Investigations of voice using aerodynamic techniques have been reported for more than 30 years. Investigators realized early on that voice production is an aeromechanical event and that vocal tract aerodynamics reflect the interactions between laryngeal anatomy and complex physiological events. Aerodynamic events do not always have a one-to-one correspondence with vocal tract physiology in a dynamic biological system, but careful control of stimuli and a good knowledge of laryngeal physiology make airflow and air pressure measurements invaluable tools.

Airflow (rate of air movement or velocity) and air pressure (force per unit area of air molecules) in the vocal tract are good reflectors of vocal physiology. For example, at a simple level, airflow through the glottis (Vg) is an excellent indicator of whether the vocal folds are open or closed. When the vocal folds are open, there is airflow through the glottis, and when the vocal folds are completely closed, there is zero airflow. With other physiological events held constant, the amount of airflow can be an excellent indicator of the degree of opening between the vocal folds.

Subglottal air pressure directly reflects changes in the size of the subglottal air cavity. A simplified version of Boyle's law predicts the relationship in that a particular pressure (P) in a closed volume (V) of air must equal a constant (K), that is, K = PV. Subglottal air pressure will increase when the size of the lungs is decreased; conversely, subglottal pressure will decrease when the size of the lungs is made larger. Changes in subglottal air pressure are mainly regulated through muscular forces controlling the size of the rib cage, with glottal resistance or glottal flow used to help increase or decrease the pressures (the glottis can be viewed as a valve that helps regulate pulmonary flows and pressures).

A small number of classic studies used average airflow and intraoral air pressure to investigate voice production in children (Subtelny, Worth, and Sakuda, 1966; Arkebauer, Hixon, and Hardy, 1967; Van Hattum and Worth, 1967; Beckett, Theolke and Cowan, 1971; Diggs, 1972; Bernthal and Beukelman, 1978; Stathopoulos and Weismer, 1986). Measures of flow and pressure were used to reflect laryngeal and respiratory function. During voice production, children produce lower average airflow than adults, and boys tend to produce higher average airflow than girls of the same age. Supraglottal and glottal airway opening most likely account for the different average airflow values as a function of age and sex. Assuming that pressure is the same across all speakers, a smaller supraglottal or glottal opening yields a higher resistance at the constriction and therefore a restricted or lower flow of air. The findings related to intraoral air pressure have indicated that children produce higher intraoral air pressures than adults, especially because they tend to speak at higher sound pressure levels (SPLs). The higher pressures produced by children versus adults reflect two physiological events. First, children tend to speak at a higher SPL than adults, and second, children's airways are smaller and less compliant than adults' (Stathopoulos and Weismer, 1986). Intuitively, it would appear that the greater peak intraoral air pressure in children should lead to a greater magnitude of oral airflow. It is likely that children's smaller glottal and supraglottal areas substantially counteract the potentially large flows resulting from their high intraoral air pressures.

Children were found to be capable of maintaining the same linguistic contrasts as adults through manipulation of physiological events such as lung cavity size and driving pressure, and laryngeal and articulatory configuration. Other intraoral air pressure distinctions in children are similar to the overall trends described for adult pressures. Like adults, children produce higher pressures during (1) voiceless compared to voiced consonants, (2) prevocalic compared to postvocalic consonants, (3) stressed compared to unstressed syllables, and (4) stops compared to fricatives.

In the 1970s and 1980s, two important aerodynamic techniques relative to voice production were developed that stimulated new ways of analyzing children's aerodynamic vocal function. The first technique was inverse filtering of the easily accessible oral airflow signal (Rothenberg, 1977). Rothenberg's procedure allowed derivation of the glottal airflow waveform. The derived volume velocity waveform provides airflow values, permitting detailed, quantifiable analysis of vocal fold physiology. The measures made from the derived volume velocity waveform can be related to the speed of opening and closing of the vocal folds, the closed time of the vocal folds, the amplitude of vibration, the overall shape of the vibratory waveform, and the degree of glottal opening during the closed part of the cycle.

The second aerodynamic technique developed was for the estimation of subglottal pressure and laryngeal airway resistance (Rlaw) through noninvasive procedures (Lofqvist, Carlborg, and Kitzing, 1982; Smitheran and Hixon, 1981). Subglottal air pressure is of primary importance, because it is responsible for generating the pressure differential causing vocal fold vibration (the pressure that drives the vocal folds). Subglottal pressure is also important for controlling sound pressure level and for contributing to changes in fundamental frequency—all factors essential for normal voice production. The estimation of Rlaw offers a more general interpretation of laryngeal dynamics and can be used as a screening measure to quantify values outside normal ranges of vocal function.

Measures made using the Smitheran and Hixon (1981) technique include the following:

Measures made using the derived glottal airflow waveform important to vocal fold physiology include the following (Holmberg, Hillman, and Perkell, 1988):

Additional measures important to vocal fold physiology include the following:

Voice production arises from a multidimensional system of anatomical, physiological, and neurological components and from the complex coordination of these biological systems. Many of the measures listed above have been used to derive vocal physiology. Stathopoulos and Sapienza (1997) empirically explored applying objective voice measures to children's productions and discussed the data relative to developmental anatomical data (Stathopoulos, 2000). From these cross-sectional data as a function of children's ages, a clearer picture of child vocal physiology has emerged. Because the anatomical structure in children is constantly growing and changing, children continually alter their movements to make their voices sound “normal.” Figures 1 through 7 show cross-sectional vocal aerodynamic data obtained in children ages 4–14 years. One of the striking features that emerge from the aerodynamic data is the change in function at 14 years of age for boys. After that age, boys and men functionally group together, while women and children seem to have more in common aerodynamically and physiologically. The data are discussed in relation to their physiological implications.

