Usually, the interference effect is understood as the fact that the resulting intensity in some points of space is greater, in others — less than the total intensity of the waves.
The addition of mechanical waves is easiest to observe on the surface of the vibrations caused by the waves, just add up. The resulting displacement of any particle of the medium is the algebraic sum of the displacements that would occur when one of the waves propagated in the absence of the other.
The amplitude of the medium oscillations at a given point is minimal if the difference in the course of the two waves exciting the oscillations at this point is equal to an odd number of half-waves and provided that the phases of the oscillations of the two sources coincide.