### Stimuli and Calculation  
Traditional acoustic measures of voice--such as jitter and shimmer--are computed during voicing steady-states, most often during sustained vowel productions. RFF is a unique voice measure because it quantifies changes in fundamental frequency (\(f_{\text{o}}\)) during voicing _transitions_. Specifically, RFF is measured from the instantaneous changes in \(f_{\text{o}}\) during vocalic devoicing and voicing gestures during a vowel-voiceless consonant-vowel (VCV) utterance. Usual RFF stimuli include VCV utterances of /ifi/, /afq/, and /ufu/ (10), or specific wording from the phonetically balanced Rainbow Passage [11]. For example, the VCV transition within the word \"beautiful\" (i.e., /ifq/) meets the criteria for RFF extraction.  
RFF is calculated from the 10 voicing cycles immediately before the voiceless consonant (\"offset cycles\") and 10 voicing cycles immediately after the voiceless consonant (\"onset cycles\"). As such, an estimate of RFF for one speaker comprises 20 individual values; each value corresponds to a change in \(f_{\text{o}}\) from steady state and can be analyzed either alone as an estimate of laryngeal tension or alongside other values to understand trends in the speaker's devoicing and/or voicing gestures. When reporting RFF findings, individual voicing cycle values are labeled sequentially from 1 to 10 as they occur in time, with offset cycle 10 and onset cycle 1 located closest to the consonant (see Figure 1 for example of labeled offset and onset cycles).  
RFF values for a given voicing cycle are determined by first calculating the instantaneous \(f_{\text{o}}\) of the cycle (i.e., the inverse of the cycle period) in units of Hertz (Hz). The cycle \(f_{\text{o}}\) is normalized to that of the voicing cycle closest to the steady-state portion of its corresponding vowel (\(f_{\text{o}}\)_ref_); for voicing offset cycles, this is the first voicing cycle (cycle 1) in the first vowel of the VCV utterance and for voicing onset cycles, this is the last voicing cycle (cycle 10) of the second vowel. Finally, all cycle values (in Hz) are converted to semitones (ST), as shown in Equation (1).  
\[\text{RFF (ST)}=12\times\text{log}_{2}\bigg{(}\frac{f_{\text{o}}}{f_{\text{o}}^{ \text{ref}}}\bigg{)} \tag{1}\]  
Figure 1: Offset and onset cycle identification and RFF cycle values for (**a**) the VCV utterance /ifi/, and (**b**) the portion /ifq/ in the word “beautiful”.  
Normalizing RFF values from Hz to ST helps to standardize the measurement to account for a speaker's own variations in \(f_{\rm o}\) production across vowels, and additionally allows for comparisons across speakers. Therefore, RFF cycle values reflect a standardized change in the frequency of cycle vibration from the steady state of each vowel. Specifically, an RFF cycle value of 0 ST reflects no change in vibratory timing from the vowel steady state, whereas a positive RFF cycle value indicates higher \(f_{\rm o}\) value relative to steady state (i.e., faster vocal fold vibration) and a negative RFF cycle value indicates lower \(f_{\rm o}\) value relative to steady state (i.e., slower vocal fold vibration).  
Importantly, [PERSON] [12] was the first to publish on the RFF measure using the normalization and conversion procedures explained above. At the time, he referred to the measures only as \"cycle values during devoicing.\" It was not until [PERSON] et al. [13] that the term \"relative fundamental frequency\" was coined, and subsequently adopted across the voice literature.