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Yesterday I noticed that if I played this video from my smartphone on speaker, one could barely hear anything. On the other hand, if I used earphones I could hear it perfectly.

Then, I tried with several devices (other phones, my PC's speakers, a Bluetooth speaker) and on some of them one could listen to the audio with a really high quality, but on others it was very low and like 'echoish'.

I believe there should be a signal processing explanation to this phenomenon. I don't really understand what's going on, but I suspect it has something to do with stereo and mono audio. Sound quality was good on those devices that were stereo (speakers from PC, earphones, headsets) but on those that were mono (speaker of a cellphone, Bluetooth speaker) I could barely hear anything.

What is really going on? Is this phenomenon a mistake from the video uploader or could it be an intended effect?

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The 2 stereo channel might be out of phase (near opposites, thus cancelling each other out when mixed to mono).

The human ear-brain doesn't just sum stereo left-right to the 2 ears, but instead uses any phase differences to help determine directionality.

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  • $\begingroup$ Indeed, out of phase by 180°. I.e. right channel = (-1)·left channel. I downloaded the audio (youtube-dl -x URL), then converted it to wav (mplayer -ao pcm:file=output.wav:fast -vo null), then loaded it into python to compare the energy in the sum of the two channels: it's negligible; I guess we're only hearing compression artifacts. I don't know who made this audio mix, but it's really just mono. Probably someone tried to cover up how badly they failed (probably lost the original audio recordings, or else they could have just done it right) by trying to fake stereo. $\endgroup$ – Marcus Müller Sep 3 '17 at 10:17

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