Effects of slow- and fast-acting compression on the detection of gaps in narrow bands of noise.

The inherent amplitude fluctuations in narrow bands of noise may limit the ability to detect gaps in the noise; 'dips' in the noise may be confused with the gap to be detected. For people with cochlear hearing loss, loudness recruitment may effectively magnify the fluctuations and this could partly account for the reduced ability to detect gaps in noise bands that is usually found for such people. Previously, we tested these ideas by processing the envelopes of noise bands to alter the amount of envelope fluctuation. We showed that instantaneous compression, implemented via processing of the Hilbert envelope, led to smaller (that is, better) gap detection thresholds for subjects with cochlear hearing loss. In the present experiment, we determined whether fast-acting compression of the type sometimes used in hearing aids could also lead to improved gap detection. A behind-the-ear (BTE) digital hearing aid was programmed to implement multi-band compression, either fast-acting or slow-acting (control condition). A reference condition using unaided listening was also used. Stimuli were delivered via an earphone placed over the hearing aid. Overall stimulus levels at the output of the hearing aid were similar across conditions. Thresholds for detecting gaps in noise bands centred at 4 kHz were measured as a function of noise bandwidth (10-500 Hz). To prevent the detection of spectral changes introduced by the gap, stimuli were presented in a broad-band background noise. Three normally hearing subjects and three subjects with bilateral cochlear hearing loss were tested. Gap thresholds varied non-monotonically with noise bandwidth, being maximal around 50 Hz. Gap thresholds were generally higher for the hearing-impaired than for the normally hearing subjects. For the latter, gap thresholds were similar for the three conditions. For the hearing-impaired subjects, gap thresholds were similar for the unaided condition and the condition using slow compression. However, fast compression led to smaller gap thresholds, especially for noise bandwidths up to 50 Hz. The results show that fast compression can improve the ability of hearing-impaired subjects to detect gaps in sounds with slowly fluctuating envelopes.