Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: When a wave meets a barrier, it reflects and travels back the way it came. The reflected wave may interfere with the original wave. If this occurs in precisely the right way, a standing wave can be created. The types of standing waves that can form depend strongly on the speed of the wave and the size of the region in which it is traveling. Answer: reflected wave
A:
A standing wave is created when what type of wave interferes with the original wave?