Using spectograms like Praat and Chrome Musiclabs Spectogram. I discovered that I can control my first formant by moving my tongue between front to back by consciously moving between the vowel [u] [ae] [a] [i] sounds. I further discovered that I could move between slightly different "l" sounds by pronouncing "l" while forcing a different vowel sound.
This seems to just be an exercise on just the placement of the body of the tongue, so it leaves the middle and tip of the tongue relatively free to move, and naively, I would assume that I could also practice those movements to produce a variety of other sound effects that would be visible in a spectrogram.
Are there reference materials on how various tongue movements will effect spectrogram readings?