If two pure tones of the same frequency and intensity occur simultaneously out of phase, they will cancel each other out.
If two pure tones are produced simultaneously, but are directly out of phase (i.e., the molecules of air are moving in exactly the opposite direction) the result will be no sound at all! That is because all the energy is cancelled out.
For example, if I am pulling on a table and you are pushing on it equally hard; and then I push on it and you pull on it equally hard, the end result will be no movement of the table.
In some auditoriums that have not been carefully acoustically designed, you can be sitting right in the front row and hear hardly anything because you are in a "dead spot" (I.e., a spot where the sound bouncing off the walls reflects back out of phase an
There are delay feedback mechanisms now on the market, which play back environmental noises out of phase creating almost a quiet world for an individual who wears the earphones.
NOTES: See more on cancellation.
NOTES: And yet more on cancellation
NOTES: And would you believe more on Cancellation
NOTES: Even more yet on Cancellation.