Perceived Naturalness and Acoustic Characteristics of Habitual Versus Clear Speech for Individuals With Hearing Loss

This study examined the impact of clear speech on perceived naturalness. We compared habitual speech (HS), untrained clear speech (UCS), and clear speech after a brief training (trained clear speech [TCS]) and examined whether acoustic variables differed between conditions and predicted naturalness.

Twelve adult female talkers read sentences in three conditions (HS, UCS, TCS). Twenty-eight normal-hearing adults rated naturalness on a 1-9 scale. Along with naturalness, acoustic measures of speech rate, number of pauses, and pause duration were obtained and compared between speaking conditions via linear mixed-effects models (LMMs).

LMMs showed a significant effect of speaking condition on the four dependent variables (p < .001), and all pairwise comparisons, except one, were significant. HS was rated most natural (3.56) and was produced at the fastest rate (3.89 syllables/s), whereas TCS was least natural (5.65) and performed at the slowest rate (1.72 syllables/s), with the most and longest pauses. All variables revealed substantial between-talkers variability in regard to baseline measures and when shifting to UCS and TCS. Speech naturalness ratings showed important between-listeners variability. Speech rate strongly predicted perceived naturalness, and the number of pauses and pause duration were also associated with it.

A 30-min clear speech training reduced speech naturalness compared to HS and UCS. A slower speech rate and increased pausing were associated with inferior naturalness ratings, although these features may not fully explain the reduced naturalness of TCS. The findings highlight an intelligibility-naturalness trade-off: Strategies that improve clarity can make speech sound less natural. Clinical protocols may benefit from promoting clarity-enhancing techniques that avoid excessive slowing or disruption of prosody and/or from providing real-time naturalness and intelligibility ratings as feedback during training.