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Compensation of Hearing Defects Using Multichannel Dynamic Compression

Rehtonen, Petri

This thesis concerns the development and evaluation of signal processing methods that compensate for the hearing disorders caused by ageing. The global ageing increases the number of elderly people with hearing impairment. In most cases this impairment is irrecoverable and cannot be medically or surgically treated. The only option is to process the sound going to the ear, so that it would be as clear and intelligible as possible. The multichannel dynamic compression is such a processing. With the multichannel compression the need for amplified sound and the problems of redused dynamic range can be solved. The compression reduces the loudness fluctuations in the sound so that when sound is amplified, loud sounds do not sound too loud. As a degree of the hearing impairment is more severe in high frequency bands and by processing the bands individually. In this work, I have implemented a three- channel dynamic compression system controllable in real-time. The system was used to conduct listening tests for hearing impaired: The goal was to found a relationship between the audiogram (which represents the hearing threshold level and uncomfortable listening level measured by using sinusoids as stimuli) and the parameters of the multichannel dynamic compression system. In the first listening test, only a week relationship between the two was found. A second test was conducted in order to find out if the adjusted parameter were actually the most intelligible. This was realised using a paired comparison test. In this tests, four different processing were compared. The results shows that the spectral shaping was judged to be the processing which made speech the most intelligible. The compression was disliked and the possible reason for this is that the stimuli were very intelligible and power-normalised already before processing. There was no room for further improvement in intelligibility and thus the distortions caused by compression were perceived unpleasant