Inclusion Criteria:  A minimum of 18 years old Experienced user of a RONDO 3 (≥ 6 months) for the ear to be tested Post-lingual onset of severe to profound sensory-neural hearing loss on the ear to be tested A minimum of 40% speech recognition in the Freiburg Monosyllables Test in quiet at 65 dB SPL for the ear to be tested (at the last time tested in clinical routine) Signed and dated Informed Consent Form (ICF) before the start of any study-specific procedure.  Exclusion criteria  Lack of compliance with any inclusion criteria CI user with contralateral hearing equal to or better than 30 dB (pure-tone average over the following frequencies: 250, 500, 1000, and 2000 Hz) Implanted with C40X, or C40C on the ear to be tested Implanted with an Auditory Brainstem Implant (ABI) or split electrode array Known allergic reactions to components of the investigational medical device (RONDO 3) or the SONNET 2 Anything that, in the opinion of the Investigator, would (a) place the subject at increased risk, (b) preclude the subject's full compliance with or completion of the study