Noise Cancellation

Wittgenstein said "Whereof one does not know, thereof one may not speak"

but then he didn't have the Max list to fall back on. So ....

I have a microphone picking up sound form the environment and
triggering off sound events from the computer (based on frequency
dependent thresholds), which are played back over speakers in the same
environment as the microphone.

The issue is that I don't want the sounds coming form the speakers to
further trigger any of the detection "circuits".

I imagine, using some kind of phase cancellation network after the mic
input, but before the threshold detectors, and combining the main
output of the computer with the mic input, that it should be possible
to reduce the amount of sound from the speakers being fed back into
the system, so that only environmental sound events act as triggers.

I'm also guessing that the phase cancellation bit needs to be broken
down into several frequency bands to at least partially cope with
different frequencies needing different amounts of delay to create
cancellation.

Am I right so far? Is this fairly trivial to do in msp? And how would
I go about doing this? (Something to do with fft~, I bet)

Thanks for any help!

David