Question

When sounds of two frequencies sound 'good' together, what does that mean? Why does one note which is an octave above another note, sound like the same note?

Answer 1

Two notes sounding “good” together sounds like a very subjective statement. The songs we like and the sounds we like are incredibly dependent on our culture, personality, mood, etc.

But there is something that feels fundamentally different about certain pairs of notes that sound “good” together. All over the world humans have independently chosen to put the same intervals between notes in their music. The feeling of harmony we get when we hear the notes C and G together and the feeling of disharmony we get when we hear C and G flat together turns out to be part of the universal human experience.

Instead of from subjective notions of “good” and “bad”, scientists call the feeling of harmony “consonance” and the feeling of disharmony “dissonance”. Some cultures and genes of music use a lot more dissonance, but most humans perceive the same relative amounts of dissonance between pairs of notes.

The most consonant pairs of sounds are two sounds that are perceived as having the same “pitch” . In other words, the G key below middle C on my piano is so consonant with the G string on my guitar that they are said to be the same note.but to really answer this question scientifically you need to look beyond acoustics and examine how the brain processes musical grammar. Traditional consonance/dissonance theories and psychoacoustics only explain why notes sound good outside of a musical context; they don't tell you anything about why they sound good in an actual piece of music.

About a year ago, I attended a talk by a neuroscientist who studies music. He had done some very interesting studies on people who had underwent very specific trauma to the brain and lost the ability to form coherent sentences. They still had their entire vocabulary, but they no longer had the grammar to put words together. He wondered whether they would also lose the ability to hear musical grammar (as in, which notes sound good together, when does a passage of music sound "complete" or "satisfying", etc).

His experiment was to play music for these people that fit the normal standards of "sounding good" and "making sense" versus music that used similar materials but was essentially randomly constructed. The patients in the study could not distinguish one from the other.

So from that, we see why musical grammar varies so much from one culture to another, and why certain people think some music sounds good while others can't stand it. What sounds good in music is a learned grammatical process. Yes, consonance and dissonance play some role on an atomistic level, but those rules are easily overridden by context: there are highly dissonant passages in very popular pieces of music from virtually any historical period and geographical area.

As children, we learn what sounds good musically in the same way we learn a language. We can also learn new musical grammars through study when we get older (what most people would call "expanding your tastes"). Either way, your brain needs to develop neural pathways that reward "good sounding music" for you to get any enjoyment out of it.