A few other vocal issues to consider in terms of intonation:
- Extremes of range - moving to the top of any singer's range will be a challenge--vocally and therefore for intonation as well. This is an area that needs the kind of long-term vocal work that I've already described: vocalizes that help singers learn how to sing high notes well and modification of vowels, especially for sopranos as they move to the top of the staff and above (closed vowels will need to be more open) - these aren't short-term fixes (although if you've already been working on these techniques, sometimes a reminder is all that's needed)
- Music written in the passaggio or break -- this is too complex a topic for a brief blog, but we're speaking of the transitions from one register to another -- suffice it to say that you need to learn how to help your singers deal with this issue and it's always an area to consider when diagnosing intonation problems -- all singers will find it challenging to sing with consistent resonance in this area (well-trained singers might not) and therefore to sing in tune as well
- Sustained or repeated notes - this is both a vocal and mental/ear issue: when a singer has to sustain a long note, it's easy to 1) fail to keep a constant flow of air/let energy flag 2) fail to keep a pure vowel sound or 3) the singer fails to listen or pay attention to pitch -- any of these mean that sustained or repeated notes can easily go flat unless . . . attention is paid to air flow/energy, mentally "repeating" the target vowel, and the singer listens for sustaining the same pitch and not allowing it to change
- "Placement" of vowels - Mike mentioned in his post on barbershop the importance of vocal resonance and placement, much as I did when discussing vowels and teaching concepts of "bright" and "dark" (which one could also consider "forward" and "back," although that's a simplfication) - in general it's easier to keep a forward, brighter vowel in tune, compared with one that is placed further back or is darker (if these terms don't mean much to you, then we may need more discussion) -- really well-trained singers will be able to sing a much richer, "darker" tone without it affecting pitch
These are specific issues, but a reminder that intonation problems have many (and sometimes, multiple) causes. Don't forget to consider all the vocal issues in both the training of your choir or in your diagnosis of why the choir's singing out of tune in a specific place in the music.