[spot5010]

A wild guess as to what is happening. I haven’t actually tested this hypothesis so I could be completely wrong.

In feedback systems, the gain is a function of frequency, and typically decreases when going from low frequency to high frequency. This is often accompanied by a phase delay.

So if the overall gain of the system is high enough, there will be some high frequency where the gain is 1, and the phase is 180 degrees. This would result in positive feedback, amplifying noise at that frequency.

Maybe that’s what’s happening in the latest AirPods? If Apple is aggressive cranking up the gain of the noise cancellation system, there’s some high frequency where the noise gets amplified rather than suppressed.

The solution would be to either reduce the gain (which reduces the noise cancellation), or to add some differential gain in the system which pushes out the unity gain frequency to higher frequencies.

[userbinator]

An even wilder guess: the speed of sound varies with air density.

https://en.wikipedia.org/wiki/Speed_of_sound#Altitude_variat...

If they were calibrated assuming a certain distance from the microphone that "hears" what the wearer's ear is hearing and the ear itself, then it's possible a change in air density could position the area of highest constructive interference at the eardrum instead of the intended destructive interference for some frequencies.

[_s]

The pressure difference shouldn’t be significant enough in modern jets? Cabin altitude is around 6000-8000’ - we would hear complaints from a few major cities. Humidity is much lower in aircraft though.

[schiffern]

The speed of sound varies with air temperature, which is what the linked graph shows.

Technically the speed of sound does vary with density, but as you change altitude there's also a change in pressure which exactly cancels that out. In the end only temperature and gas composition alter the speed of sound.

As long as you're inside the plane (and hopefully it's not 217 K or -70 °F, per the graph) then the speed of sound should be unchanged.

[em3rgent0rdr]

Sounds like you are stating https://en.wikipedia.org/wiki/Barkhausen_stability_criterion (1921):

"if A is the gain of the amplifying element in the circuit and β(jω) is the transfer function of the feedback path, so βA is the loop gain around the feedback loop of the circuit, the circuit will sustain steady-state oscillations only at frequencies for which:

1: The loop gain is equal to unity in absolute magnitude, that is, |βA|=1, and 2: the phase shift around the loop is zero or an integer multiple of 2π: ∠βA=2πn,n∈{0,1,2,…}"

[em3rgent0rdr]

But you said "phase is 180 degrees" which is 1π, while Barkhausen instead says integer multiple of 2π.

[spot5010]

Found an article which talks about the phase margin, referencing 80 degrees (pi). https://en.wikipedia.org/wiki/Phase_margin

[Fwirt]

I just had a thought, it's possible to completely disable ANC in settings, turning them into "dumb" bluetooth headphones. (Enable "Off Listening Mode" in Airpods Settings and the option will become available in Control Center.) If some of us who are able to replicate this effect consistently could try turning ANC off and seeing if the effect still occurs, that would narrow it down to being feedback related from Transparency/ANC or being something external like back EMF.

I just tested this myself and the two ways that I am able to get consistent squealing (stroking the upper body when in-ear and cupping them in the hand) both fail to replicate when ANC is off. So this does point to a feedback issue.

My other thought is that the APP3 may have microphones located next to the drivers in the ear canal, both for measuring fit, and for the new "own voice amplification" feature that appears in hearing control center if you enable Hearing Assistance. Maybe vibration is leaking through the body to the inner microphone.

[ilt]

Own voice amplification is nothing new and has been present at least since iOS 18 and hence was/is present in Airpods Pro 2 as well.

[layer8]

That doesn’t explain why the issue seems to be specific to left APPs.