Effect of Compression on Musical Sound Quality in Cochlear Implant Users
Authors: Gilbert Melanie , Jiradejvong Patpong , Limb Charles 
Published: April 2019

Objectives:
Cochlear implant (CI) users frequently report poor sound quality while listening to music, although the specific parameters responsible for this loss of sound quality remain poorly understood. Audio compression, which reduces the dynamic range (DR) for a given sound, is a ubiquitous component of signal processing used by both CI and hearing aid technology. However, the relative impact of compression for acoustic and electric hearing on music perception has not been well studied, an important consideration especially given that most compression algorithms in CIs were developed to optimize speech perception. The authors hypothesized that normal-hearing (NH) listeners would detect increased levels of compression more easily than CI users, but that both groups would perceive a loss of sound quality with increasing compression levels.

Design:
The present study utilizes the Cochlear Implant-MUltiple Stimulus with Hidden Reference and Anchor to evaluate the listener sensitivity to increasing levels of compression applied to music stimuli. The Cochlear Implant-MUltiple Stimulus with Hidden Reference and Anchor is a tool used to assess relative changes in the perceived sound quality of music across increasingly degraded listening conditions, in both CI and NH subjects. In this study, the authors applied multiple iterations of an aggressive compression algorithm to the music clips using Adobe Audition. The test conditions included 1, 3, 5, and 20 iterations sound tokens, with the 20-iteration samples serving as the Anchor stimuli. The compressed excerpts were 5 sec in length, with five clips for each of the five common musical genres (i.e., Classical, Jazz, Country, Rock, and Hip-Hop). Subjects were also presented with a Reference excerpt, which was the original music clip without any additional compression applied. CI recipients (n = 7, 11 ears) and NH listeners (n = 10) were asked to rate the sound quality of additionally compressed music as compared to the Reference.

Results:
Although both NH and CI groups could detect sound quality differences as a function of compression level, the discriminatory ability of the CI group was blunted compared to the NH group. The CI group had less variability in their responses and overall demonstrated reduced sensitivity to deterioration caused by excessive levels of compression. On average, the CI group rated the Anchor condition as only “Slightly worse” than the Reference. The music clips that were most affected by the compression were from Jazz and Hip-Hop genres and less so for Rock and Country clips. Corollary to this was a small but statistically significant impact of DR of the music clips on sound quality ratings, with narrower DR showing an association with poorer ratings.

Conclusions:
These results indicate that CI users exhibit less sensitivity to sound quality changes in music attributable to high levels of compression. These findings may account for another contributing factor to the generally poor music perception observed in CI users, particularly when listening to commercially recorded music.