Developing an Empirical Acoustic Transfer Function Model for the Upper Airway

In this pilot study, we introduce a novel approach for the upper airway acoustic modelling aimed at developing a patient-specific transfer function of the upper airway. We modelled the upper airway as an acoustic filter, and hypothesized the parameters of such model would correlate with the anatomical features.

The method involved generating a signal with known frequency characteristics at the mouth while recording the output at the suprasternal notch. Five distinct protocols were tested, and a consistency study was conducted to identify the most suitable protocol. The protocols varied in terms of input sound type and breathing maneuvers. Ten healthy subjects participated in this pilot study over three days with four recordings per day.

The results indicated that the most consistent protocol utilizing white noise as the input sound while the participant breathed passively. The standard error of the difference between the detected peak frequencies was less than 10%, and the standard error for the amplitude difference was less than 5 dB. Additionally, the transfer function for each participant was modelled as a cascade of six 2nd order systems. A strong negative correlation of - 0.91 (p = 0.0003) was found between the first natural frequency and the participants' height, while a positive correlation of 0.69 (p = 0.027) was observed between the transfer function gain and the participants' BMI.

This study presents an initial step toward developing an acoustic transfer function of the upper airway, which could ultimately be used to classify and diagnose respiratory disorders.