In many ways the round vowels the very core of singing. Later in the book you will be dealing with the concept of Foundation Vowel, which as its name implies is the foundation of the singing voice. The Round Vowels encompass the Foundation Vowel, and here is the info you need.
The round vowels result from unrestricted resonance occurring within both Head and Chest resonance channels simultaneously.
When the throat is entirely relaxed, the ‘uh’ vowel is produced. If the upper part of the pharynx is slightly narrowed, the ‘oo’ vowel is produced. If the upper part of the pharynx is slightly widened, the ‘o’ (posh English or London accent eg hot or bottle) is produced.
Note that in British English this vowel is normally only used in a short form. For singing we simply lengthen it. To many British ears this lends it a 'American' flavour but this is however an oversimplification and simply not true.
Fundamental to understanding the voice is understanding that the round vowels are not three isolated and distinct sounds, each made in its own way. Rather, the round vowels are formed when the vocal tract is positioned in a certain way, and this position can be subtly varied from narrow to wide, thereby producing a continuum of sounds from oo to o.
It is important to understand that language flows from the sounds the body can make, and not the other way round. When we allow the body to produce sound in its 'built-in' way, we can produce a wide enough variety of sounds to cover any word in any language.
The lips should not be used to produce the oo (they should be in ‘uh’ position), and the tongue should be flat for all round vowels.
Round vowels are the primary sounds. All others are built by modifying the sound of round vowels.