We set up two loudspeakers 2 metres apart on the front bench on the front bench. Each speaker was emitting circular wavefronts of sound of the type shown in the ripple tank picture below.
When you get two sets of overlapping ripples, you get constructive interference when waves meet in phase - which occurs when the path difference is a whole wavelength. Destructive interference occurs when the waves meet 180 degrees out of phase. This is when the path difference is half a wavelength. The result is the stripey pattern of fringes seen below. The two speakers were correctly cross-wired so that the waves had a fixed phase relationship when they left the loudspeakers - they were COHERENT.
We had calculated a frequency that would put the fringes for sound about half a metre apart - the fringes have to be further apart than the distance between your ears! As we moved along we could hear clear maxima of loud sound with minima between. It never does get fully down to zero volume! And lastly, the picture of the surface of the ripple tank.
