Hint
All vowels involve free passage of lung air through the upper vocal tract.
(This is what distinguishes them from consonants, for which, as you began
to see in the last chapter, there is always some obstruction above the level of the
larynx).
The impediment to the airstream for vowels is located at the glottis, not in the supra-glottal tract: the vocal folds are in close enough contact for vibration to occur. The sound wave that this generates is amplified by the resonance of the cavities above the larynx.