Classification of steady state gain reduction produced by amplitude modulation based noise reduction in digital hearing aids

Digital noise reduction systems in modern digital hearing aids aim to improve either speech intelligibility or listening comfort. Little is known, however, about the actual gain reduction produced by different noise reduction systems. The aim of this study is to compare the gain reduction of different noise reduction systems. We applied a systematic variation of different presentation levels of the input signal, different signal-to-noise ratios, and different target hearing loss configurations. The gain reduction of 12 different hearing aids was compared both qualitatively and quantitatively by means of principal value decomposition. The results show that these hearing aids differ considerably in their responses. These differences are presented qualitatively by plotting contour maps that give the gain reduction as function of signal-to-noise-ratio and frequency. The quantitative analysis shows that the gain reduction produced by most hearing aids can be characterized by two transfer functions. The most important being an overall gain reduction. In addition to this, the frequency response is tilted to achieve either more low-frequency reduction and less high-frequency reduction, or the opposite