I have been learning to play guitar, and I wanted to look at it from a more physical point of view, because that is something I like to do. Maybe a similar question has been asked here, but I could not find it. If this has already been asked, and if you have the link, please share.
What is the similarity between the frequencies 130.81Hz (C3) and 261.63 (C4), with standard reference as A4 = 440Hz, that makes us call both the frequencies as C? Why do they sound similar? Say I played both the Cs together in unison while playing fingerstyle, it sounds good, and they resonate really well, but when played separately they do sound different. Why is that?