i found this
  The movie at left shows how a standing wave may be created from two travelling waves. If two sinusoidal waves having the same frequency (wavelength) and the same amplitude are travelling in opposite directions in the same medium then, using superposition, the net displacement of the medium is the sum of the two waves. As the movie shows, when the two waves are 180 out-of-phase with each other they cancel, and when they are in-phase with each other they add together. As the two waves pass through each other, the net result alternates between zero and some maximum amplitude. However, this pattern simply oscillates; it does not travel to the right or the left. I have placed two dots on the string, one at an antinode and one at a node.  http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html

is it possible to make an applet showing this with a frequency and opposite phase calculator
sine wave within normal hearing range