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The previous explanations of “modulation” are fine if you want technical. But allow me to demonstrate another way of saying this:
Stand up. Sing a sustained mid-range note by saying “AAAAAAAAAAAAAAHHHHH!!”
While singing the single tone of “AAAAHHH!!!” start thumping on your chest like a jungle gorilla. “thump, thump, thump, thump, thump.”
Notice how the “AAAAAHHHH!” gets shifted and sounds like “AAAuuh AAAAeee AAAuuh AAAeee!” You are modulating yourself.
The sustained “aaahh” is the carrier. The thump thump is the modulation.
Try not to cause your heart to stop beating.
You can bypass the chest thumping by simply singing vibrato, a form of self-modulation without physical abuse.