The high vault produces the dark timbres, while the lower arch the clear ones.—M. García

So wrote Manuel García in Hints on Singing in 1894.  It's true of course. But good luck telling this to a student and their being about to demonstrate the difference.

Kinesthetic awareness is a funny thing. More often than not, the student feels what isn't working long before they can identify what is. Ask them what they feel and hear when everything is going well, and they will tell you the voice is free.

Well, of course it is. But detailed information? What are the physiological conditions leading to freedom; to different sounds, to clear and sombre timbre? Not on their radar.

This is why I assert 'singing' is a language. You learn to 'speak' singing by hearing it, afterwards learning its grammar—the physiological stuff. Some never really get a handle on it. And you know what? Not everyone has the wherewithall to be the vocal equivalent of a wordsmith. I've quoted her before, but García's student Anna. E. Schoen-René is worth remembering on this point.

Scientific explanations can only be grasped by singers already educated in the principals of their art- from America's Musical Inheritance (1941) by Anna E. Schoen-René

What were the principals taught by Schoen-René? Pure vowels for one thing—though it's not a term you are likely to hear much since it's considered obsolete, 'formant-speak' being all the rage.  

Students were taught to 'call' on /a/, which 'opened' the throat. My contention is: if you can't hear what this means, you won't be able to feel it either. In that sense, Schoen-René is talking about the education of the singer's listening faculty, one that is active rather than passive.

I find myself telling students that “listening is a feeling,” which turns the mechanical approach on its ear. Why do I do this? Because instruction to 'move the parts' all too often leads the student's attention away from listening to mechanically manipulating the tone—which results in bungling.

Here's the thing. Get a student to experiment with sound. To imitate clear vowels and then guttural and nasal ones and observe the body while doing it again and listening and feeling.

We learn by contrast, by listening and feeling, not by trying to mechanically make something happen.

Back to /a/. The English language has the tendency to gutturalize it, which is far from what García is writing about. Whether the vowel is clear or sombre, it's 'pureness' depends on the vowel being 'rounded'. How can this rounding be experienced? By practicing a very clear /o/ vowel.  How it this done? By being a kid again and exclaiming:  Oh boy! 

/o/ is a very high vowel.  It embodies, demands and invokes awe and wonder. The spine extends, its muscles gently yet firmly rounding. The naso-pharynx arches in response, rising even higher for /u/.

/a/ is held from heaven.  

Photo: The Cloisters, Norwich Cathedral