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Exploring the limits of frequency lowering.

PURPOSE
This study examined how frequency lowering affected sentence intelligibility and quality for adults with postlingually acquired, mild-to-moderate hearing loss.


METHOD
Listeners included adults aged 60-92 years with sloping sensorineural hearing loss and a control group of similarly aged adults with normal hearing. Sentences were presented in quiet and babble at a range of signal-to-noise ratios. Intelligibility and quality were measured with varying amounts of frequency lowering, implemented using a form of frequency compression.


RESULTS
Moderate amounts of compression, particularly with high cutoff frequencies, had minimal effects on sentence intelligibility. Listeners with the greatest high-frequency hearing loss showed the greatest benefit. Sentence intelligibility decreased with more compression. Listeners were more affected by a given set of parameters in noise than in quiet. In quiet, any amount of compression resulted in lower speech quality for most listeners, with the greatest degradation for listeners with better high-frequency hearing. Quality ratings were lower with background noise, and in noise, the effect of changing compression parameters was small.


CONCLUSIONS
The benefits of frequency lowering in adults were affected by the compression parameters as well as individual hearing thresholds. The data are consistent with the idea that frequency lowering can be viewed in terms of improved audibility versus increased distortion trade-off.