## 4 Discussion
### Intraglottal and Vocal Fold Collision Pressure Waveform Characteristics  
To the authors' knowledge, the results of this study represented the first time that direct measurement of in vivo vocal fold intraglottal/collision pressures resulted in expected waveform characteristics in a human speaker. In previous work [8], it was found to be more challenging to place a sensor between two vibrating vocal folds in a vocally healthy individual than it was to perform this procedure in an individual with one functional vocal fold (following a hemilarynectomy surgical procedure). This previous proof-of-concept study demonstrated that it was possible to safely acquire estimates of vocal fold collision forces in one vocally healthy individual (with two functional vocal folds) and three individuals with a hemilarynectomy. Conclusions from that work were that: (1) simultaneous measurement of subglottal pressure is necessary to accurately interpret the vocal fold collision data, and (2) measures for the individuals with a hemilarynectomy were easier to obtain and were more reliable and valid than those obtained for the vocally healthy speaker. However, the intraglottal force sensor used in that study had a relatively large sensing area and yielded triangular waveforms that did not appear to exhibit the signature impulse-like component at the time instants of vocal fold collision (an electroglottography signal provided verification of the instants of glottal closure).  
Building on this previous work, in the current study, verification of the correct placement of the intraglottal pressure sensor was accomplished using novel, concurrent recordings of laryngeal high-speed videendoscopy and a subglottal pressure signal. Our prior investigations of a hemilarynx model took advantage of a dual high-speed videendoscopy setup that offered visual verification of sensor placement from a top-down, endoscopic view and a medial, en face visualization of the of the hemilarynx setup [13]. That work concluded with the following guidelines for using the ISP probe in vivo:  
1. Endoscopic visualization is necessary to guide placement of the ISP probe such that the distal pressure sensor at the probe tip is positioned subglottally and the proximal sensor is positioned in the glottis in the phonatory strike zone to sense vocal fold impact collision pressure during phonation.
2. In individuals with a hemilarynectomy, the ISP probe should rest on the medialized scar band that replaces the excised vocal fold tissue, such that the pressure-sensing element comes into direct contact with the functioning vocal fold.
3. The positioning of the intraglottal pressure sensor is in the phonatory strike zone if the following waveform characteristics are exhibited: 1. An impulsive peak in the direction of increasing pressure at the instant of vocal fold contact; 2. A more rounded peak following the impulsive peak that senses aerodynamic pressure build-up during the open phase; and 3. A minimum value approaching zero or negative pressure immediately preceding the impulsive peak of the subsequent phonatory cycle, reflecting rapidly decreasing intraglottal pressure as airflow accelerates.  
Figure 7 displays exemplary intraglottal pressure waveforms observed in the literature according to numerical, physical, excised, and in vivo animal models of phonation. The expected waveform characteristics were observed during the recording in this study as shown in Figure 4.