Consonance/Dissonance and the Critical Band
Now, let's see what happens if the same two 500 Hertz sine waves are combined "out of phase" or in "phase opposition", that is, when one waveform goes positive, the other goes negative and vice versa.
First, one signal alone:
And now the second tone will be added momentarily and then taken away:
That dead spot in the middle was caused by adding the second, equal 500 Hertz tone out of phase. When the two signals are in "phase opposition", one cancels the other and the resultant output is zero.