# Changes in Articulatory Contact Pressure as a Function of Vocal Loudness
## 4 Discussion  
This study evaluated the influence of vocal loudness on the tongue-to-palate contact pressure generated during speech. Overall, the findings supported the hypothesis that ACP differs as a function of loudness with significantly greater values in the loud condition compared to the others for all three phonemes. The habitual condition tended to have higher ACP than the soft and whisper conditions, although these differences were not all statistically significant, depending on the phoneme. The novel finding from this study is that altering vocal loudness is associated with articulatory changes reflected in ACP measures. The magnitude of the change in ACP was large, with it being at least 36% greater in the loud condition compared to the habitual condition. Conversely, the ACP values for the three phonemes were at least 37% smaller in the soft compared to the habitual condition. There are no other benchmark data in the literature for comparison, but the current results indicate that the alterations to tongue-palate pressure are substantial when a person either increases or decreases their loudness. Previous studies have identified other influences besides loudness on ACP, such as the phoneme class [3] and syllable structure [7]. The fact that ACP varies across loudness conditions is consistent with other studies documenting changes in oral articulatory activity when adjustments are made in the phonatory system. For example, loudness manipulation has been shown to result in associated changes in speech-related movement strokes and spatial variation of the trajectory of articulator movements [34]. Others have reported that  
Figure 4: Articulatory contact pressure (ACP) for each phoneme produced in the four loudness conditions plotted for individual participants ordered from the least to the greatest ACP in the habitual condition. increasing loudness resulted in increased articulatory displacement and velocity of the upper and lower lips during bilabial stop consonants [46]. The current ACP results are congruent with these studies, and together, the literature highlights that the phonatory and articulatory subsystems influence one another. In a clinical context, increasing a person's vocal loudness may increase their ACP, although this remains to be determined in future studies that include individuals with speech disorders from conditions such as PD. High values of ACP have been associated with increased oral air pressure during stop consonant production [47]. One outcome of an increase in oral air pressure is likely to be a more intense burst or friction on phonemes, which could have a positive impact on the clarity of speech [23]. Although this needs to be confirmed empirically, one reason that loud-focused interventions often help people with PD and those with non-progressive dysarthria may be the concomitant increase in ACP with associated increases in oral air pressure on stops and fricatives. In turn, more perceptually salient bursts and friction might result in perceptions of greater speech precision and intelligibility that have been reported when such therapy is applied to these patient populations [26; 48; 49]. Future research about the relationship between ACP and vocal loudness is needed on several fronts. Because this is the first reporting of the impact of loudness on ACP, confirmation of the results is needed with a larger subject pool and with a broader set of speech stimuli, in particular, speech that more closely reflects typical daily conversation. Second, a multiparameter assessment of speech produced in the four loudness conditions is needed to more robustly characterize the articulatory parameters that change and how these parameters relate to one another.