Vocal membrane model equations

System of coupled harmonic oscillators

The derivation of the equations of motion follows the sketch of the mechanical model for nonhuman mammalian vocal membrane systems. Details of the deriva-tion of the model equaderiva-tions from Newton’s laws can be found in Appendix C.1, page 193.

The dynamics of the model masses was described by a system of coupled

har-monic oscillators. The lower masses (vocal fold) were coupled with each other only by linear elastic restoring forces. Due to the rigid connection of the upper mass and the vocal membrane plate, the upper subsystem contained both dynamic coupling (due to conservation of linear momentum and angular momentum) and aerodynamic coupling (due to forces from the air flow acting on the vocal mem-brane).

Left and right model masses interacted due to the airstream through the glottal channel and restoring forces during vocal fold contact. The equations of motion are given for one side of the model only as the model is assumed to be left-right symmetric.

The dynamics for the lower mass m₁ of one vocal fold could be written as:

m₁x¨₁+ r₁˙x₁+ k₁x₁+ k_c(x₁− x₂) = ld₁p₁+ H(−a₁)c₁|a₁|

2l (5.1)

with ˙x = _dt^dx and the Heaviside function H(a). The length of the vocal folds was given by l, the height of the lower masses was d₁. The cross-sectional area of the glottal channel between the lower masses was a1 = 2l(x1+ x01). The aerodynamic pressure p₁ acted on the walls of the lower glottal channel. During contact of the lower masses, the model masses overlapped. The additional restoring force during contact was proportional to the overlap length ^|a_2l^1|. Thus, after collision, the elastic restoring force −k₁|x₁|, (x₁ < 0) became −k₁|x₁| − c₁^|a_2l^1| which corresponded to an abrupt change of the stiffness constant k₁.

The equation of motion for the upper masses m₂–m₃ of one vocal fold were given by: The height of the upper mass m₂ was d₂; the height of the vocal membrane plate was d3. θa was the absolute vocal membrane angle, as given below. The aero-dynamic pressures p₂ and p(y) acted on the walls of the uniform upper glottal channel and on the reed-like vocal membranes respectively. Due to the rigid con-nection between upper mass m2 and the vocal membrane plate m3, the dynamic coupling term −¹₂m₃d₃ ¨θ cos(θ_a) − ˙θ²sin(θ_a)

appeared. The first part ¨θ cos(θ_a) was due to conservation of linear momentum in the upper subsystem, the second part ˙θ²sin(θ_a) described the centrifugal force of the rotating vocal membrane on the upper mass.

The dynamics for the vocal membranes could be derived as:

where the absolute vocal membrane angle was given by:

θ_a(t) = θ₀ + θ(t) (5.4)

The expression m₃^d₃²³ was the moment of inertia of the vocal membrane plate with constant line mass density m₃/d₃ with respect to the given pivot point (see Fig. 5.8). The dynamic coupling −¹₂m3d3x¨2cos(θa) was due to the conservation of angular momentum in the rigidly coupled upper subsystem. The aerodynamic pressure p(y) on the vocal membrane walls resulted in a torque on the vocal mem-brane.

dynamic coupling of vocal membrane to vocal fold

The coupling terms between the vocal membrane m3 and the upper mass m2 of the two-mass system are briefly discussed. Setting the driving pressures to zero, neglecting contact between left and right model masses, and setting the upper masses and the vocal membranes at rest at their resting positions x02 and θ0,

˙x₂ = x₂ = x₁ = p_i = p(y) = ˙θ = θ = 0 , (5.5) then the equations of motion for the upper subsystem were given by:

1. (m₂+ m₃) ¨x₂ = ¹₂m₃d₃ ¨θ cos(θ_a) − ˙θ²sin(θ_a)

: Here the upper mass was accelerated in the positive direction due to an acceleration ¨θ of the vocal membrane, a consequence of the conservation of linear momentum. For finite angular velocity ˙θ, the upper mass was accelerated in the negative direction due to the centrifugal force of the vocal membrane acting on m₂. Both contributions depended nonlinearly on the absolute angle θ_a = θ₀+ θ(t) of the vocal membrane.

2. m₃^d₃²³θ =¨ ¹₂m₃d₃x¨₂cos(θ_a): Due to the conservation of angular momentum, the vocal membrane was accelerated in the positive direction by a positive acceleration ¨x₂ of the upper mass.

Aerodynamic driving forces and torques

The vocal membrane model was driven by the aerodynamic pressure from the air flow acting on the walls of the glottal channel. Using Bernoulli’s equation, valid along a streamline in the open glottis, the pressures of the quasi-steady, laminar, inviscid and incompressible flow could be derived. A jet was assumed to separate from the glottal walls at the narrowest point in the glottis. Downstream of the jet separation point the air outside of the jet was assumed to be stagnant, whereas the flow inside the jet was assumed to be laminar. Turbulence effects, such as mixing layers and vortex shedding, were neglected. Details on the derivation could be found in Appendix C.2, page 198.

The driving pressure p₁ on the massless plate connected to the lower mass m₁ could be written as:

p₁ = p_s 1 − a_minH(amin) a₁

2!

H(a₁) (5.6)

The Heaviside functions H(a) assured the open glottis condition for Bernoulli’s equation while the glottal area a1 = 2l(x1 + x01) and the minimum glottal area a_min could become negative.

