The influence of attack time and release time on speech intelligibility. A study of the effects of AGC on normal hearing and hearing impaired subjects.

Automatic gain control may be used in hearing aids intended for different types and degrees of hearing loss; to average the levels, to reduce the dynamics of speech or to set the maximum output level of the hearing aid without adding nonlinear distortion. Our study concerns hearing impaired listeners with recruitment. A group of normal hearing subjects was used as a control group. The compression was introduced over a laboratory unit. The compression ratio was set to 30:5 dB. A fixed ratio between the attack time and the release time, tr = 200 x ta, was used. The values used varied from 0.05 to 5 ms for ta and 10 - 1000 ms for tr. The speech material was nonsense syllables of the CVC type presented with S/N = 60 dB and S/N = 5 dB. With S/N = 5 dB a significant deterioration of the discrimination of the nonsense syllables was found at time constant combination ta = 5 ms and tr = 1000 ms. No statistically significant difference in discrimination was found between the different combinations of attack and release times for the S/N = 60 dB condition neither for the normal hearing nor for the hearing impaired group.