Inertagrams for a Variety of Semi-Occluded Vocal Tracts

Objective This investigation addressed the desirable source-airway interaction obtained with vocalization through a semi-occlusion at the mouth. The semi-occlusion was a flow-resistant tube of varying length and diameter. Method The methodology was strictly computational. Airway shapes resembling those obtained from magnetic resonance or computed tomography imaging were stylized. Supraglottal and subglottal acoustic impedances were calculated, from which inertagrams were plotted over a frequency range of 0-4000 Hz. The inertagrams predict which harmonics are likely to be strengthened. Results The combination of epilaryngeal airway narrowing and lengthening, a pharyngeal expansion, and an oral narrowing behind the tube produce the best overall inertagram for pitch glide exercises. This configuration supports harmonics of the source evenly over a range of 300-4000 Hz. Conclusions For clinical applications, theory predicts that the length of a tube or straw does not matter if the inner diameter is on the order of 3 mm or less. For wider open-ended tubes, greater length can compensate for the wider diameter for the beneficial inertance effect, but the desired steady pressure in the airways for vocal fold posturing cannot be maintained.