using various speech tests, but many tests are not sensitive to changes in high-frequency audibility.

Purpose: A Phoneme Perception Test (PPT), designed for the phonemes /s/ and /ʃ/, has been developed to investigate whether detection and recognition tasks are able to measure individual differences in phoneme audibility and recognition for various hearing instrument settings. These capabilities were studied using two different fricative stimulus materials. The first set of materials preserves natural low-level sound components in the low- and mid-frequency ranges (LF set); the second set of materials attempts to limit the audibility to high-frequency fricative noise (nLF set). To study the effect on phoneme detection and recognition when auditory representations of /s/ and /ʃ/ are modified, a too strong nonlinear frequency compression (NLFC) setting was applied.

Research Design: Repeated measure design was used under several different conditions.

Study Sample: A total of 31 hearing-impaired individuals participated in this study. Of the 31 participants, 10 individuals did not own HAs but were provided with them during the study and 21 individuals owned HAs and were experienced users. All participants had a symmetrical sensorineural hearing loss.

Data Collection And Analysis: The present study applied a phoneme detection test and a recognition test with two different stimulus sets under different amplification conditions. The statistical analysis focused on the capability of the PPT to measure the effect on audibility and perception of high-frequency information with and without HAs, and between HAs with two different NLFC settings ("default" and "too strong").

Results: Detection thresholds (DTs) and recognition thresholds (RTs) were compared with respective audiometric thresholds in the free field for all available conditions. Significant differences in thresholds between LF and nLF stimuli were observed. The thresholds for nLF stimuli showed higher correlation to the corresponding audiometric thresholds than the thresholds for LF stimuli. The difference in thresholds for unaided and aided conditions was larger for the stimulus set nLF than for the stimulus set LF. Also, thresholds were similar in both aided conditions for stimulus set LF, whereas a large difference between amplifications was observed for the stimulus set nLF. When NLFC was set "too strong," DTs and RTs differed significantly for /s/.

Conclusions: The findings from this study strongly suggest that measuring DTs and RTs with the stimulus set nLF is beneficial and useful to quantify the effects of HAs and NLFC on high-frequency speech cues for detection and recognition tasks. The findings also suggest that both tests are necessary because they assess audibility as well as recognition abilities, particularly as they relate to speech modification algorithms. The experiments conducted in this study did not allow for any acclimatization of the participants to increased high-frequency gain or NLFC. Further investigations should therefore examine the impact on DTs and RTs in the PPT as well as the contrasting effects of strong setting of NLFC to DTs and RTs because of (re)learning of modified auditory representations of /s/ and /ʃ/ as caused by NLFC.