Arbeitsgruppe Medizinische Physik, Fachbereich Physik, Carl-von-Ossietzky Univ., D-26111 Oldenburg, Germany
To compensate for the frequency-dependent, nonlinear component of the hearing loss (recruitment), multichannel dynamic compression yields advantages over broadband compression. However, two unsolved problems remain: Due to the nonlinear additivity of perceived loudness across critical bands, the compression characteristic cannot be fitted correctly for narrow-band and broadband signals likewise. Furthermore, independent compression in different frequency bands introduces artifacts, which may limit the benefit of the algorithms. To overcome the problems, a 24-channel compression algorithm was developed and tested using critical bandwidths and time constants of ~10 ms. The compression characteristic for broadband signals of arbitrary spectral shape is calculated by a loudness estimation model based on data from normal listeners. The algorithm should provide correct gain factors for narrow-band signals as well as for broadband signals. Furthermore, it introduces a psychoacoustically motivated interaction between frequency bands that reduces the processing artifacts. The complete algorithm was implemented in real time on a multi-signal processor setup and tested with sensorineurally impaired patients. First results of speech intelligibility measurements in quiet show the capability of the algorithm to compensate for the ``distortion component'' of the hearing loss. Further results of speech quality assessments and speech intelligibility tests in noise are presented.