RESUMO
A multichannel signal-processing hearing aid in which the gain is controlled by the level of the minima in the sound envelope [outlined by Festen et al., 1990] was evaluated with hearing-impaired listeners. This evaluation is an extension to the work reported by van Dijkhuizen et al. (1990). A first experiment focused on the speech-reception threshold (SRT), i.e. the S/N ratio for 50% intelligibility. The greatest benefit in terms of the SRT from frequency-dependent control of the amplification is expected in conditions where the spectrum of noise exceeds strongly that of the speech in a limited frequency region. In these conditions frequency-dependent amplification may reduce upward spread of masking. We investigated the upper limit of this benefit in conditions of intense frequency-limited interfering noise. Speech and noise were both spectrally shaped according to the line bisecting the listener's dynamic range; however, the level of the noise in one octave band (0.25-0.5 or 0.5-1 kHz) was increased by 20 dB. The results show that frequency-selective attenuation of the signal in the octave band with the 20-dB increase of noise is more beneficial than wide-band gain control, and gives a decrease in SRT of up to 4 dB relative to a condition without gain control. In a subsequent experiment we investigated, for several very common interfering sounds, the effect of controlling the gain by the minima in the signal envelope on both the SRT and the perceived noisiness. Results show that the condition with gain control does not affect the SRT for sentences in the presence of everyday interfering sounds having spectra that are roughly comparable to that of the speech signal; however, it substantially reduces the perceived noisiness. In line with our expectations, the effect of the gain control on the signal was very small for a single voice, and it was greatest in case of sounds with a more or less continuous character (e.g. stationary noise, music). For these last sounds it was found that the growth in perceived noisiness with the increase of input level is equivalent to the growth produced by only about one-fifth of the increase in input level (in decibels) in a condition without gain control.