Noise Canceled

When I was in the 'states I got myself a pair of cheap noise canceling headphones. I wasn't expecting them to be up to much, but it turns out that they are quite effective. You turn them on and they do nothing for a few seconds and then there is a kind of un-thump and the sounds outside you vanish. It is a bit like having a wet flannel suddenly wrapped around your head.

I've been wondering what would happen if I played the same music into some speakers and the headphones, sat next to the speaker, and then turned the noise canceling on.......