[trashfindhunter]

Yeah, you're right about ear damage coinciding with exposure time - short bursts would have to be very loud to cause real damage.

And yes, it would be weird if a frequency range spontaneously inverted - the only scenario I can imagine that happening in is some jerk doing it on purpose.

The reason I became interested in ANC was because every night I would hear a terrible frequency being emitted from the air conditioner units above me (top floor apartment building), and during my experiments I quickly realized how hopeless it would be to effectively combat them due to the varying intensity of the sound throughout my apartment, the dynamic interactions of the sound with itself within my echoey wood floor studio, and my location at any one time. All valid points though, thanks for chiming in. I learned more about ANC :)

Edit: Btw my goal was ANC via speakers, not headphones. Headphones would be much easier since they only have a single, summed audio source.

[trashfindhunter]

Then again... won't that summed audio source be at the mercy of my loud speakers?

[solarkraft]

If you mean by that that it couldn't be louder than what the speaker could produce: No, because you're adding to the original noise entering your ear from the outside.

[trashfindhunter]

Will the physical sound waves that I produce with my loud speakers cause Active Noise Control enabled headphones to "engage" with me? Could I adversarially engage with them at that point?

[solarkraft]

I think I understand what you mean now.

While the noise cancellation is active it will attempt to neutralize (destructively interfere with) sounds from the outside, including those generated by your speaker. You could indeed adversarially engage through something like a spontaneous phase shift (so the interference will become constructive, making the resulting signal louder) or generating a frequency the ANC can't compensate.