Suppression of reverberation in hearing aids
Johan Nilsson · Epubl LTU · 2006
Reverberation is a sound degradation that cause reduced speech intelligibility. For a hearing impaired person wearing an hearing aid, this may cause communication problems and thus, the motivation for a dereverberation algorithm for hearing aids is clear. Reverberation is generated by propagation delays from reflective boundaries around the source and the receiver of a sound, and is perceived as temporal smearing by the listener. Three basic approaches for dereverberation are considered and evaluated theoretically. While inverse filtering and speech coding methods are dismissed because of time delay requirements not met and time constraints of the project, beamforming approaches are investigated further. One additive beamformer is implemented as well as an adaptive subtractive beamformer, both operating in frequency bands. To evaluate the performance, much effort is put into creating a test setup for the reverberation problem. Simulated impulse responses are generated with the image method and real impulse responses are recorded with different methods, the effect on speech intelligibility by reverberation and noise is investigated and a test program for speech intelligibility by listening tests is implemented. The results imply that articulation-index signal-to-noise ratio quantify dereverberation best among the common objective measures of speech intelligibility investigated. The listening tests show that the subtractive beamformer approach is an efficient algorithm for increased speech intelligibility by dereverberation while the additive algorithm is not. The resulting increase in speech intelligibility is quantified to +2.0 dB for the subtractive beamformer algorithm while the additive beamformer algorithm decreases the speech intelligibility with -1.4 dB.