Figure 11. I line up three pulses so that they coincide. However, in this figure, I separate them for easier discussion. Points A, C, and E coincide, and points B, D, and F coincide. In traditional waves, reverse direction does not reverse the amplitude. Here I equate the amplitude to a pulse's phase change (Δθ). At point A the pulse's amplitude is going down, and at point B it is going up. Hence, going along pulse P from point B to A, the amplitude changes from going from up (↑) to down (↓), which gives Δθ↑↓. This Δθ↑↓ represents the amplitude and it means that as the phase changes, the amplitude goes up, then down a crest. The symbol Δθ↓↑ means a trough. Going along pulse P from point B to point A gives the symbol Δθ↑↓, which is a crest. Going along pulse P from point A to B gives –(Δθ↑↓) = Δθ↓↑, which is a trough. In other words, at point A the wave is going down, and at point B the wave is going up. And going from A to B, the amplitude goes down, then up, which is a trough. Therefore, pulse P is a crest going to left, but going to the right, it is a trough.

Now I examine how the other pulses interfere with pulse P. Pulse Q is represented in Figure 11 as moving in the opposite direction with the same amplitude as pulse P. While pulse P is moving to the left, it interferes with pulse Q, which is moving to the right. Hence, relative to P, Q's amplitude is –(Δθ↑↓) = Δθ↓↑, which is a trough. These two amplitudes cancel. Nonetheless, P constructively interferes with R. Once again, while pulse P is moving to the left, it interferes with pulse R. And relative to P, R’s amplitude is –(Δθ↓↑) = Δθ↑↓, which is a crest. Hence, pulse P and R add up constructively.