Voiced sounds generation in the vocal tract
How does it work ? (in less than 300 words)
The human vocal folds are soft structures located within the larynx (the region between the trachea and the pharynx). The slit between the two folds is called the glottis.
Initially the vocal folds are placed close together and the glottis is almost closed leaving just a narrow passage for the air coming from the lungs.
As the flow passes through the glottis it exerts a pressure force, tending to move both folds apart.
Because the vocal folds are made of elastic tissues, an elastic force will appear tending to move the vocal folds back to their rest position.
In addition to these two forces, a third one must be added :
The following equations can be obtained under the assumption of an inviscid steady incompressible flow (for the sake of simplicity) :
u.A = constant (1)
p + 1/2ru2 = constant (2)
where u and p are the local flow velocity and pressure and r is the air density coefficient.
From equation (1) one can see that when the glottal area A decreases, u increases.
From equation (2) one can see that when u increases p has to decrease as u2. As a result one can even expect negative pressures within the glottis (Bernoulli effect) leading thus to a third force that tends to "suck" the vocal folds together.
In summary, due to the hydraulic forces and the mechanical reaction of the tissues, the vocal folds will move apart (the glottis opens) up to the point where the elastic restoring force counterbalances the pressure force. The glottis starts to close then ...
... and will be reopened due to the pressure and elastic forces ...
... and so on ...
