Multidimensional Timbre Spaces of Cochlear Implant Vocoded and Non-vocoded Synthetic Female Singing Voices.

Molly L Erickson,Patti M Johnstone,Mark S Hedrick,Katie Faulkner,Taylor Stone

doi:10.3389/fnins.2020.00307

Abstract

Many post-lingually deafened cochlear implant (CI) users report that they no longer enjoy listening to music, which could possibly contribute to a perceived reduction in quality of life. One aspect of music perception, vocal timbre perception, may be difficult for CI users because they may not be able to use the same timbral cues available to normal hearing listeners. Vocal tract resonance frequencies have been shown to provide perceptual cues to voice categories such as baritone, tenor, mezzo-soprano, and soprano, while changes in glottal source spectral slope are believed to be related to perception of vocal quality dimensions such as fluty vs. brassy. As a first step toward understanding vocal timbre perception in CI users, we employed an 8-channel noise-band vocoder to test how vocoding can alter the timbral perception of female synthetic sung vowels across pitches. Non-vocoded and vocoded stimuli were synthesized with vibrato using 3 excitation source spectral slopes and 3 vocal tract transfer functions (mezzo-soprano, intermediate, soprano) at the pitches C4, B4, and F5. Six multi-dimensional scaling experiments were conducted: C4 not vocoded, C4 vocoded, B4 not vocoded, B4 vocoded, F5 not vocoded, and F5 vocoded. At the pitch C4, for both non-vocoded and vocoded conditions, dimension 1 grouped stimuli according to voice category and was most strongly predicted by spectral centroid from 0 to 2 kHz. While dimension 2 grouped stimuli according to excitation source spectral slope, it was organized slightly differently and predicted by different acoustic parameters in the non-vocoded and vocoded conditions. For pitches B4 and F5 spectral centroid from 0 to 2 kHz most strongly predicted dimension 1. However, while dimension 1 separated all 3 voice categories in the vocoded condition, dimension 1 only separated the soprano stimuli from the intermediate and mezzo-soprano stimuli in the non-vocoded condition. While it is unclear how these results predict timbre perception in CI listeners, in general, these results suggest that perhaps some aspects of vocal timbre may remain.

Highlights

Many post-lingually deafened adults who use cochlear implants (CIs) report that they no longer enjoy listening to music, and poor music perception is often reported as a significant negative factor in self-reported quality of life (Migirov et al, 2009)
While dimension 1 separated all 3 voice categories in the vocoded condition, dimension 1 only separated the soprano stimuli from the intermediate and mezzo-soprano stimuli in the non-vocoded condition. While it is unclear how these results predict timbre perception in CI listeners, in general, these results suggest that perhaps some aspects of vocal timbre may remain
Multidimensional scaling (MDS) Results For both non-vocoded and vocoded conditions at the pitch C4, dimension 1 separated stimuli according to vocal tract transfer functions M, I, and S, while dimension 2 organized the stimuli according to excitation source spectral slope

Summary

Introduction

Many post-lingually deafened adults who use cochlear implants (CIs) report that they no longer enjoy listening to music, and poor music perception is often reported as a significant negative factor in self-reported quality of life (Migirov et al, 2009). Cochlear implant signal processing favors the encoding of speech cues and allows users to perceive speech remarkably well using limited spectral and temporal acoustic information (Limb and Roy, 2014). While CI listeners may perceive speech well, some acoustic factors related to the perception of vocal timbre may not be adequately represented in the CI signal. While rhythm cues are mostly preserved, CI users show deficits in the perception of pitch, melody, and timbre (Limb and Roy, 2014; Drennan et al, 2015; Jiam et al, 2017). Timbre is defined as that auditory attribute that distinguishes two sounds of equal pitch and loudness (ANSI, 1973). Vocal timbre is a perceptual attribute that is related to the acoustic characteristics of the output vocal signal and, is a function of the interaction of the glottal excitation source with the vocal tract transfer function (Cleveland, 1977; Sundberg, 1994, 2013; Roers et al, 2009)

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