Voice and Resonance Disorders 69the larynx. The abductor muscle, the posterior cricoarytenoid muscle, is primarily responsible for
opening the glottis, the opening at the level of the larynx. The lateral cricoarytenoid, transverse
arytenoid, and oblique arytenoid muscles are the primary adductors of the vocal folds (Stemple,
1984; Zemlin, 1998). During voicing, respiratory support is adjusted for the impedance (re sis tance)
in the larynx; when the impedance to airf low changes, there is a consequent adjustment in respira-
tory support. Without respiratory support adjustments, utterances would have inappropriate pitch
and loudness changes.
Age and Gender Differences in Phonation
The vocal cords vibrate very rapidly, and the vibratory sound produced at the level of the
larynx is called phonation. In women, the vocal cords vibrate about 250 cycles per second, and in
men, about 130 cycles per second. The vocal cords vibrate more frequently in women because the
female larynx tends to be smaller and is usually shaped somewhat differently from that in men.
This difference in laryngeal shape also contributes to the prominent thyroid notch in many men.
In addition, Zemlin (1998) notes that with increasing age, the laryngeal cartilages tend to ossify,
contributing to pitch alterations and airway re sis tance, which are greater in women than in men.
According to Kent (1997), besides having a higher pitch, women tend to have a breathy voice qual-
ity. From about age 4 to 10 years, the fundamental frequency is stable and typically the same for
both boys and girls. At about age 12, the male fundamental frequency changes and remains about
one octave below that of the female (Kent, 1997). Falsetto, the extreme upper limit of the pitch
range, occurs in both men and lower- pitched women (Zemlin, 1998). With advanced age, there
is typically a small increase in male fundamental frequency, whereas in women, there is a small
decrease.
Pitch-Changing Mechanism
The pitch- changing mechanism is a complex interaction between respiratory support and
laryngeal activities (Table 4-1). Several variables affect pitch. Adjustments affecting the mass per
unit length of the vocal folds account for decreases and increases in fundamental frequency. When
the mass per unit length of the vocal folds increases, the vocal folds become thicker and shorter,
with a consequent decrease in pitch. Conversely, when the mass per unit length of the vocal folds
decreases, the vocal folds become thinner and longer, with a consequent increase in pitch. Vocal
fold tension also affects pitch; relative increases and decreases in tension result in corresponding
increases and decreases in pitch. Fi nally, when all other variables are held constant, an increase in
subglottal air pressure increases pitch and vice versa.
Figure 4-1. Anterior and posterior views of the larynx.