Consider two counter-propagating waves on a string with equal frequencies but 180 degrees out of phase so that there is complete destructive interference. Each wave carries energy - one to the left and the other to the right, let's say. But when they interfere we get a stationary string and, apparently, no energy. Where did the energy go? I'd like to nail down the answer to this question. If you look on the internet, there are all kinds of different explanations. Let's see if we can do better at sciforums.