Figure shows sound waves of wavelength $\lambda$ are emitted by a point source $S$ and travel to a detector $D$ directly along path $1$ and via reflection from a panel along path $2$. Initially, the panel is almost along path $1$ and the waves arriving at $D$ along the two paths are almost exactly in phase. Then the panel moved away from path $1$ as shown until the waves arriving at $D$ are exactly out of phase, the path difference $∆x$ of the waves along the two paths.