The primary vowel series may be modified by the addition of a ‘third resonator’ (temporary resonator) formed by the establishment of a resonant cavity bounded in front by the teeth, above by the front of the hard palate, and to the rear by the tongue. The tongue is positioned with the tip behind the lower teeth, forming a hump under the arch of the hard palate, the blade of the tongue being positioned somewhat vertically.
This relatively small cavity acts upon the primary sound to produce the typical high frequency components (2nd formant) of the acute vowels.
The position of the tongue is adjusted to accommodate the incoming primary vowel.
oo is modified to ee: tongue almost vertical, close to teeth, very small 3rd resonator cavity.
uh is modified to eh: tongue slightly less vertical, slightly larger 3rd resonator cavity.
o is modified to agh: tongue only half-humped, relatively large 3rd resonator cavity.
Once again we must recognise that the ‘in-between’ vowels are part of our sonic palette. Not only the vowels in the series ee-eh-agh, but also the vowels between the primary vowel and the fully formed acute vowel.
The most important of the latter is between ‘o’ and ‘agh, namely ‘ah’ (Italian sound). However, the angle of the blade of the tongue is of great importance is styling. A slight down angle adds a lot of brightness to the sound.