### Relative Fundamental Frequency (RFF) as an Estimate of Laryngeal Muscle Tension  
In recent years, relative fundamental frequency (RFF) has been suggested as an acoustic indicator of laryngeal muscle tension. Estimated from short-term changes in instantaneous \(f_{o}\) during intervocalic offsets and onsets, RFF is a non-invasive, objective measure that shows promise in estimating the degree of baseline laryngeal muscle tension. RFF can be calculated from a vowel-voiceless consonant-vowel (VCV) production as in Figure 1. The instantaneous \(f_{o}\) of the ten voiced cycles preceding (\"voicing offset\") and following (\"voicing onset\") the voiceless consonant are each estimated and normalized to a steady-state \(f_{o}\) of the nearest vowel (\(f_{o}^{ref}\)) to produce an RFF estimate in semitones (ST):  
\[\text{RFF (ST)}=12\times\log_{2}\!\left(\frac{f_{o}}{f_{o}^{ref}}\right) \tag{1}\]  
Recent work exploring the clinical utility of RFF for assessing laryngeal muscle tension suggests that RFF correlates with severity of vocal symptoms in speakers with dysphornia [9; 10; 11] and can distinguish speakers with and without voice disorders characterized by excessive laryngeal muscle tension, including individuals with vocal hyperfunction (VH) [9; 12; 13], Parkinson's disease [14; 15], and adductor-type laryngeal dystonia [11]. Specifically, those with voice disorders characterized by excessive laryngeal muscle tension tend to exhibit lower average RFF values, perhaps due to increased baseline laryngeal muscle tension that impedes their ability to leverage tension as a strategy for devoicing (voicing offset) and reinitiating voicing (voicing onset). RFF also normalizes (increases) in individuals with VH following voice therapy (i.e., a functional change; [9; 16]), but not in individuals with vocal nodules or polyps following therapy (i.e., structural intervention; [16]). This suggests that RFF reflects functional voice changes rather than structural voice changes. It has also been demonstrated that RFF captures within-speaker changes in vocal effort [17], or the perceived exertion of a vocalist to a perceived communication scenario (i.e., vocal demand; [18]), as well as indirect kinematic estimates of laryngeal tension [19]. Despite the promise of RFF as an acoustic estimate of laryngeal muscle tension, this measure requires refinement before it will be appropriate for routine clinical use. Figure 1: Acoustic waveform of the vowel–voiceless consonant–vowel production, /ifi/. Voicing cycles preceding the voiceless consonant, /f/, are marked as voicing offset cycles, whereas those following the /f/ are indicated as voicing onset cycles. The first and tenth vocal cycles are highlighted for each transition. RFF can currently be calculated in two ways: manually or semi-automatically. The current gold-standard method of computing RFF is through manual estimation techniques using Praat software [20]. Due to the time- and training-intensive procedures that are necessary to reliably perform manual RFF estimation, a semi-automated RFF algorithm, called the \"aRFF\" algorithm was developed [21; 22]. Both manual and aRFF estimation techniques use autocorrelation to estimate \(f_{o}\), which occurs by comparing a segment of the voice signal with itself when offset by a certain period. Despite being a relatively fast \(f_{o}\) estimation method, autocorrelation assumes signal periodicity and \(f_{o}\) stationarity, requiring 2-3 complete pitch periods to examine the physiological \(f_{o}\) ranges encountered in speech [23]. These characteristics are not ideal for estimating \(f_{o}\) during the voicing offset and onset transitions examined in RFF, which specifically capture rapid changes in \(f_{o}\). Indeed, [PERSON] et al.