Children form tense and lax vowel classes under aerodynamic constraints.

Abstract

In English, lax vowels contrast with tense vowels as follows: (1) They are always “checked” by a following consonant, (2) they require only moderate displacement of the tongue from its resting position, and (3) they are “short.” If these characteristics are arbitrary and independent–as currently believed–then it is a remarkable coincidence that each divides the vowel inventory into classes with the same membership. Alternatively, these characteristics emerge together under constraints in child speech that do not appear in the adult model. Aerodynamically, the pressures in child speech are higher, flows are similar, and airways are smaller. Mechanically, the respiratory drive that supports speech is pulsatile rather than smooth. This latter difference is heightened in stress‐accent languages, such as English, where a child must reinforce pulses for greater loudness on stressed syllables. The constraints created by these factors require a child to check vowels made with open articulations and lengthen those made with close ones. The former behavior protects the subglottal pressure head. The latter is the indirect result of limiting airflow to avoid unwanted turbulent noise at the point of maximum oral constriction. Increasing laryngeal resistance to do this prolongs the time it takes to dissipate a pulse.