Compression and its effect on the speech signal

Compression systems are often used in hearing aids to increase the wearing comfort. A patient has to readjust frequently the gain of a linear hearing aid because of the limited dynamic hearing range and the changing acoustical conditions. A great deal of attention has been given to the static parameters but very little to the dynamic parameters. We present a general method to describe the dynamic behavior of a compression system by comparing modulations at the output with modulations at the input. The use of this method resulted in a single parameter describing the temporal characteristics of a compressor, the cut-off modulation frequency. In this paper its value is compared with known properties of running speech. A limitation of this method is the use of only small modulation depths, and the consequence of this limitation is tested. The use of this method is described for an experimental digital compressor developed by the authors, and the effects of some temporal parameters such as attack and release time are studied. This method shows the rather large effects of some of the parameters on the effectiveness of a compressor on speech. This method is also used to analyze two generally accepted compression systems in hearing aids. The theoretical method is next compared to the effects of compression on the distribution of the amplitude envelope of running speech, and it could be shown that single-channel compression systems do not reduce the distribution width of speech filtered in frequency bands. This finding questions the use of compression systems for fitting the speech banana in the dynamic hearing range of impaired listeners.