Development of a loudness normalisation strategy for combined cochlear implant and acoustic stimulation

Users of a cochlear implant together with a hearing aid in the non-implanted ear currently use devices that were developed separately and are often fitted separately. This results in very different growth of loudness with level in the two ears, potentially leading to decreased wearing comfort and suboptimal perception of interaural level differences. A loudness equalisation strategy, named 'SCORE bimodal', is proposed. It equalises loudness growth for the two modalities using existing models of loudness for acoustic and electric stimulation, and is suitable for implementation in wearable devices. Loudness balancing experiments were performed with six bimodal listeners to validate the strategy. In a first set of experiments, the function of each loudness model used was validated by balancing the loudness of four harmonic complexes of different bandwidths, ranging from 200 Hz to 1000 Hz, separately for each ear. Both the electric and acoustic loudness models predicted the data well. In a second set of experiments, binaural balancing was done for the same stimuli. It was found that SCORE significantly improved binaural balance.