Scat Singing, is merely improvising without words. We aim to approach it the same way as an instrumentalist. Scat Syllables are only to serve the punctuation of the line and I feel strongly that the melody and rhythm used over given chord changes dictate the syllables, not the other way around. The scat syllables flavour the rhythmic inflection synonymous with Jazz.
As any good instrumentalist develops his/her language and vocabulary, singers must do the same. This comes from continual listening and transcribing. There isn't a shortcut. Harmonic and rhythmic elements must be studied, for example outlining the relevant scales and arpeggios, patterns and extensions of the chords using a variety of rhythms. Aural training plays a big part in this and because singers rely so much on their ears when it comes to improvising, developing a sense of pitch memory is essential.
The more one develops the ear to hear certain sounds the more creative and interesting the improvisation will be. Some people have a natural tendency to hear the harmony and understand rhythmic concepts, but collating and assimilating the vast amount of information, displaying it in performance, on the spot, anticipation of chords, implying multiple chord changes, using altered extensions, interaction and reaction and being emotionally honest in the performance takes a lifetime to hone.
Feel, articulation and flow are important factors to consider. All instrumentalists work on this constantly. Having observed vocal improvisers, I"ve noticed difficulties regarding these aspects when using scat syllables. The eighth note feel (continuous line) in particular, is the hardest to develop. Attaining smooth and relaxed swung eighth notes (for bop lines) demands consistent attention and practice
Like woodwind and brass instruments we use our tongues to articulate sounds. We'll take for instance- ba (off beat) with the lips and doo (on beat) with the tongue. The off beat (ba) is often articulated too hard and the doo (on beat) is too short, this coupled with over- expressed and affected syllables presents problems with the flow and feel of the line and doesn't swing.
I feel that scat singing should be conversational, an extension of the speaking voice, not laboured and over- exaggerated to the point where it is no longer musical.
Finally, I'd like to add that in recent years, many have become aware of the marked increase of highly talented jazz vocal improvisers who continue to raise the level of this growing art form. These vocalists regard themselves as musicians and consider it a priority to develop their improvisational skills. It's also a reflection of dedicated Jazz vocal educators with improvising skills, on a mission to encourage the search for educational opportunities in this field.