Why can’t phone audio (media) be heard through phone call?

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How does the phone distinguish the sound I am making from the phones own audio. And why, for example, can’t the receiver of the phone call hear when I listen to a YouTube video while talking to him?

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The phone knows what sound it’s making, and subtracts the sound it itself makes from the sound it sends over the wire (well, wireless, by now). This also is the mechanism that makes sure people don’t echo back if you have the speaker on when talking to someone.

The simplest reason is because of the fact that the phones microphone and speaker point in different directions.

The speaker projects sound outward, away from the microphone – this means that for the microphone to pick up any of this sound, the sound needs to travel away from the phone, reflect off of something and bounce back towards the mic. Generally any sound that does this will have been attenuated enough as to be a lot, lot quieter than someone intentionally speaking towards the phone, so it won’t be heard by a mic tuned for human voice levels.

The second trick that can be used are various forms of active noise cancellation. Because the phone knows what audio signal it is producing, that means it is able to listen to the incoming audio and cancel out audio that matches the sound it is making.