The present paper reports the design process and the experimentation of a teaching-learning sequence about the concept of mechanical wave propagation and the role played by media where waves are propagating. The sequence focuses on the central issue of the relationships between observable phenomena, like macroscopic behaviours of waves, and their interpretation and/or explanation in terms of corpuscular characteristics of media. We describe the design process with respect to the general framework of the Educational Reconstruction Model and the pedagogical tools used. Results of a teaching/learning experiment, involving a sample of 75 high school students, are also reported. Data analysis is mainly based on qualitative research methods. The main focus is on students' representations of phenomena and on the cognitive strategies put in action in order to modify or support their descriptive and interpretative mental models. Results are discussed by pointing out the efficacy of strategies focusing on the process of constructing predictive conceptual models and by identifying the concept of "level of analysis" as different ways to look at the same phenomenon. (Contains 5 figures, 8 tables and 2 notes.)