  The effect of hearing impairment on speech perception was described by Plomp
(1978) as a sum of a loss of class A, due to signal attenuation, and a loss of
class D, due to signal distortion. While a loss of class A can be compensated
by linear amplification, a loss of class D, which severely limits the benefit
of hearing aids in noisy listening conditions, cannot. Not few users of hearing
aids keep complaining about the limited benefit of their devices in noisy
environments. Recently, in an approach to model human speech recognition by
means of a re-purposed automatic speech recognition system, the loss of class D
was explained by introducing a level uncertainty which reduces the individual
accuracy of spectro-temporal signal levels. Based on this finding, an
implementation of a patented dynamic range manipulation scheme (PLATT) is
proposed, which aims to mitigate the effect of increased level uncertainty on
speech recognition in noise by expanding spectral modulation patterns in the
range of 2 to 4 ERB. An objective evaluation of the benefit in speech
recognition thresholds in noise using an ASR-based speech recognition model
suggests that more than half of the class D loss due to an increased level
uncertainty might be compensable.
