Once registration is working reasonably well, one can start trying to maximize one’s vowels, also called “optimizing resonance”, among other things. I’m defining “maximized” as: a vowel that has a strong fundamental plus strong overtones, which is subjectively called things like “full”, “rich”, “complete”, “chiaroscuro”, “resonant”, and other positive terms that imply there is a lot of agreeable sound with the least effort. There is no perfect definition of this complex area called “resonance”. In spite of what I may have left out, I want to talk about how we learn more about resonance, or “maximizing our vocal emission”, in our own voices.
Experimentation and play are required. Your teacher cannot hand you resonance. Your teacher can help you get to the point where you can do the experiments, but they can’t make the sounds for you. Your body, including your vocal tract, is unique, and adjustments that you make for your most beautiful sound will not be exactly the same as someone else’s. You will have to try different mouth openings, different configurations of the squishy parts in your mouth, and different mental concepts. One has to be very, very careful not to stiffen the throat in any way when doing these experiments. One also has to learn what sounds you may own safely and which you cannot. You cannot do all the things, but you can do many, many things!
Dolora Zajick has some great ideas on forming vowels. I have observed her in a masterclass, as well as studied her interview with Terry Gross from 2014 where she gives two short lessons on vowels. Check out 15:52-18:00 and 32:00-36:00 especially, on the podcast: http://www.npr.org/sections/deceptivecadence/2014/03/19/291410969/dolora-zajick-singing-is-connected-to-the-body (I encourage you to download and study it – free!). She advocates having the tongue be the primary former of vowels and she shows clearly how to do so. She said, “resonance is all manipulated between the middle of the tongue and the hard palate.” This will keep your throat and jaw out of it and let your larynx do what it needs to do, reflexively and freely.
Enjoy learning about all the different kinds of sounds you can make. Learn how to make your own richest, easiest, biggest bang for the buck. THAT should be your sound. That is where “opera singing” came from. Trying to “sound like an opera singer” isn’t quite the same thing. The greatest opera singers sound like they do because they maximize their own unique voices, rather than imitating other singers.