In the series 1-8 the tongue is highest in the centre for ee and gradually descends until it lies flat in the mouth for ah.
The upper pharynx is most closed in 1, most open in 8, and closes more and more in the descending series 7'-1'.
The lower pharynx gradually opens in the descending series 7'-1'.
The researches of Helmholtz, Koenig, Willis, Wheatstone, Appunn, Bell, and others have shown that each vowel sound has its own characteristic pitch. The Scale of Vowel Sounds given above corresponds closely to the order of resonance pitch from the highest ee to the lowest oo. In the natural resonance of the vowels ee is highest in the head, ah is midway in the scale, and oo is lowest in resonance.
LIP POSITION
Figure 2 shows the best position of the lips to give the sound of ee. Hold the under jaw without stiffness and as far from the upper teeth as is consistent with delivery of the pure sound of this vowel.
Figure 2.
Figure 3 shows the best position of the lips to produce the vowel oo.
