## 6 Discussion
### Comparison of Results to Prior Work  
Prior attempts at measuring in vivo vocal fold collision pressure using a single sensor have yielded mixed results, including uncharacteristic intraglottal pressure waveshapes with no impulsive peak signature [9] or impulsive pressure signatures without information regarding sensor positioning or concomitant subglottal driving pressures [7]. In the current work, a dual-sensor ISP probe was developed to mitigate issues related to the uncertainty in sensor positioning. Incorporating multiple pressure taps in physical models of phonation is common [37], and adding a second sensor to the in vivo ISP probe was a natural step to provide important information to the endoscopist regarding subglottal pressure characteristics. Emphasis was placed on characterizing the intraglottal and subglottal pressure waveshapes using a human hemilarynx model such that these waveform properties can then be translated to measurement in the human in vivo setting.  
In agreement with prior work that employed animal hemilarynx models [12; 37], silicone vocal fold models [10; 36], and numerical modeling [40], the intraglottal pressure signal in the current human excised larynx study exhibited both contact/collision and aerodynamic components separated in time. In particular, sweeping the vertical (superior-inferior) position of the pressure sensor from a supraglottal to a subglottal location was performed to mimic the experimental protocol of [10]. In that study, the pressure sensor position was swept in increments of 0.5 mm from 2 mm above to 2 mm below a silicone hemilarynx model. Results of that work indicated that measurement of peak collision pressure may exhibit underestimation of up to 20% relative to the true collision pressure when the sensor was as close as 0.5 mm above or below the strike zone. This underestimation was hypothesized to be due to the sensor simultaneously capturing both impact and aerodynamic pressures. In addition, the pressure waveform at the offset location was similar to that in the strike zone such that one would not know of the offset sensor position without independent verification. Similar results were observed in the current study when the pressure sensor was swept from above the glottis to a subglottal location (see Figures 8 and 9). However, knowledge of the simultaneous recorded subglottal pressure waveform enabled immediate feedback as to any differences from the intraglottal pressure waveform. Another  
Figure 11: Effect of increasing subglottal pressure (SGP) on the mean peak collision pressure (PCP). Also displayed for each trial is the PCP/SGP ratio.  
telling signature that the pressure sensor was in the strike zone was a strong return-to-zero-pressure property of the negative-going peak.  
In the hemilarynx experiment, the PCP/SGP ratio was approximately one at higher subglottal driving pressures, with PCP/SGP decreasing to half that value at lower driving pressures (Figure 11). The PCP/SGP ratio has been reported in a previous excised canine hemilarynx experiment to range from 1.1 to 3.8 [12]. Recent work employing a silicone hemilarynx model exhibited a PCP/SGP ratio of 1.15, with medial compression properties of the laryngeal configuration having a significant impact on the ratio [10]. Further work is needed to understand the variables (mode of vibration, rheometric tissue properties, etc.) associated with the conversion of aerodynamic power to vocal fold collision.