Notes and the staff · Music theory
The qualities of sound, the names of the notes, the staff, ledger lines and how octaves are numbered.
Sound and what gets written
A musical sound has four qualities: pitch (low or high, depending on its frequency), duration, loudness and timbre, which is what tells a violin from a flute playing the same note.
A score writes down mainly the first two: pitch, by where the note sits on the staff, and duration, by its note value. Loudness is shown with dynamics (p, mf, f…) and timbre, by the instrument the music is written for.
Sound and noise
What sets a musical sound apart from noise is the regularity of the vibration. A string or a column of air vibrates periodically, so the ear gives it a pitch. A knock or a scrape vibrates irregularly, and does not. But music uses both: percussion without definite pitch is part of it too.
What is inside a sound
A sound is a vibration. Its pitch depends on frequency, measured in hertz: the reference A vibrates 440 times per second, and an octave higher, twice that. Intensity depends on the amplitude of the vibration, and duration on how long it lasts.
Timbre is the most curious. A note on an instrument is not a pure vibration: a series of harmonics sounds with it, exact multiples of its frequency. How strongly each one sounds is what makes us hear a clarinet or a violin, even on the same note.