Why are there only 7 musical notes? Was it decided to divide sounds like that or are there no more in nature?
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Sound is vibrating air. The vibration can be faster or slower, and you’ll perceiving it as higher or lower pitch. For example, if you play A on a tuned piano, it will cause a string to vibrate 440 times a second, producing a sound wave at 440 Hz. If that string were not tuned correctly, it could vibrate 439 times or 441 times a second.
You might not be able to tell the difference immediately, because the human ear is not very sensitive to the absolute number of vibrations in a second — but it can readily tell the difference between two notes played together, or shortly separated in time. Some intervals will sounds good, and others will sound bad.
It turns out that notes that are related to each other by simple fractions (2/1, 3/2, 4/3, etc) tend to sound “good”. That’s because a string vibrating at 440 Hz is also producing *overtones*, which are sounds at integer multiples of 440 Hz (880 Hz, 1320 Hz, etc) — not as strongly, but in a way that your ear and brain will notice. This is why if you play the “A” at 440Hz together with the “A” one octave higher, at 880 Hz, they will sound almost perfect together — as if they are the same note, because they are producing the same overtones. In fact they are so similar that in music we call them the same note, “A”.
Another note that sounds good with A is E, at or around 660Hz. That’s because E is producing sounds at 1320 Hz and 1980Hz, which A is also producing. The ratio between E and A is that simple fraction, 3/2.
So: you can play a note at any pitch you want, but music is about playing notes that sound good together. In Western music, we’ve standardized on a scale of seven notes, which mostly have simple fraction relationships with each other, or close to it.
It’s not the only possible scale, but it’s by far the most common. Even cross-culturally, music from regions in the world have a lot of the same notes as in Western music, because of the physical principles involved.
Now, the choice to call 440Hz “A” is a perfectly arbitrary one, because again, the human ear is not sensitive to the exact number of vibrations, only the relationships between notes. Historically in some countries, “A” was something else, like 432 Hz. So long as **every** note is lowered by the same proportion, the overall music might sound lower in pitch, but the melody would still sound essentially unchanged — it would not suddenly sound discordant. Musical arrangements sometimes will raise or lower the pitch of the entire piece to accommodate the range of a vocalist, for example replacing all “A”s with “B”s. This is why pianos have 12 keys in an octave, because the notes A-G are not all equally spaced and sometimes you’ll need to have access to a note “in between” when you’re changing the key of the music.
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