# Investigation of Vocal Fatigue Using a Dose-Based Vocal Loading Task
## 6 Discussion and Conclusions  
The primary finding in the SAVRa results was that the ratings show an arch-shaped variation trajectory from QE1 to QE8. This trend was also observed in the CAPE-V rating results. In the SAVRa ratings, the rapid increase of the vocal effort, the vocal discomfort level, and the severity of the soft phonation quality indicated a voice adjustment period. Over this period, the participants' vocal folds were abruptly exposed to a heavy vocal loading, and thus the subjects' feelings of vocal fatigue were strong over this period. After this period, the degrees of the participants' perceived vocal fatigue remained constant at a high level, or increased moderately from QE2 to QE7. The vocal loading intensity for each VLT session was identical. The participants gradually adapted to the vocal loading intensity, thus the increasing rate slowed down after this period. After the S1, the accumulation effect of the vocal loading led to a slow and persistent increase of the participants' perceived vocal fatigue in the late sessions. This indicates that, given constant vocal stimuli for different sessions, the participants' perceived vocal fatigue increased over time (or vocal distance dose). The notable decrease of the SAVRa scores from QE7 to QE8 reflects the effect of vocal rest on participants' vocal fatigue feelings. This indicates that the participants felt much better about their vocal functionalities after the 15-min rest session, but the rest was not sufficient to completely recover to the original status at QE1. A longer rest session was thus presumed to enhance the vocal recovery process. The findings in the CAPE-V rating results show that the overall severity, roughness, and strain ratings have the same arch-shaped variation trend with the SAVRa results. This indicates that vocal fatigue degraded the participants' voice performance by increasing roughness and strain. The data for the fatigue-indicative symptoms obeyed different trends than the SAVRa results. Previous studies showed that these symptoms were frequently observed on vocally fatigued participants. However, a quantitative relationship between the number of occurrences of these symptoms and the degree of vocal fatigue could not be established. Individual discrepancies in the counts of these symptoms showed varied sensitivities to vocal fatigue. Some participants coughed frequently during the VLT, while others did not cough at all. Swallowing was much more frequently observed than other symptoms for all participants. This finding further validated the results of previous studies on the effect of hydration on vocal fatigue [44; 48], i.e., superficial vocal fold hydration (swallowing or drinking) could help relieve vocal dysfunction and improve vocal efficiency. Fifteen voice metrics (features) were studied to track voice quality variation across VLT sessions. The statistical analysis results in Figure 7 did not include the rest session. The \(f0\), SPL_MIC, duty ratio, shimmer_MIC, and shimmer_NSA rapidly increased in early sessions, and remained constant afterwards. The variation of \(f0\) in Figure 7 was consistent with the results of previous studies, which found that participants' \(f0\) increased with vocal fatigue [49; 50]. The shimmer_MIC and shimmer_NSA showed an increase after the VLT session. This finding contradicts [PERSON]'s one-day vocal fatigue study results [51], but it is consistent with [PERSON]'s 60-min vocal fatigue study results [49]. This might be caused by the different phonation durations. One limitation of the present study was that the voice type was not considered as a parameter in the voice use quantification. For example, breathy voice obviously had a different vocal loading intensity from pressed voice when the \(f0\), SPL, duty ratio, and duration were identical for these two voice types. The impact stress between vocal folds for these two voice types were different. The feelings of vocal fatigue that the breathy and pressed voice brought to participants were therefore different.