Why do speech-enhancement algorithms not improve speech intelligibility?

While most speech enhancement algorithms improve speech quality, they do not improve speech intelligibility in noise. The reasons for that remain unclear. In this paper, we present a theoretical framework that can be used to analyze potential factors influencing the intelligibility of processed speech. It is hypothesized that if distortions are properly controlled, then large gains in intelligibility can be achieved. To assess the perceptual effect of the various distortions that can be introduced by speech enhancement algorithms, intelligibility tests are conducted with human listeners. The results indicated that certain distortions are more critical than others. The result of listening tests suggested that when these distortions are properly controlled, substantial gains in intelligibility can be obtained.