The aerodynamic pressure p₂ on the massless plate connected to the upper mass m2 read:

p₂ = p_s 1 − a_minH(a_min) a₂

2!

H(a₁) H(a₂) H(a₁− a_vm) H(a₂− a_vm) (5.7)

The Heaviside functions H(a) provided the case differentiations with respect to the glottal areas a₁, a₂ = 2l(x₂+ x₀₂) and the area at the tip of the vocal membranes avm.

The pressure distribution along the vocal membranes m₃ was given by:

p(y) = ps 1 − a_minH(a_min) a₃(y)

2!

H(a1) H(a2) H(a1− avm) H(a2− avm) (5.8)

The area function a₃(y) for the glottal channel formed by the vocal membranes could be written as:

a₃(y) = a₂− 2l tan(θ_a) y, y ∈ [0, d₃cos(θ_a)] (5.9) The area at the tip of vocal membranes was given by:

a_vm= a₃(y = d₃cos(θ_a)) = a₂− 2ld₃sin(θ_a) (5.10)

-6 -4 -2 0 2

-6 -4 -2 0 2

0 30 60 90

absolute vocal membrane angle Θ_a [degrees]

-6 -4 -2 0 2

a₁ a₂₃ α

Convergent glottis Parallel glottis Divergent glottis

linear acceleration [cm/ms2 ], angular acceleration [rad/ms2 ]

Figure 5.9: Aerodynamic accelerations for different glottal configurations. Vocal membrane parameters were set at m₃ = 0.25 and d₃ = 0.05, otherwise standard parameter values were used.

Thus, the minimum glottal area could be calculated as:

amin = min(a1, a2, avm) (5.11) Details on the evaluation of the integrals for the aerodynamic force and torque contributions can be found in Appendix C.2, page 198. Here the forces and torque were visualized for different glottal configurations. The forces and torque were scaled by the associated masses and moments of inertia. Therefore, the following linear and angular accelerations were plotted:

a^{airf low}₁ = ld₁p₁

where a^{airf low}₁ denoted the acceleration of the lower mass m₁, and a^{airf low}₂₃ described the acceleration of the upper mass–vocal membrane system m₂–m₃. The angular acceleration of the vocal membrane was α^{airf low}.

In Fig. 5.9 the aerodynamic accelerations a^{airf low}₁ , a^{airf low}₂₃ and α^{airf low}for differ-ent glottal configurations are shown. For these plots, the chosen parameter values were m₃ = 0.25, d₃ = 0.05. All other parameters were set to their standard values (Table 5.1 on page 122).

1. In the case of a divergent glottis, i.e. divergent vocal folds (e.g., here: a₁ = 0.025, a₂ = 0.05), all aerodynamic forces vanished until the vocal membrane area a_vm(θ_a) was the minimum area at θ_a > 0. Then, there was a smooth increase of the force on the lower masses. The forces and torques on the upper masses and the vocal membranes jumped abruptly to finite values. When the vocal membrane area was zero, the full subglottal pressure p_s acted on the lower masses. Due to decreasing effective vocal membrane height d^{ef f}₃ after vocal membrane contact, the forces and (absolute) torques on the upper masses and the vocal membranes decreased gradually.

2. For a parallel glottis (here, e.g., a₁ = a₂ = 0.05), all aerodynamic forces smoothly increased for θ_a≥ 0.

-4 0 4

-4 0 4

-4 0 4

-4 0 4

-4 0 4

0 30 60 90

absolute vocal membrane angle Θ_a [degrees]

-4 0 4 linear acceleration [cm/ms2 ], angular acceleration [rad/ms2 ]

d₃ = 0.01

d₃ = 0.05 d₃ = 0.10 d₃ = 0.20 d₃ = 0.40

d₃ = 0.80

Figure 5.10: Aerodynamic accelerations for different vocal membrane heights. Vo-cal folds were chosen parallel: a₁ = a₂ = 0.05. Vocal membrane parameters were set at m₃ = 0.25 and d₃ = 0.05, otherwise standard parameter values were used.

3. When the glottis was convergent, i.e. the vocal folds were convergent (e.g., here: a₁ = 0.10, a₂ = 0.05), the aerodynamic pressure on the lower masses was finite for negative vocal membrane angles θ_a < 0. All other forces and torques on the upper masses and the vocal membranes were zero until the vocal membrane tip area a_vm(θ_a) became the minimum area in the glottis.

Fig. 5.10 shows the influence of the vocal membrane height on the aerodynamic accelerations. The glottis was chosen parallel: a₁ = a₂ = 0.05. The vocal mem-brane mass was m3 = 0.025, all remaining parameters were set to their standard values (see Table 5.1 on page 122). If the vocal membranes were short enough not to be able to close the glottal channel completely (here: d₃ = 0.01), the aerody-namic forces and torques changed smoothly with varying vocal membrane angle θa. For increasing vocal membrane height the increases of the forces and torques on the upper masses and the vocal membranes for θ_a > 0 became steeper and steeper.

The height d₃ had no effect on the driving force for the lower masses m₁. For very long vocal membranes (here, e.g., d₃ = 0.80) the aerodynamic acceleration of the upper mass m₂ could become arbitrarily large, even larger than the acceleration of the lower mass m₁.