Inclusion criteria: Newly Implanted Group:

• Ability to provide informed consent
• No previous cochlear implant experience in either ear
• 18 years of age or older
• Up to a moderate sensorineural hearing loss in the low-to-mid frequencies (pure tone average ≤65 dB for 125, 250, 500, and 1000 Hz) and a severe-to-profound sensorineural hearing loss in the high frequencies (pure tone average ≥ 70 dB HL for 2,000, 3000, 4,000, and 8,000 Hz) in the ear to be implanted
• Aided CNC word recognition score up to 50% in ear to be implanted
• Up to a moderate sensorineural hearing loss in the low-to-mid frequencies (pure tone average ≤65 dB for 125, 250, 500, and 1000 Hz) and a severe-to-profound sensorineural hearing loss in the high frequencies (pure tone average ≥ 70 dB HL for 2,000, 3000, 4,000, and 8,000 Hz) in the contralateral (non-implanted) ear
• Aided CNC word recognition score up to 80% in the contralateral ear
• English language proficiency
• Willingness to use an ear-level sound processor postoperatively for the duration of the study trial
• Willingness to participate in all scheduled procedures outlined in the protocol

Exclusion Criteria: Newly Implanted Group:

• Preoperative audiometric conductive overlay of 15 dB or greater at two frequencies or more in range of 500-1000 Hz in the ear to be implanted
• Congenital hearing loss (for purpose of this study, onset prior to age 2 years*). *Based on critical period for speech and language development
• Duration greater than 30 years of severe-to-profound high-frequency hearing loss
• Cochlear malformation or obstruction that would preclude full insertion of electrode array in the ear to be implanted
• Medical or psychological conditions that contraindicate surgery or impact the ability to manage an implanted device or the study related procedures as determined by the investigator
• Deafness due to central auditory lesion or cochlear nerve deficiency, diagnosis of auditory neuropathy/dys-synchrony in either the ear to be implanted or the contralateral ear
• Active middle-ear disease/infection in the ear to be implanted
• Unrealistic expectations regarding potential benefits, risks and limitations inherent to implant surgical procedures as determined by the investigator
• Unwillingness or inability of subject to comply with all investigational requirements as determined by the investigator

EAS Fitting Inclusion Criteria: Newly Implanted and Existing Implanted Groups

• Ability to provide informed consent
• 18 years of age or older, unilaterally implanted with HiRes90K™ Advantage cochlear implant with HiFocus™ Mid-Scala electrode and fit with an ear-level sound processor (i.e. Naída family device)
• Postoperative unaided low frequency audiometric hearing thresholds in the treated ear up to 80 dB HL for at least one frequency from 125 - 1000 Hz
• English Language Proficiency
• Willingness to participate in all scheduled procedures outlined in the study protocol

EAS Fitting Exclusion Criteria: Newly Implanted and Existing Implanted Groups

• Unilaterally implanted with HiRes 90K™ Advantage cochlear implant with HiFocus™ Helix® and HiFocus™ 1j electrode
• Exclusive use of a body worn external sound processor
• Deafness due to central auditory lesion or cochlear nerve deficiency, diagnosis of auditory neuropathy/dys-synchrony in either the implanted or the contralateral ear
• Postoperative unaided low frequency audiometric hearing thresholds in the treated ear exceeding 80 dB HL at each frequency from 125 - 1000 Hz Unwillingness or inability of subject to comply with all investigational requirements as determined by the investigator