Every planning of an efficient teaching has the aim of achieving satisfactory learning outcomes. From a constructivistic point of view, it is a commonly accepted fact that such a planning has to take into account the prior ideas that students bring to the class. In order to know them, we carried out a survey about the prior ideas on the nature of sound that our fifteen third-year engineering students had at the begin of an elective subject on acoustics. We used a questionnaire where the students had to express their prior ideas with their own words. Although the students expressed scientifically accepted ideas in about 2/3 of the individual questions on a whole, a cross comparison between each student’s answers for the different scenarios revealed a great number of inconsistencies in the mental model of the nature of sound (wave model): only about 1/3 of our students were consistent in all these scenarios. The inconsistency in their reasoning was still clearer when each student had to apply his/her respective mental model about sound to several properties of sound, in particular the relationship between pitch and distance travelled by sound. We analyse the state of the art in the literature about the issue of students’ consistency, and we consider some proposals suggested in the literature, which we apply in part in our own teaching resources, in order to overcome this inconsistency problem.