How does noise canceling headphones work?

for christmas, i got a kinda decent pair of headphones. without the batteries, it sounded as if bass was a little too high. but i just got batteries today, and they...sound...great! i could hear the difference, without music playing. just putting them on, and hearing my background. switching it on...i heard... show more for christmas, i got a kinda decent pair of headphones. without the batteries, it sounded as if bass was a little too high. but i just got batteries today, and they...sound...great!

i could hear the difference, without music playing. just putting them on, and hearing my background. switching it on...i heard almost nothing.

my question is, how they work. how do they cancel out outer noise?
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