This isn't officially a capstone (I'm only doing them officially with my AP class), but it's a great project that she did to demonstrate her Advanced-level understanding of timbre and FFT analysis.
Inspired by a website, Grace constructed some of these vowel resonators and compared the aural and FFT results to those in the article and to recorded vowel sounds. After that, she revised her resonators a bit, using reeds made from straws instead of the duck calls, and decided that these sounds closer to the original vowels (the duck calls were too bright).
Read the paper here!
It's also posted at Capstone Learning.