You might be looking for the term polyphonic. Its basically the
only term to describe the technique.
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polyphonic
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it is POLYPHONIC
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Polyphonic means many sounds intertwining or interweaving.
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It's done by reshaping your Larynx, Pharynx, and Mouth to change
how the end result sounds. You essentally amplify specific
frequencies in the overtones of your voice.