In order to get a big and powerful singing voice, the singer needs to incorporate many singing techniques together.
Now to open up further for bigger singing sound, the singer has to open the area near the base of the tongue. This can be down by imagining the beginning of a yawn. This imaging opens up the singing pathway, keeps the tongue in the correct position and lifts the soft palate making your mouth shaped like a megaphone. A megaphone produces a bigger sound doesn’t it?
With this mouth structure, the back of the throat will naturally open up making the critical link between the vocal tone colors of the upper and lower resonances possible. While you are singing, the inner structure of your mouth is constantly working, moving to fine tune the oral cavity until the feel and sound of any sustained note just right.
The correct position of the mouth should thus be a slight oval like the picture above, with the corners of the mouth drawn in and the jaw down. It is critical that your lips are relaxed. If your lips are not relaxed, tension will accumulate around the jaw and your mouth making your singing tone sounding tight and tensed.
When your singing mouth is in this position, the jaw, tongue and the palate will not be so rigid and will be responding to the direction of the inner ear. As you gain more experience, you will be sensitive to the feeling of the mouth cavity subtly adjusting to each vowel as it fits into the available space in your mouth.
All this amazing mechanism takes place just because of your imaging of the beginning of a yawn. However of you generate a full yawn, then the voice box will be lowered too much and that creates a new obstruction in your singing pathway rather than freeing up your voice so that you can have a big singing voice.
Another source singing pathway blockage is your tongue. Just make sure that your tongue is relaxed and sits comfortably against the back of your bottom teeth. If your tongue pulls back, the airflow will be obstructed and will divide the mouth and throat into 2 resonators. On the other hand, it the tongue lift up too high, the mouth will be separated into two resonator, front and back.
Therefore the optimal mouth position when sustaining tone on a vowel for the big singing voice is a flat forward relaxed tongue position just behind the lower teeth oval mouth position and the imaging of the beginning of a yawn. What follow naturally from here will be a bigger and a more powerful singing voice. To get that big powerful singing voice, regular vocal exercises are crucial.