I arranged a song for my acapella group. I have chords, the solo, and a basic background which I can change easily but, I need to know a method or a direction to look for background syllables.
In my a capella group in college, I did some arranging and picking "vocables" as we called them, was a frequently overlooked (by me) detail. I think we mostly tried to mimic the instruments that were being arranged, for instance distorted guitar could be "wah" or "bow", bass was frequently "dum", etc.
In hindsight though, those syllables would rarely sound cohesive or good together and sometimes they could contribute to pitch being perceived differently ("ee" can sound higher in pitch than "uh" even if they're the same note because of the different balance of overtones).
Because of this challenge, some other ideas for what to do might be using words that everyone sang together or all on one syllable. These blend well, but words may distract from the main singer, so space them out (a word every whole note or half note). All singing "ah" or another pure vowel sounds very choral and pretty, but that may or may not be the effect you're going for.
My biggest piece of advice would be to listen to professional arrangements (maybe King's Singers or Swingle Singers) or very high quality collegiate recordings (like those on the BOCA or CASA albums) for ideas and to listen for what sounds best to your ear.
I'm assuming your question is about arranging the non leading voice parts. If you don't know even where to start I suggest a 4 part harmony homophonic approach in the classic way. Whole volumes have been written on this subject only, but a simple basic approach is not so hard if you know your way around basic chords and scales.
- 4 part harmony assumes 4 voices with ranges of Soprano, Alto, Tenor and Bass (see e.g. wikipedia for a definition of these ranges). Usually the higher register, Soprano, takes the lead melody.
- Homophonic means that the different voices move with the same rythm of the lead melody, so for each note in the lead melody each of the other 3 voices will have its own note.
For each note in the melody:
1 - Give the chord root note to the bass.
2 - Assign notes to the remaining two voices the following way, depending on what is the lead melody note:
- If it's the root of the chord, distribute the remaining notes of the chord (assuming it's a diatonic triad) by the remaining two voices.
- If it's not the root, but it's part of the chord, double the root in one of the remaining voices and give the remaining note of the chord to the remaining voice.
- If it's not part of the chord (what's called a Non Chordal Tone, or NCT) just distribute the remaining notes of the chord (other than the root) by the remaining two voices.
If the chord is a tetrachord (normally in classic harmony, a seventh chord) and the additional note (normally a 7th above the chord root) is not the lead melody note, discard the 5th degree ("3rd note") of the chord and insert the additional 7th degree note instead.
In the above process one should look for voice leading, i.e., the assignment of notes to the different voices is such that each voice has minimum movement (sometimes reamining in the same note) between steps of the melody.
Now these simple principles are the tip of the iceberg, they will not overcome all possible harmonization situations that you may encounter and will not get you very far in terms of variety and creativity. But they may suffice to get you started and achieve some interesting results on your own (what your band mates may think of it, that's another matter :-)
No experience with that type of composing myself, but each syllable has up to two parts: the vowel and the consonant. In general, the more open the vowel and the more percussive the consonant, the louder the effect will be, e.g. moo moo moo will be much less dramatic than da da da. Voiced consonants will also be more noticeable than their unvoiced counterparts, e.g. ka ka ka is quieter than ga ga ga.
Personally, I subscribe to the theory that the vowel sounds exist on a spectrum. Try closing your mouth and making it as wide as possible, then say/sing:
You may want to go somewhere where no one can hear you before doing this. But you may have noticed that it feels like you're going back and forth along a spectrum, from the deep and wide "muu" sounds up to the tight and narrow "iiiiii" sounds, and back down again. This is because the vowel spectrum is changing your resonating cavities to change the volume of the overtones (aka partials, harmonics) relative to the fundamental frequency of your voice.
The same harmonics are present in everyone's voices, but their relative volume is what allows one to tell vowels apart, and vice versa. The deep "u" or "ooo" sound has quieter high partials when compared to a more nasal "ah" or "eeeee" sound, which has louder high frequency content. But enough of the acoustic theory and phonetics; how does one use background syllables?
When I arrange, as far as vowels go, I like to use the deeper syllables (like "ooh", "oh", or humming as well) for moments where the background vocals should be in the background, and when the background vocals need to momentarily jump into the foreground of the arrangement, that's the time to go for the bright and high-energy "oh", "whoa", "ah", and other syllables. If you use energetic syllables during a moment where the background vocals aren't being highlighted, you run the risk of stepping on the soloist's toes and muddying up the sound. If you use laid-back syllables when the background parts are doing something interesting, your arrangement may feel a bit dimmer than you want it to feel.
Consonants are also important. "Wa" and "doot" feel very different from "bap" or "hoo". There are a lot of different consonants that can be used in singing background parts, and you'll have to play around and see what's most comfortable for you and what sounds best. And don't be afraid to give the bass singer different syllables than the rest of the group! In fact, they may prefer to come up with their own syllables, so they may know better than you do what the song needs in their part's syllables.
Often, your choice of background syllable will be limited by the lyrics of the song. If the soloist is holding out a particular vowel, you may want to match that in your background parts. As any choir director will tell you, vowel blending is really important, so pay attention to how the background parts and regular lyrics interact. And hey, none of this matters when your background parts have their own specific lyrics, like echoing the soloist, obviously. If you don't already do this, try having the background parts sing some real words at times for some interest. It's a great way to get the background into the foreground, and it also keeps your background singers happier and more invested in the piece, I find - who wants to spend two and a half minutes just saying "doo"?
If you haven't seen it already, I recommend giving this video by Deke Sharon a run. He's done a lot of really big-time acapella arranging, and I found his video to be helpful to me back when I was asking myself the same question you are.