Measurement and Analysis of Feedback and Nonlinearities for the Codacs Direct Acoustic Cochlear Implant

Acoustic feedback is a very common problem in hearing instruments. Not only does it occur in common behind-the-ear or in-the-ear hearing aids, it also affects bone conduction implants, middle ear implants, and more recent devices, such as the direct acoustic cochlear implant (DACI). In this paper, we present the data and analysis relating to the feedback path characterization of the Cochlear™ Codacs™ DACI, performed on fresh frozen cadaver heads in four different measurement sessions. The general objectives were the following: 1) To measure and analyze the feedback path of the system and check for possible specimen-dependent variabilities; 2) To assess whether this feedback path is affected by an incorrect implantation; 3) To check for nonlinear behavior; and 4) To determine differences between tissular and airborne feedback. The data analysis reveals that the feedback seems to be dependent on the specific head morphology of the implanted specimen, and that an incorrect implantation might strongly affect the feedback path; additionally, the analysis reveals that some nonlinear behavior at high stimulus levels can be expected and, finally, that the feedback path is characterized by a tissular feedback component with a rather different frequency content compared to the airborne feedback component.