In fact, a negative pressure is produced in the wake of an air column that continues to move forward due to its inertia (quantified as acoustic inertance
In this model, the impedance of the waveguide is represented by the acoustic inertance [L.sub.A] given by 
Additionally, the SHS can be modeled with acoustic inertance
Let us suppose that the waveguide has a larger cross section area and therefore the effective acoustic compliance [C.sub.eff] is significantly greater than acoustic inertance [L.sub.A] at near resonance frequency.
With the increasing of frequency, the effective bulk modulus is decreasing, because the acoustic inertance of SHS begins to take more effect.
The first resonance frequency is lowered to around 200 Hz, which means that the lower-frequency harmonics can benefit from vocal tract acoustic inertance in a range from 200-1500 Hz.
The acoustic inertance of the vocal tract also lowers the phonation threshold pressure.
A longer tube produces slightly more acoustic inertance, but the effect is secondary because a semi-occluded vocal tract is already inertive.
The hypothesis here is that whistle voice makes use of a source-vocal tract interaction based on acoustic inertance
below the third formant.