Transcript Phonation

Phonation
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Physiology
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Phonation = series of openings and closings
of the vocal folds
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Two phases
1.
Prephonation phase: period during which
VFs move from abducted to either
adducted or partially adducted position.
Prior to onset of phonation, folds are in
the abducted position.
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Vocal fold approximation: folds
approximate and pressure beneath
folds builds. Velocity of air through
the glottal constriction is raised
sharply, and pressure drops.
Extent to which v. folds are
approximated call medial compression
(force with which folds brought
together), caused by action of
adductor muscles.
2.
Attack phase
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Vocal folds are adducted and go thru
initial vibratory cycles.
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Initiation of phonation:
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As folds begin to approximate, a
critical closure value is reached.
VFs begin to vibrate before they have
actually approximated.
Folds undergo a # of vibrations before
they meet completely to obstruct the
air stream.
As long as subglottal pressure is
adequate, medial compression of folds
will be overcome and they will be blown
apart, releasing a puff of air into
supraglottal area.
These air puffs are source of voicing.
The elasticity of folds and Bernoulli
effect causes folds to snap back to
midline (myoelastic aerodynamic theory
of VF vibration)
– Theories of VF vibration
• Myoelastic aerodynamic theory: Phonation
made possible due to aerodynamic and tissue
force of laryngeal muscles.
– BERNOULLI EFFECT: as air pressure
decreases, velocity increases
» Air has a constant velocity until
reaches constriction. Used to
explain why airplanes rise in the air.
Velocity will increase as air passes
thru glottal opening. The result is
negative pressure between the
medial edges of folds and they are
drawn toward each other.
– Folds also approximate due to myoelastic
properties.
» Muscles adduct folds and apply
tension or stiffness to folds
» Viscous (elastic) force created by
density of VF tissue
» More viscous, more resistance to air
flow.
» Mechanical coupling stiffness =
outermost layer of VFs functions as
if composed of two different
masses. Lower mass pulls upper
mass away from midline when air
pressure is great enough.
• Three mass model of VF vibration
– Problem with myoelastic aerodynamic
theory:
» Doesn’t explain how VFs sustain
oscillation
– This model adds the supraglottal area to
the equation.
» Air pressure from lungs always has
to be positive
» Air flows from positive to negative
pressure locations
» Air pressure has to be negative at
the level of the glottis, but how is it
created?
» Supraglottal airflow continues
outward, even during VF closure,
creating negative pressure just
above the folds
» Air pressure is the driving force of
sustained oscillation
http://www.ncvs.org/ncvs/tutorials/voiceprod/
tutorial/model.html
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Characteristics of the Vibratory Cycle
1.
Glottal area (at normal pitch and loudness
levels)
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opening phase-around 50% of one cycle
– Subglottic pressure?
closing phase-@ 37% of one cycle
closed phase-@ 13%
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Subglottic pressure?
2. Mode of Vibration: (in modal register)
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A. Opening/closing: v. folds begin to
open (normally) posteriorly, with chink
moving anteriorly. Closure begins with
entire medial edge moving toward
midline. Posterior part last to close.
• B. Vibration: At conv. pitch and
intensity levels, v. folds vibrate almost
in their entirety.
• C. Vertical phase difference: v. folds
begin to be forced open from beneath
or at bottom of closure. Lower edges
first to be blown apart, upper edges last
to be blown apart. During closing phase
lower edges close first, then upper
edges.
Important factors governing
vocal fold vibration
1) vocal fold position - degree of adduction
glottal adductors:
a) lateral cricoarytenoid m
b) transverse
interarytenoid m.
c) oblique interarytenoid
2) vocal fold myoelasticity - length + tension
glottal tensors/relaxors:
a) cricothyroid m.
b) thyroarytenoid m.
3) “amount/size” of pressure drop along the folds
Fundamental Frequency Change
1.
VF length
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At modal frequencies, as VF length increases,
frequency increases.
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2.
Seems to go again what happens with stringed
instruments
Reason for difference: the vibrating portion of
the VFs decreases as frequency increases.
VF mass
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3.
As mass increases, frequency decreases
VF tension
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As tension increases, so does frequency
Loudness Change
Degree and timing of VF closure
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If VF closed time is increased, then there is
more time to build up pressure beneath then.
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Louder sound occurs when subglottic air pressue
is adequate enough to blow VFs apart (overcome
the resistance).
Resistance is the important factor for
intensity.
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The more resistance there is, the more
pressure is needed to overcome the resistance.
Thus, when the resistance is overcome, the
pressure disturbance is greater = louder