In the case of CIs, an additional processing is needed to adapt the acoustic signal to the CI electric stimulation. Spectral subtraction, based on the estimation of the noise, followed by the attenuation of the band spectrum containing the noise by widely used Wiener filtering, may improve the signal-to-noise ratio (SNR) ( Van den Bogaert et al., 2009 Kallel et al., 2012 Yang and Fu, 2005). Several binaural algorithms are classically used for noise reduction, such as the Beamformer, which favors a direction through delay ( Bouchard et al., 2009 Kompis and Dillier, 2001). This strategy is worthy in the case of CI coding ( Loizou, 2006). Furthermore, some reinjection of the input signal has been suggested by Van den Bogaert in the case of hearing aids ( Van den Bogaert et al., 2008), in order to restore some source localization of the sound source this effect will be seen in the current work. The aim of noise reduction algorithms is to improve speech intelligibility, but localization should be considered as well when an algorithm improves speech perception (for instance, by focusing on a source situated in the front), the localization of other sound sources may be affected. Several noise reduction algorithms favor the signal coming from the front ( Van den Bogaert et al., 2008, 2009). The ITD can be employed in a coordinated process ( van Hoesel et al., 2009 Lawson et al., 1998). In the case of binaural cochlear implants (BCIs), two CIs are used, and only one processor drives them. This effect is mostly perceptible with high frequencies (above 3 kHz). The signal is more or less attenuated by the head shadow. ILD is related to the intensity reaching the two ears.
When the source is situated in the front (azimuth 0°), the so-called front target, the ITD is 0 ms. It can be reminded that a sound coming from the side at 90° has an ITD of 0.6 ms. It is due to the envelope of the signal reaching the two ears. It is efficient for low frequencies (below 850 Hz). Binaural hearing uses two main acoustic cues: interaural time difference (ITD) and interaural level difference (ILD) ( Doerbecker and Ernst, 1996 Francart et al., 2011):
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