Effects of an adaptive filter hearing aid on speech recognition in noise by hearing-impaired subjects

In a digitally controlled adaptive filter, ongoing detection and spectral analysis of quasi steady state noise are performed, and analog filter parameters are adjusted to provide greatest attenuation in frequency regions where noise energy is greatest. Attenuation introduced by the filter affects both speech and noise signals in the attenuation region. Effects of a master hearing aid incorporating an adaptive filter chip were assessed by measuring aided speech recognition threshold in noise with and without the filter. Performance of four hearing-impaired subjects showed that filter effects were related both to the frequency spectrum of the noise and to the distribution of speech information by frequency. No improvement in speech recognition threshold was seen for broadband speech materials in broadband noise. Better performance was observed for low-frequency noise in combination with speech materials for which the important information was confined to high frequencies.