This would allow students to observe that the foil does not silence the sound waves. It has an even more useful role, however. My goal is not just to measure my students' achievement, but also to help them grow. Formative assessment allows me to give feedback to my students that guides and pushes them to achieve, to close the gap between his starting point and where I wish him to be. Our real goal is to build students' capacity to judge the quality of their own work, and internalize high standards.
Students tend to do a job of work, then sit back and let the teacher evaluate it.
I have been working this year to turn this around. Last semester I assigned a research project on the planets. I gave students a detailed assignment so they would know what I expected. I shared a draft I had written myself and invited them to critique it in a class discussion. This gave them a little practice, as I emphasized the type of feedback that is most helpful to an author.