Noise-based acoustic features of Polish retroflex fricatives in children with normal pronunciation and speech disorder

This study addresses Polish retroflex sibilants /ʂ, ʐ/ produced by preschool children. The aims of our research were (1) to explore acoustic characteristics of normal and distorted (dental and interdental) articulation patterns of retroflex fricatives and (2) to define and verify new acoustic features of frication noise. We extracted and analyzed a set of 80 acoustic features, including full-spectrum-based metrics (linear cepstral coefficients, mel-frequency cepstral coefficients, spectral moments) and noise-based metrics (noise energies, fricative formants, and original features: noise cepstral coefficients and fricative formant relations). The analysis involved linear mixed-effects models and Spearman's rank correlation over speech samples from 42 Polish children (21 with normal and 21 with distorted pronunciation). Normal articulation of Polish retroflex sibilants proved to be acoustically distinguishable from non-normative interdental and dental articulation. Significant acoustic differences between the classes considered were found both in the full-spectrum-based and noise-based features, including our proposed measures (p<0.05). Thirty-six of 80 analyzed features proved significantly correlated (Spearman's |ρ|>0.5,p<0.05) with tongue position and front-cavity size. More evident cues for articulation pathologies were found in the voiceless sibilant /ʂ/ than in /ʐ/. Our study confirms that metrics describing the structure of frication noise bring information distinctive in particular articulatory oppositions for a more comprehensive acoustic description of sibilants.