## 4 Discussion
### Interpreting RFF in Clinical Practice
A second recommendation would be to incorporate RFF measures into the therapeutic assessment protocols and tracking for patients with listener-perceived vocal strain (defined as excessive vocal effort [65]) or self-reported vocal effort. Historically, vocal strain and effort have proved difficult to quantify via acoustic measures [66, 67, 68]. This may be due to the variation of strained and effortful productions across speakers, as well as the potential overlap of percepts in individuals with voice disorders, in which a person with a strained voice can also present with a breathy voice. Our review of the literature indicates that RFF may be an appropriate clinical tool related to the percepts of strain and effort with offset cycle 10, in particular, being significantly correlated with either speaker or listener-ratings [4, 7, 8, 27]. Perhaps the most substantial evidence for RFF as an objective indicator of vocal strain was described in [PERSON] et al. [9]. The study design controlled for the problems of speaker variation and overlapping voicing percepts by synthetically altering RFF cycle values in speech samples gathered from those with typical voices. The authors manipulated RFF cycle values in two ways: (i) artificially lowering RFF cycle values during typical speech, which was hypothesized to increase the perception of strain due to the lower values seen in individuals with tension-based voice disorders, and (ii) artificially raising RFF cycle values in productions of maximal vocal effort so that the cycle values would resemble those found in typical voice productions. The hypotheses were supported when these manipulations resulted in changes to perceived strain in the speakers, wherein lowering RFF values acted to increase the perception of strain, and increasing RFF values reduced perceived strain. The authors incorporated an additional condition into the study in which they artificially increased mid-to-high frequency noise in the signal (which they quantified via a harmonics-to-noise ratio) and, subsequently, saw an increase in perceived strain. This study provides a foundation for future perceptual work on RFF, in that it may prove beneficial to study RFF in combination with other acoustic measures [8] in order to increase the correspondence between acoustic and perceptual measures. Finally, a thorough understanding of the relationship between RFF and underlying laryngeal physiology is needed to appropriately interpret RFF values. That is, in order to use RFF as a clinical outcome measure, it should be validated as an indicator of laryngeal tension. In our review, few studies investigated the laryngeal factors proposed by [PERSON] et al. [24]; however, the results of these studies provide preliminary evidence for RFF being related to measures of laryngeal stiffness [1,44], aerodynamics [7], and vocal fold adduction parameters [44]. Our review did not delve into the physiological basis of RFF, partially due to the fact that we limited our search to studies that enrolled human subjects, excluding modeling and theoretical studies. Recent work on modeling voicing offset behaviors [69] suggests a physiological relationship between abduction duration and RFF values; however, these findings should be expanded upon, and evaluated with empirical testing in human subjects. Combining computational modeling strategies with human subject research is an important step towards isolating the differential components associated with intervocalic voicing offset and onset (i.e., laryngeal kinematics, aerodynamics, and tension) and, in turn, may provide useful insights into the clinical utility of RFF for assessing and treating vocal disorders.