Figure 1..  

Estimated subglottal pressure as a function of age and sound pressure level.

Estimated subglottal pressure: Children produce higher subglottal pressures than adults, and all speakers produce higher pressures when they produce higher SPLs (Fig. 1). Anatomical differences in the upper and lower airway will affect the aerodynamic output of the vocal tract. The increased airway resistance in children could substantially increase tracheal pressures (Muller and Brown, 1980).

Airflow open quotient (OQ-20%): Open quotient has traditionally been very closely correlated with SPL. In adults, it is widely believed that as SPL increases, the open quotient decreases. That is, the vocal folds remain closed for a longer proportion of the vibratory cycle as vocal intensity increases. As seen in Figures 2A and 2B, which show data from a wide age span and both sexes, only adults and older teenagers produce lower open quotients for higher SPLs. It is notable that the younger children and women produce higher OQ-20%, indicating that the vocal folds are open for a longer proportion of the cycle than in men and older boys, regardless of vocal intensity.

Figure 2..  

A, Airflow open quotient as a function of age and sex. B, Airflow open quotient as a function of age and sound pressure level.

Maximum flow declination rate (MFDR): Children and adults regulate their airflow shut-off through a combination of laryngeal and respiratory strategies. Their MFDRs range from about 250 cc/s/s for comfortable levels of SPL to about 1200 cc/s/s for quite high SPLs. In children and adults, MFDR increases as SPL increases (Fig. 3). Increasing MFDR as SPL increases affects the acoustic waveform by emphasizing the high-frequency components of the acoustic source spectra (Titze, 1988).

Figure 3..  

Maximum flow declination rate (MFDR) as a function of age, sex, and sound pressure level.

Alternating glottal airflow: Fourteen-year-old boys and men produce higher alternating glottal airflows than younger children and women during vowel production for the high SPLs (Fig. 4). We can interpret the flow data to indicate that older boys and men produce higher alternating glottal airflows because of their larger laryngeal structures and greater glottal areas. Additionally, men and boys increase their amplitude of vibration during the high SPLs more than women and children do. Greater SPLs result in greater lateral excursion of the vibrating vocal folds; hence the higher alternating glottal airflows for adults. Younger children also increase their amplitude of vibration when they increase their SPL, and we would assume an increase in the alternating flow values. The interpretation is somewhat complicated by the fact that younger children and women have a shorter vocal fold length and smaller area (Flanagan, 1958), thereby limiting airflow through the glottis.

Figure 4..  

A, Alternating glottal airflow as a function of age and sex. B, Alternating glottal airflow as a function of age and sound pressure level.

Fundamental frequency: As expected, older boys and men produce lower fundamental frequencies than women and younger children. An interesting result predicted by Titze's (1988) modeling data is that the 4- and 6-year-olds produce unusually high f₀ values when they increase their SPL to high levels (Fig. 5). Changes in fundamental frequency are more easily effected by increasing tracheal pressure when the vocal fold is characterized by a smaller effective vibrating mass, as in young children ages 4–6 years.

Figure 5..  

Fundamental frequency as a function of age, sex, and sound pressure level.

Laryngeal airway resistance: Children produce voice with higher Rlaw than 14-year-olds and adults, and all speakers increase their Rlaw when increasing their SPL (Fig. 6). Since Rlaw is calculated by dividing subglottal pressure by laryngeal airflow, the high Rlaw for high SPL is largely due to higher values of subglottal pressure, since the average glottal airflow is the same across age groups. A basic assumption needs to be discussed here, and that is, that glottal airflow will increase when subglottal air pressure increases if laryngeal configuration/resistance is held constant. The fact that subglottal pressure increases for high SPLs but flow does not increase clearly indicates that Rlaw must be increasing. Physiologically, the shape and configuration of the laryngeal airway must be decreasing in size to maintain the constant airflow in the setting of increasing subglottal pressures. In sum, children and adults alike continually modify their glottal airway to control the important variables of subglottal pressure and SPL.

Figure 6..  

Laryngeal airway resistance as a function of age and sound pressure level.

The cross-sectional aerodynamic data, and in particular the flow data, make a compelling argument that the primary factor affecting children's vocal physiology is the size of their laryngeal structure. A general scan of the cross-sectional data discussed here shows a change in vocal function at age 14 in boys. It is not merely coincidental that at 14 years, male larynges continue to increase in size to approximate the size of adult male larynges, whereas larynges in teenage girls plateau and approximate the size of adult female larynges (Fig. 7). Regardless of whether it is size or other anatomical factors affecting vocal function, it is clear that use of an adult male model for depicting normal vocal function is inappropriate for children. Age- and sex-appropriate aerodynamic, acoustic, and physiological models of normal voice need to be referred to for the diagnosis and remediation of voice disorders.

Figure 7..  

Length of vocal fold as a function of age and sex.

See also instrumental assessment of children's voice.