Lets go through the single formant vowels. These vowels exploit our voices ability to create warm vocal qualities. They are formed in the pharynx, starting just above a low seated larynx, and rising above mouth and upper pharynx with the tongue remaining flat. The most closed single formant being [u] and the most open being [ɔ] or [ə]. This is of course assuming that [a] is considered neither single or double formant, as it is fully open and demonstrates qualities of both types of vowels. The larynx should be low, and you should be able to make the [u] with no rounding of the lips, if you can't make this sound without your lips rounding, your larynx is too high, and you are off of your cords. If allowed to develop from the wrong laryngeal position and engagement of the embouchure, as you develop, your attempts to warm or color the tone will either be too breathy, or too pressed. This must be learned before you begin the development of the [i], or the most closed "double-formant" vowel.