Hello Aaron!
Look at
csounds.com and the udo section. There is a udo called vowel, which
looks exactly like what youwant. Or you can try Fofilter which is still based
on grains, but you can feed it an audio signal from which the grains are
taken. At least, that is, how I understood it.
In any case: You will need resonating filters. So I think the buterworth
filters are out.
Warmly yours
Julien
--------
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