Nice trick, but don't forget that uncorrelated noise is always additive. I know a few people who have built noise cancelling headphones (either for fun or for a

Nice trick, but don't forget that uncorrelated noise is always additive. I know a few people who have built noise cancelling headphones (either for fun or for a project class I once TAed) who discovered (after dealing with lots of fun feedback issues) that their "quiet" output was noisier than the original output. There's usually a way to make this trick work, but it's important to have accurate models of your various noise and interference sources.