Well, I have a theory which has no actual physical observation for the singer, but rather a feeling (or hearing) of what happens when you sing either sound. Because of this I always prefer the "oo." Here's why.

A slightly open jaw and "o" shaped lips bring the "oo" vowel sound forward in the mouth. The formation of the vowel is also concentrated in the front of the mouth, behind the front teeth, and not back in the throat. The lips need to take on a circular shape. All of this makes the sound light and in tune/on pitch. It is also easier to take a quick breath without greatly moving the lips or mouth. (Keep the shape exactly the same as much as possible--even when breathing.) One of the strong points of "oo" is that it uses less air.

When you have a sustained "mm" the sound tends to be back in the throat, and in most cases a bit flat in pitch. (May sound right to the singer, but isn't.) An open jaw also gives a very different sound than a closed jaw. Which do you want? Breathing in the "mm" also requires you to open your mouth more, with more space, and a greater need for air. The "mm" sound is also very nasal in character whether or not the jaw is open or closed. Either way, it is heavy--not the lighter sound of "oo" which is the easiest vowel to sing (air wise) and keep in tune.

Another whole ballgame is "ah." I would NEVER use this sound for long sustained passages! It takes too much air, is hard to shape uniformly, will probably be too loud which makes it go flat, and is usually placed in the back of the throat making it not very pleasant to listen to. Use either an "oo," (as in tune) or if you need a bigger sound, an "o" (as in boat). The only time an "ah" would be used is when there are contrasting sections using a different sustained sound for each section. (LORD, LISTEN TO YOUR CHILDREN is a good example of this.)

All of these sounds are used as accompaniments in some way or style. They may cover for a vocal soloist, a piano solo (GREEN GROW THE LILACS), or it may be simply to set a quiet, subdued tone and mood. When using sustained sounds, the composer/arranger has made a choice, and you need think carefully and decide if that choice fits your group. You are not changing a text, but creating an accompaniment sound. What matters is that it should make your chorus sound good. The notes (pitches and rhythms) will be the same, but not necessarily the "mm" or vowel sound. Make sure it is in tune, is uniform in production/shape (among your singers), and it doesn't distract from the music of which it is a part. No matter how it is used, it is a secondary sound to something else. Always keep that in mind--and in tune!

DWCrouch
June, 2002