I always assumed that AdaptiveEQ needed either transparency or ANC to be on as both of those utilize the internal microphones of the APM to function and get rid of the uneven sound pressure in the acoustic chamber (for the APM it would be the area around your ear and ear canal, with the APP it’s the ear canal). That pressure comes back if I turn both systems off. I am curious how well it’ll function with the AP3 since it won’t have a perfect seal. I do have a pair of those delivering on Tuesday to play with.
The internal mic is still in use, but the ANC circuit itself off. Apple has a patent on the subject I've already linked to in this thread somewhere

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You can assess whether or not the feedback mechanism is in use by applying varying degrees of compression on the cups and measure the FR, something a bit like what I did here with ANC on :
https://www.audiosciencereview.com/...ad-fi-and-sean-olive.27017/page-4#post-927518
The other clue is that when ANC is off the APM may show some deviation from the ANC on FR when using sweeps for measuring them (although in the case of my own on-head measurements I fail to get the large deviations that I've seen in some places), but not when using white noise (a large signal).
And finally on the APP - at least a new copy where the inside mesh hasn't been clogged up with skin oil / wax (but not the APM), you can use a little test to get an idea of how the feedback mechanism works when ANC is off :
- switch the APP to ANC off
- play pink noise for a few seconds
- stop playing pink noise
- start playing a single tone below a few hundred hertz, for example at 200Hz
- while the tone is playing, switch to ANC on. The sound level should be the same.
- while the tone is playing, switch back to ANC off. Now the sound level should be different.
- stop playing the tone. Play pink noise for a few seconds.
- stop playing pink noise. Play the tone again : now it should be back at the same level as with ANC on or ANC off
after playing pink noise.
I am also very curious to see how Apple's Adaptive EQ will perform on a pair of HPs without a seal

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