Making Sense of How Students Make Sense of Mechanical Waves
  We report on our study of student understanding of the physics of mechanical waves, specifically the propagation and superposition of simple wavepulses traveling on long, taut strings. We introduce the terms "particle pulses mental model" to describe the reasoning approach that students use to guide their thinking in wave propagation and superposition. Student responses on free response and multiple-choice, multiple response questions dealing with the same physics show inconsistent student thinking, where they may have both correct and incorrect ideas about the concepts. 
Michael C. Wittmann, Richard N. Steinberg, Edward F. Redish