What happens when a sound wave interfers with another wave of same frequency and constant phase difference?

When two waves with same frequency and constant phase differenc interfere,

When two waves with same frequency and constant phase difference interfere,

When two waves with same frequency and constant phase difference interfere,

Two waves having the same wave length and amplitude but having a constant phase difference but having a constant phase difference with time are known as

The resultant amplitude, when two waves of two waves of same frequency but with amplitudes a_(1) and a_(2) superimpose at phase difference of pi//2 will be :-