I am trying to understand how our brain interprets sound and I am bumped on a few things. We can differentiate between different frequencies of sound (athough if they are close enough we might get confused). And there is something called pitch which is a subjective realisation of frequency of sound by our brain. Now there is something called an octave which I don't understand. When you double the frequency you go an octave higher. For eg:- If the frequency is x Hz and you make it 2x Hz then it is an octave higher. But it turns out that 2x Hz and x Hz sound similar to us? Why is that? Also why the factor two? Is it something related to music or does it relate to how our brain is structured?