The basic equations that have been derived in the study of one-dimensional sound propagation are well suited to solve a multitude of problems concerning reflection and sound transmission. In the following, we shall assume that the incident wave arrives from — ∞, and travels in the positive x-direction. The particle velocity in the incident wave then is simply its pressure divided by + ϱ c. The reflector will be positioned at x = 0 so that at the reflector, the exponentials e ±j k x are equal to unity. This convention simplifies writing considerably. The disadvantage of this notation is that points in front of the reflector correspond to x < 0.