Let's say I have an acoustic duct setup and the contaminating sound is propagating through it. I can hear it at the end where I'm standing.
I now start the ANC algorithm and it cancels out the contaminant within the duct so that I can no longer hear it where I'm standing. Success!
But where did all that energy go? I read some posts here and it seems like there should be localized cancellation (near the error microphone), but not global. As far as I can tell, being at the output end of the duct, the cancellation on my side is complete. I can't audibly tell a difference on the other side (but I might not be a good measurement equipment). This is a metal pipe embedded in concrete.
Did the incoming sound reflect and travel back to its source? But then why didn't the cancelling wave reflect towards me? Or did it?
Or are they cancelling "globally" in my direction because they are travelling in the same direction and it is possible for them to always cancel from some point on? Seems like magic though :/