intro algorithm sounds

# Coherence-based dereverberation - algorithm details

The dereverberation algorithm studied in the HearCom project is a binaural coherence filtering based approach. It estimates the coherence (i.e. the signal similarity) between the signals captured at the left and the right ear. The estimate is computed in different frequency bands using an FFT-based filterbank with a non-linear frequency mapping that approximates a Bark-scale. As a coherence estimate, a time average of the interaural phase difference is computed. If the signals are coherent in a specific frequency band, the sound is expected to be directional, hence the gain in the frequency band is set to a high value. If on the other hand the coherence is low, a diffuse sound field is present, and accordingly, the frequency band is attenuated. The frequency-dependent gains are derived from the phase difference vector strength by applying an exponent (between 0.5 and 2) to the coherence estimate. High values for the exponent provide efficient filtering, but lead to more audible artifacts. Because of the head geometry, the coherence is high always at low frequencies, independently of the type of signal. At medium and high frequencies, on the other hand, the coherence is low for reverberated signal components (late reflections) and for diffuse babble noise, while it is high for the direct-path contribution of the signal of interest. Hence, by applying appropriate gains reverberated signal components and diffuse noise can be suppressed with respect to direct-path signal components.

## References:

Wittkop, T. and Hohmann, V. (2003). *Strategy-selective noise reduction for binaural digital hearing aids.* Speech Communication, 39:111–138.