Is it like the visible spectrum and we just can’t physically hear anything else?
What determines the dividing line between each note?
I know this is more than one question but I just don’t understand the science behind music
In: Physics
With sound you have lots of harmonics. Basically if you generate a sound wave it will usually also generate a wave double the frequency, and three times the frequency, and four times the frequency, etc. So we have learned to perceive these sounds as the same, so we name them the same. If you start with an A and double the frequency, you still have an A, just at a higher pitch. So the A-G notes just repeats as you get higher and higher in pitch.
As for the specific notes they are the sound waves which fit well together because they share harmonics. To start with A and E have a frequency ratio of 3:2. This means that the third harmonics of A is the same as the second harmonic of E. This is called a perfect fifth. We also use the minor third interval of 6:5 which in the case of A is a C. We can also go down a fifth and a third to get D and F. I can not remember the B and G intervals but it uses similar concepts of matching harmonics.
This gets us the 7 notes that sound good together and are the only notes you need to write a song. Any notes not using these specific 7 frequencies are probably not going to sound good. But you do not have to use these specific 7 notes. If you do the same harmonics calculations but instead of starting with A you start with C you get C,D,F and G from your A scale. But if you go 6:5 up from C you get a note that is higher then E but lower then F. We call this Eb or F#. Similarly you get Ab and Bb. If you go through all base notes this way you actually get 12 notes, not 7.
But there is more. Instead of using a ratio of 6:5 for the third, which sounds kind of somber, we can use 5:4. This is called a major third and sound more happy. You go through the same calculations of the ratios and you end up with the exact 12 notes. And if you start with C you end up with the original A-G notes that is used for A-minor.
This is the basic music theory used in western music. There are an infinite amount of frequencies of sound, it is just that we have found out that these sets of 7 notes out of 12 sounds best together as they have a lot of harmonics. Other music styles use different notes. This is why music from for example the Middle East or China sounds so different, they literally use different notes. And even things like Lo-Fi deliberately use other notes then those on the western music scale to sound different.
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