1. Have your choir slowly sing through a major scale which is in a comfortable range for all. While they are doing that have different sections stop at places to form chords. Have them listen to each other and tune the chords.
2. Have your choir sing a minor chord. Basses and altos at the octave, tenors up the fifth and sopranos on the minor third. After they pitch it properly, have them transpose it by a semi-tone down or up as you want all while maintaining the tuning.
You may also find this helpful - here's a (rather long) compilation of vocal warmups I gathered as a project for my choral methods class. It includes a bibliography at the end should you wish to purchase more warmup books!
For blend - have the weakest voice in your choir sing fa with a forward and non nasal ah on a given pitch. Have the person hold the pitch and renew it as long as necessary for each single voice in the choir to match that pitch without overpoweriing the original voice. Do this one voice at a time until all voices are singing fa and still hearing the original voice and vowel quality.
Then sing scales, arpeggios, chords, or whatever without altering the original blend From this basis you can add crescendi or diminuendi with all voices keeping to the original blend.
Blend is when one has all voices singing the same quality of each and every vowel, the same loudness, etc. This exercise will help one to achieve that 'one voice' blend.
Equally important is achieving equal and uniform consonant quality. Especially challenging is pronouncing each of two consonants (e.g. ...d t..., or ...s p.., etc) distinctly without running them together. This takes work and applies whether within a word or between two words. Have your choir practice two consonants in varying degrees of rapidity with equal clarity on each one.
Remember that in vocal excercises one is training ears as well as voices, and one's choristers are learning to think and mentally hear the quality of every note and or group of notes and use their vocal aparatus to produce it.
I suggest you a simple yet very useful exercise: They all sing in unison a very long note, first a bocca chiusa (closed mouth) then you start with vowels one at the time, then 2 then 3 and so on, then you go up as many semitones as you need, of course pianissimo. You may add a consonant at the beginning and at the end, or do that in a polyphonic way do the same with each of the sections.
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