The mouth actually does not make a good resonator due to its overall fleshiness.
In fact, it must be “de-tuned” as a resonator. This means that the vibrating column of air should not be directed into the mouth as a feel or thought form. There should be no attempt to place the tone “well-forward.”
Rather, the feel of the step on which we stand, even on a low pitch, will still be lifted and not “dropped” into the front and bottom area of the mouth.
Imagine there is a straight line dividing the face from the back of the head. The low pitch steps will be in front of the line and the high pitch steps will be back of the line and up as one sings higher. This is both a feel and a physical reality due to registration and subsequent sound column. On the lower pitches, the feel of the step will be in front of that imaginary line and right underneath the hard palate, still on the same level as soft palate.
The area of the mouth should be set free to do the primary job of “articulation”; moving to shape consonants and finish vowels. When the registration is functioning properly with the air column being of a proper height, diameter, direction, the mouth will be effectively detuned. We will have the feel that the step is lifted above the mouth area, and our articulation improves as a result.