Aerodynamic and Acoustic Power in Infant Cry.

OBJECTIVES: Acoustic and aerodynamic powers in infant cry are not scaled downward with body size or vocal tract size. The objective here was to show that high lung pressures and impedance matching are used to produce power levels comparable to those in adults.

STUDY DESIGN AND METHODOLOGY: A computational model was used to obtain power distributions along the infant airway. The parameters were cross-sectional areas of the larynx canal and the velar region, which tend to be variable in infant cry when there is no specific vowel structure.

RESULTS: Aerodynamic power can reach 1.0 W with 70 cm H2O lung pressure, while acoustic power radiated from the mouth reaches about 1.0 mW, corresponding to 90 dB sound level at 30 cm from the mouth. Acoustic evidence of potential roughness is introduced with a narrow larynx canal (epilaryngeal airway).

CONCLUSIONS: Infants are born with the ability to produce high lung pressures to compensate for reduced airflow in a small vocal tract. Airflow impedances are about 100 times higher than in adults. Maximum power transfer from the source to the airway (and ultimately to the listener) appears to be an innate ability.