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How do ANC in headphones actually work?

vertumno

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Aug 17, 2021
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Hello,
I’m trying to understand better how ANC headphones noise canceling works (especially AirPods Pro), the literature I’ve found is confusing me.

As far as I understand a mic picks up noise and the then a cpu inside the headphones creates a flipped phase of the aforementioned noise thus achieving phase cancellation. Is that correct?

I also don’t understand how the flipped phase signal is used, does it enter your ears or it’s done before any sound reaches your ear?

I’m trying to understand if those AirPods Pro 2 can be used as effective hearing protection, I normally use ear plugs when walking in noisy city streets, but sometime I also have to engage in conversations and I have to take the earplugs out (you can hear a voice through earplugs but it’s not ideal) so I was wondering if ANC could be worn constantly, cancel noise and still be able to have a conversation.
 
As far as I understand a mic picks up noise and the then a cpu inside the headphones creates a flipped phase of the aforementioned noise thus achieving phase cancellation. Is that correct?
Yes.

I also don’t understand how the flipped phase signal is used, does it enter your ears or it’s done before any sound reaches your ear?
It's played back through the same ear speakers that play your music.

I’m trying to understand if those AirPods Pro 2 can be used as effective hearing protection
To a certain point, yes.

Both passively and actively, Airpods can reduce the noise pressure level that reaches your ears.

However, I'm unaware of any case where AirPods were certified as PPE for the job site.

So, you can use them that way, but don't let OSHA see and switch to proper hearing protection when things get really loud.
 
if ANC could be worn constantly, cancel noise and still be able to have a conversation.
In practice ANC is good in blocking continuous noise like roar from an engine. Fluctuating noise like somebody talking is to challenging. Often ANC earbuds come with a hear through function to enable conversations. On this case the mics are used without applying ANC
 
I’m trying to understand if those AirPods Pro 2 can be used as effective hearing protection, I normally use ear plugs when walking in noisy city streets, but sometime I also have to engage in conversations and I have to take the earplugs out (you can hear a voice through earplugs but it’s not ideal) so I was wondering if ANC could be worn constantly, cancel noise and still be able to have a conversation.
There are active hearing protection earphones that sort of do that (but these Etymotics aren't ANC based).
 
Yes.


It's played back through the same ear speakers that play your music.
What about if I'm not playing music, will I hear a phase inverted version of the 'outside' noise?

This is what Apple says that really confuses me: Noise-cancelling microphones and a rear vent are optimally placed to quickly detect sound coming in, working together to counter noise before it reaches your ear.

Yes when things can potentially get really loud I use 3m Peltor and earplugs, but I avoid those situations 99% of the time ;)
 
What about if I'm not playing music, will I hear a phase inverted version of the 'outside' noise?
No.

Noise cancelling happens completely independently of music playback.
 
Is ANC help in ears have better call quality or is another technology embedded to the mics for this work and ANC is only to block external noises?
 
What about if I'm not playing music, will I hear a phase inverted version of the 'outside' noise?
My recent ANC reading describes the following, which I had not previously thought of concerning the technology. I have not investigated if there is now a different ANC implementation so am merely posting a screen shot from 2022. My surmise based on the below sentence is that when you are not playing music you will have phase inversion of ambient frequencies up to 1kHz plus simply muffled higher frequencies (which frequencies are not then or for that matter ever actually phase inverted when using ANC).

IMG_1005.jpeg
 
So without music being played what I will actually hear? ANC approximation of a quite environment, low rumble filtered and some of the higher frequencies passing through?

I hope it’s better now and deals with frequencies above 1,000 Hz ( thinking about ear splitting ambulance sirens and the likes)
 
From my own experience, noise-cancelling earbuds do not work well to filter out screaming babies on airplanes. But they do cut down a lot on the ambient noise, including rushing air on a motor bike. They should come with a noise reduction curve, or at least a verbal description (-x dB <1000 Hz, -y dB up to z kHz).
 
Not all ANC are created equal. There is more than one way to implement Active Noise Cancelling

There is feed-forward ANC which is used in IEMs.
This uses a small mic on the outside of the IEM and that signal is 'inverted' in polarity and filtered in a similar way as the natural attenuation of the IEM itself.
That signal is mixed with the music signal and thus has good attenuation of outside noises and can even operate above 1kHz.
The original signal passes without changes made so switching ANC on and off should not affect tonality nor distortion but still could.
Talk-through feature is possible.

