Regulation of Fundamental Frequency (F0)
The fundamental frequency (F0) of voice is the lowest harmonic component in voiced sounds, which conforms
to the natural frequency of vocal fold vibration. F0 changes depending on two factors: regulation of the
length of the vocal folds and adjustment of aerodynamic factors that satisfy the conditions necessary for vocal
fold vibration. In high F0, the vocal folds become thinner and longer; while in low F0, the vocal folds become
shorter and thicker. As the vocal folds are stretched by separating their two attachments (the anterior commissure and vocal processes), the mass per unit length of the vocal fold tissue is reduced
while the stiffness of the tissue layer involved in vibration increases. Thus, the mass is smaller and the stiffness is greater for higher F0 than lower F0, and it follows that the characteristic frequency of vibrating tissue increases for higher F0. The length
of the vocal folds is adjusted by relative movement of the cricoid and thyroid cartilages. Its natural length is a determinant factor of individual difference in F0. The possible range of F0 in adult speakers is about 80–400 Hz in males, and about 120–800
Hz in females.
The thyroid and cricoid cartilages are articulated at the cricothyroid joint. Any external forces applied to this
joint cause rotation and translation (sliding) of the joint, which alters the length of the vocal folds. It is well known
that the two joint actions are brought about by the contraction of the cricothyroid muscle to approximate the two cartilages
at their front edges. Figure 6shows two possible actions of the
cricothyroid muscle on the joint: rotation by the pars recta and translation of the pars obliqua.
Questions still remain as to whether each part of the cricothyroid conducts pure actions of rotation or translation,
and as to which part is more responsible for determiningF0.
Figure 6a-c: Cricothyroid joint andF0 regulation mechanism.(a)The
cricothyroid joint is locally controlled by the thyroarytenoid and two parts of the cricothyroid muscles: Pars recta (anterior) and pars obliqua (posterior). AsF0 rises, the thyroid cartilage
advances and cricoid cartilage rotates to the direction to stretch the vocal folds, which leads to the increases in the stiffness of vocal fold tissue and in the natural resonance frequency of the vocal folds.
(b)Rotation
of the cricothyroid joint is caused mainly by the action of the pars recta to raise the cricoid arch.(c)Translation of the joint is produced mainly by the pars obliqua
The extrinsic laryngeal muscles can also apply external forces to this joint as a supplementary mechanism for regulatingF0.
The most well known among the activities of the extrinsic muscles in this regulation is the transient action of the
sternohyoid muscle observed as F0falls. Since this muscle
pulls down the hyoid bone to lower the entire larynx, larynx lowering has long been thought to play a certain role in F0lowering.
Figure7shows a possible mechanism ofF0lowering
by vertical larynx movement revealed by magnetic resonance imaging (MRI).
As the cricoid cartilage descends along the anterior surface of the cervical spine, the cartilage rotates in a direction
that shortens the vocal folds because the cervical spine shows anterior convexity at the level of the cricoid cartilage.
Aerodynamic conditions are an additional factor that altersF0,
as seen in the local rises of the subglottal pressure during speech at stress or emphasis. The increase of the subglottal air pressure results in a
larger airflow rate and a wider opening of the glottis, which causes greater deformation of the vocal folds with larger average
tissue stiffness. The rate ofF0 increase due to the subglottal
pressure is reported to be about 2–5 Hz/cmH2O
when the chest cavity is compressed externally, and is observed to be 5–15 Hz/cmH2O,
when it is measured between the beginning and end of speech utterances.
Figure
7a,b: Extrinsic control of
F0. Actions of the cricothyroid joint are determined
not
only by the cricothyroid muscle but also by other laryngeal muscles. Any external
forces
applied to the joint can activate the actions of the joint.(a)InF0raising,
advancement
of the hyoid bone possibly apply a force to rotate the thyroid cartilage.
(b)In
F0 lowering, the cricoid cartilage rotates as its posterior plate descends along
the
anterior convexity of the cervical spine
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