## 1 Introduction
The authors acknowledged that the force sensor signal could also reflect acoustic or aerodynamic pressure variations due to the size of the sensor extending above and below the plane of vocal fold contact. Both of these in vivo studies demonstrated the critical role of endoscopic image guidance to aid in the verification of intraglottal sensor placement. In practice, these types of experiments require skillful ambidexterity to position two probes at the same time (one hand holding the intraglottal sensor probe and the other holding the imaging endoscope) and to accurately position the intraglottal sensor with minimal tactile feedback from the laryngeal tissue while viewing a two-dimensional video image; laryngologists develop these skills to perform office-based transoral vocal fold injections to treat vocal fold paralysis and other pathologies. Equally challenging is having to take into account that the coronal plane of vocal fold collision is higher (more superior) during phonation than when the vocal folds are at rest during breathing. Relying solely on visual feedback potentially misses capturing the true intraglottal pressure signal during phonation. Multi-channel techniques using data from multiple pressure sensors can aid in the real-time monitoring of intraglottal positioning, e.g., multi-channel electroglottography applies a similar concept to monitor and verify correct electrode positioning at the glottal level in real time [11]. Essentially, if the signals captured by two adjacent pressure sensors are in phase and correlated, then they are both positioned in the same location--supraglottally, subglottally, or intraglottally. If the two sensor signals have waveshapes distinct from each other, their difference signal, polarity, and degree of correlation help confirm that one of the sensors is positioned as desired between the glottis and in the strike zone of collision during phonation [12; 13]. Expectations with respect to the waveform shape of the intraglottal pressure signal come from numerical models of phonation [14; 15; 16; 17; 18; 19; 20], self-oscillating physical models of synthetic vocal fold-like material [12; 14; 21], aerodynamically driven excised larynges or hemilarynx models [13; 22; 23; 24; 25; 26; 27; 28], and in vivo animal models [29]. For example, in a hemilarynx model, the superior-inferior position of a pressure sensor was systematically varied as the model was driven into self-sustained oscillation with an external airflow source [13]. The position of the sensor was confirmed to be in the strike zone of vocal fold collision using a dual-imaging technique that provided en face imaging of the medial vocal fold surface and top-down imaging of the vibrating vocal fold. Taken together with the other modeling results, this body of literature provides strong evidence that the in vivo intraglottal pressure signal during phonation should have three primary components: (1) an impulsive peak in the direction of increasing pressure at the start of the phonatory closed phase (collision/impact component), which is followed in time by (2) a more rounded peak during the phonatory open phase (acoustic/aerodynamic pressure component), and (3) a minimum value around zero immediately preceding the impulsive peak of the subsequent phonatory cycle. To date, these expected characteristics of the intraglottal pressure waveform have not been directly observed in human speakers. Direct measurement of vocal fold collision pressures during bilateral vocal fold phonation has proven to be challenging in vivo [8; 9]. Positioning a sensor against one vocal fold in a typical larynx can result in disrupted vocal fold vibration and irregular glottal closure characteristics. However, these issues can be mitigated in a group of individuals who have undergone surgical treatment for laryngeal cancer but still exhibit functional voice outcomes due to the cancer being largely limited to one vocal fold [30; 31; 32].