The interplay between syllable-based predictability and voicing during closure in intersonorant German stops

Contextual predictability has pervasive effects on the acoustic realization of speech. Generally, duration is shortened in more predictable contexts and conversely lengthened in less predictable contexts. There are several measures to quantify predictability in a message. One of them is surprisal, which is calculated as S(Uniti) = -log2 P (Uniti|Context). In a recent work, Ibrahim et al. have found that the effect of syllable-based surprisal on the temporal dimension(s) of a syllable selectively extends to the segmental level, for example, consonant voicing in German. Closure duration was uniformly longer for both voiceless and voiced consonants, but voice onset time was not. The voice onset time pattern might be related to German being typically considered an ‚aspirating‘ language, using [+spread glottis] for voiceless consonants and [-spread glottis] for their voiced counterparts. However, voicing has also been reported in an intervocalic context for both voiceless and voiced consonants to varying extents. To further test whether the previously reported surprisal-based effect on voice onset time is driven by the phonological feature [spread glottis], the current study re-examined the downstream effect of syllable-based predictability on segmental voicing in German stops by measuring the degree of residual (phonetic) voicing during stop closure in an inter-sonorant context. Method: Data were based on a subset of stimuli (speech produced in a quiet acoustic condition) from Ibrahim et al. 38 German speakers recorded 60 sentences. Each sentence contained a target stressed CV syllable in a polysyllabic word. Each target syllable began with one of the stops /p, k, b, d/, combined with one of the vowels /a:, e:, i:, o:, u:/. The analyzed data contained voiceless vs. voiced initial stops in a low or high surprisal syllable. Closure duration (CD) and voicing during closure (VDC) were extracted using in-house Python and Praat scripts. A ratio measure VDC/CD was used to factor out any potential covariation between VDC and CD. Linear mixed-effects modeling was used to evaluate the effect(s) of surprisal and target stop voicing status on VDC/CD ratio using the lmer package in R. The final model was: VDC/CD ratio ∼ Surprisal + Target stop voicing status + (1 | Speaker) + (1 | Syllable ) + (1 | PrevManner ) + (1 | Sentence). Results: In an inter-sonorant context, we found a smaller VDC/CD ratio in voiceless stops than in voiced ones (p=2.04e-08***). As expected, residual voicing is shorter during a voiceless closure than during a voiced closure. This is consistent with the idea of preserving a phonological voicing distinction, as well as the physiological constraint of sustaining voicing for a long period during the closure of a voiceless stop. Moreover, the results yielded a significant effect of surprisal on VDC/CD ratio (p=.017*), with no interaction between the two factors (voicing and surprisal). The VDC/CD ratio is larger in a low than in a high surprisal syllable, irrespective of the voicing status of the target stops. That is, the syllable-based surprisal effect percolated down to German voicing, and the effect is uniform for a voiceless and voiced stop, when residual voicing was measured. Such a uniform effect on residual voicing is consistent with the previous result on closure duration. These findings reveal that the syllable-based surprisal effect can spread downstream to the segmental level and the effect is uniform for acoustic cues that are not directly tied to a phonological feature in German voicing (i.e. [spread glottis]).