Ohm proposed it and Helmholtz elaborated it in a book he published about "sound sensations", which I suppose would be called "psychoacoustics" in modern parlance.

I found it online [1] and let Google translate it [2].

[1] https://psychologie.lw.uni-leipzig.de/wundt/opera/helmhltz/t...

[2] https://psychologie-lw-uni--leipzig-de.translate.goog/wundt/...