Move the slider to change the position of the two speakers. The purple wave shows the sum of the red and the blue waves.
The path length difference changes the interference of the waves. If the speakers are separated by an integer number of wavelengths, then the waves will constructively interfere. If they are separated by a half wavelength, i.e. λ/2, then they will destructively interfere.
Link to stand-alone sim: Here is a link to view the sim on its own, with no distractions.