AudioScope
Can One Guitar Disappear?
Try stereo headphones at a comfortable volume. Listen to what changes when the two channels are added together.
Why the guitar disappears
Two identical channels at the same level sound centred. Reverse one channel’s polarity, and the matching samples add to zero in mono. In stereo, the two channels still reach separate ears. Two real performances can give width because they differ slightly in timing and tone.
Noise-cancelling headphones use microphones and an opposing signal to reduce noise near your ear. The hum in the video is an illustration. It is not a headphone test.
Audacity on polarity · Yamaha on double-tracking · Bose on noise cancellation