As I started to outline potential posts in this series I came up with a ridiculous number--only some of which may actually happen before the summer hiatus! One of the problems is that so many areas overlap--you can't talk about vowel without talking about vocal technique or your idea of an ideal sound. So some posts will end up relating to others. The nature of the beast, I think.
So to begin by asking, "what is good intonation?" "What are your standards?"
I think all of us will have differing ideas, depending on our experience (how we're trained, who our models are) and the level of our ensembles.
Our ears (or more accurately, our brains) will adjust perception of what is acceptable--in a very average (or slightly below average) choir, for example, the range of pitches perceived as "in tune" will widen. On the other hand, in an advanced group, especially if they sing with much closer tolerances of pitch, small deviations from what is now established as "in tune" will be very noticeable. So the consequence of singing with narrower tolerances in pitch is that deviations are heard more easily! It's a risk I'm willing to take, however!
That rather wide tolerance of deviation from a central pitch is also, however, what creates the sort of "gray" lack of color one hears in many amateur choirs (or amateur bands--that sort of washed out sound in a large group of flutes, none of which are zeroed in on the center of the pitch). The lack of a clearly focused pitch means that overtones are not re-inforced and the sound doesn't project nearly as well. Having heard the Swedish Radio Choir up close (and prepared them for a Brahms Requiem, among other works), they have the ability with just a few added singers (48 when I prepared them) to sing the Brahms with a full orchestra and yet be heard clearly. The combination that makes it possible is 1) big, trained voices who sing with full "singer's formant" 2) great ensemble and ability to unify vowel and 3) minimal vibrato (but they do sing with it!) and absolutely focused, unified pitch.
My experiences with Swedish choirs, plus a long interest in early music, mean that I look for less vibrato than some, but singers still singing with full singer's formant, and clearly defined pitch. Additionally, if you are to tune with either "just" intonation or earlier tuning systems (quarter-comma meantone) with very pure thirds (lower than the thirds on the piano), you have to use less vibrato to hear the difference (too much vibrato blurs the two types of thirds).
That means I want a narrow definition of what a good unison is and how much tolerance there is for chords to be in tune. Of course, that changes with style--if I conduct the Verdi Requiem, I'll expect more vibrato and the consequent widening of what is allowed in a unison on one or multiple parts. Some research shows that with vibrato, humans perceive the mean frequency as the center of the pitch. One study of modern performances of Schubert's Ave Maria showed a mean variation of plus or minus 71 cents (100 cents is a half step) and more variation in a Verdi aria. Of course, there is also a difference between the perception of a solo singer with orchestra and a group of singers. If you're like me, you've heard some performances of the a cappella quartet in the Verdi Requiem where you couldn't recognize the chords!
So, opening salvo!
Next, on to tuning systems.
If you want to hear what my groups do (hard to discuss sound without hearing--and being able to say, "Oh, THAT'S what he means!" by the amount of vibrato or something else), here are two examples with my Collegium Singers:
Handel Dixit Dominus from last Thursday: http://www.youtube.com/watch?v=jz4ZVusFsTo
Victoria Requiem from the Berkeley Early Music Festival last summer with just 14 singers from last year's group: http://www.youtube.com/watch?v=EUBIxg6ZpKE
In the Victoria, in particular, because it's a small group (3 each on soprano, 2 on other parts) singing with very minimal vibrato, you'll see that small deviations in pitch are more noticeable. In the Handel, because of the virtuosic nature of the vocal writing, my singers have freer range in terms of vibrato, although there may still be some who think I'm restricting vibrato more than their taste -- and taste does enter into it!