40.5 GHz hyperfine transition frequency for hydrogen:
https://ntrs.nasa.gov/api/citations/19880003303/downloads/19880003303.pdf
https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.971036/
“The Sounds of Hydrogen” WSU article:
https://hydrogen.wsu.edu/2018/07/18/the-sounds-of-hydrogen/