Listen to the tones. Yes, you are playing a sound file for the
third graph. Note that the peaks in the first graph correspond to the troughs in
the second graph. The phase difference is not exactly 180°. So, a small
resultant wave is visible, but it is below the threshold of hearing. This is a
classic example of 1 + 1 = 0. Of course, it's actually 1 + (-1), but your ear
doesn't want to believe that.
