Speech intelligibility declines at near- and above-conversational levels in aided hearing-impaired listeners

Background: At low levels, speech intelligibility (SI) typically improves when the presentation level increases. At high levels, a level increase can lead to decreased SI. Termed rollover, this effect has been observed in listeners with normal and elevated audiometric thresholds.
In a previous study, we observed rollover in normal-hearing listeners when tested with context-free but not context-rich speech materials. We therefore concluded that semantic context information can mask rollover because of compensatory top-down mechanisms. However, other studies conducted with aided hearing-impaired listeners observed rollover with context-rich speech materials.
In view of these discrepant findings, the current study investigated the influence of semantic context information on SI at near- and above-conversational levels in aided hearing-impaired listeners.

Method: Aided SI scores were measured for two groups of older hearing-impaired adults (N = 2 x 20) in noise with context-rich and context-free speech materials. The two groups were tested at either near-conversational (65 and 75 dB SPL) or above-conversational (aided most comfortable speech level + 10 dB and 85 dB SPL) levels. A research hearing aid was used to provide individual linear amplification. The scores were analyzed with a linear mixed-effects model to test for level-dependent changes in SI for the two speech materials.

Results: For both speech materials, SI decreased at near- and above-conversational levels (by approx. 7% and 10%, respectively; both p < 0.02) as the presentation level increased. No influence of speech material was observed (both p = 0.99).

Conclusion: Contrary to our findings for young normal-hearing listeners, semantic context does not seem to mask rollover in older hearing-impaired listeners. SI in noise can decline at near-conversational levels already.