Chapter 15

https://www.scienceworld.ca/resource/sound/

https://www.physicsclassroom.com/class/sound/Lesson-2/Pitch-and-Frequency

https://www.thoughtco.com/inventing-musical-instruments-1992156

https://www.britannica.com/art/musical-instrument/Technological-developments

https://www.wqxr.org/story/how-was-musical-notation-invented-brief-history/

https://www.loc.gov/collections/moldenhauer-archives/articles-and-essays/guide-to-archives/music-history/

https://charm.rhul.ac.uk/history/p20_4_1.html

https://www.youtube.com/watch?v=-bVketPj5to

http://stockhausenspace.blogspot.com/2015/01/opus-8-gesang-der-junglinge.html

https://www.classical-music.com/features/works/six-best-works-stockhausen/