Impact of Hearing Aid Processing Delay on Stop Consonant Voicing Perception in Open Fittings

Hearing aid (HA) processing delay results in asynchronous overlap of the input sound and the delayed amplified sound at the eardrum in open-ear fittings. This may distort the temporal cues used for stop-consonant voicing distinctions. The current study evaluated the impact of HA processing delay on voiced-voiceless categorization of syllable initial consonants /d/ and /t/ for a range of voice onset times (VOTs).

Nineteen older listeners (M_age = 73 years) with mild-moderate sensorineural hearing loss participated. All listeners performed the voiced-voiceless categorization task in double-blind within-subjects design. Thirteen stimulus tokens along the /di:/ - /ti:/ continuum were created by varying VOTs. Stimuli were then processed using an HA simulator, which simulated the overall sound pressure levels measured at the eardrum in open-ear fittings with four processing delay times (0, 0.5, 5, and 8 ms). Individualized stimuli were generated for each listener based on their audiogram and presented via calibrated ear inserts at the most comfortable listening level. Performance across all VOT intervals was fitted with psychometric functions, which were then used to estimate the voiced-voiceless crossover point and the slope parameter for each simulated delay condition.

The crossover point along the voiced-voiceless continuum shifted systematically with increased processing delay toward voiced /di:/ over unvoiced /ti:/ percepts. The shift in the crossover point between the 0-ms reference condition and the 8-ms processing delay condition corresponded to 5.8 ms of change in VOT. The 8-ms processing delay condition resulted in significantly shallower categorization slopes compared to the 0- and 0.5-ms delay conditions.

Temporal distortions created by HA processing delay in open-ear fittings impacted voicing perception of syllable initial stop-consonant stimuli near the voiced-voiceless crossover point. Short HA processing delay should be considered for open-ear fittings to preserve the natural VOT cues used for voicing detection.