Then there is ANC in over-ear headphones which works differently.
This can be done fully analog but is also done digitally which can have slight advantages.
It works by using a microphone that is positioned right in front of the driver.
This microphone thus picks up the noises in the earcup + the music that is being played.
(Below AKG N60NC)
driver-nc60.jpg


As the microphone picks up both signals, the wanted and not wanted, the ANC circuit is also fed the music signal.
Based on this input (mic that detects all) and the music one can simply 'null' these signals (by subtraction, i.e. phase reversal at the correct level for the 'correction' signal) and thus the speaker inside emits the 'contra sound' for what is coming from the outside (unwanted noises) AND also corrects the frequency response at the same time thereby improving (flattening) bass response which is often wonky in closed designs. (QC35-II ANC on, ANC off)
nc-off-nc-on.png

A downside is that, because of the mic-driver distance and speed of sound, the correction is limited to to frequencies below 1kHz (or so). Attenuation of higher frequencies is done purely passively.

Some headphones also add a 'target' curve (EQ) so they can shape the sound as well. below WH-1000X mk3
passive-mode-n.c.-on-n.c.-off.png

(N.C. on, N.C. off) and the passive mode)
Distortion also could be improved by ANC
QC35-II without ANC:
dist-nc-off-r-percent.png


QC35-II with ANC:
dist-nc-on-r-percent.png


But alas this isn't always the case. There are also ANC headphones that increase distortion levels when switched on.

Some ANC headphones have inverse (flipped) polarity, some do not. It highly depends on the circuits used.
Below the PXC550-II in passive mode (polarity inverted)
phase-passive.png


PXC550-II with ANC on (correct polarity)
phase-active.png



Then there are newer ANC versions that use a combination of both techniques described above and over-ears that use the IEM tech. The latter does not correct for driver distortions so the 'sound signature' does not change when switching it on and off.
These do work a bit higher than 1kHz.
The biggest upside is that one can easily make it have 'talk thru' by simply using the outside microphone as a source and mute or attenuate the music signal (if present) but of course ANC does not work. The 'mic' signal can be shaped (limited frequency response) as well.

Not all ANC is created equal as the microphone circuits have (add) noise which in some headphones becomes audible when no music is playing. The noise 'spectrum' (tone) can vary as well between models from higher pitched noise to lower pitched noise.


Downsides:
Requires (charged) batteries
background hiss (low level) may be audible without music playing.
Often the digital ones are limited in FR to 20kHz or 22kHz and have extremely poor anti-alias filtering. (PXC550-II)
l-passive-vs-active-lin.png

and Beats:
20k-to-30k-fr-beats-wired.png

Something to consider when playing 'hi-res' music on such a phone when using them wired.
Not all of the digital ones are limited to 22kHz or so.

upsides:
The feedback types are less seal dependent (up to a point)
NC off
seal-nc-off.png


NC on (same seal breaking):
seal-nc-on.png


No amplifier needed.
Bluetooth is easily integrated.
OE's often also have distortion lowered and better frequency response (tone) at the expense of light background noise. The better ones have inaudible or barely audible noise.
 
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So without music being played what I will actually hear? ANC approximation of a quite environment, low rumble filtered and some of the higher frequencies passing through?

I hope it’s better now and deals with frequencies above 1,000 Hz ( thinking about ear splitting ambulance sirens and the likes)
The "reference'' signal processing is apparently what ANC technology has focused on improving (as per the 2024 report's screen shot below). That signal fidelity is crucial to how cleanly what is generated cancels some sound. My surmise here is that ANC headphones' ear cups are not suitable for housing additional microphones ('reference" plus "error") for the frequencies above 1kHz in order to be "capturing" them for "processing" even supposing an algorithm for those higher frequencies can process them.

IMG_1009.jpeg
 
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Great discussion. I also want to add that ANC is better at cancelling low frequencies just because they tend to have a slower rate of change and they always have a larger wave cycle. This means that it is easier to generate a phase-flipped tone within that given time, and also that there is a greater "margin of error" in which the generated flipped tone will cause a significant null, even if it isn't perfect.
 
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