In spite of advances in physics pedagogy, the lecture is by far the most widely used format of instruction. We investigated students' understanding and perceptions of the content delivered during a physics lecture. A group of experts (physics instructors) also participated in the study as a reference for the comparison. During the study, all participants responded to a written conceptual survey on sound propagation. Next, they looked for answers to the survey questions in a videotaped lecture by a nationally known teacher. As they viewed the lecture, they indicated instances, if any, in which the survey questions were answered during the lecture. They also wrote down (and if needed, later explained) the answer, which they perceived was given by the instructor in the video lecture. Students who participated in the study were enrolled in a conceptual physics course and had already covered the topic in class before the study. We discuss and compare students' and experts' responses to the survey questions before and after the lecture.