the invisible piano
play it, and watch what sound actually is. not the wavy line — the air itself, shoving and thinning as the note travels through it.
click the keys, or use your keyboard: a s d f g h j k for the whites, w e t y u for the blacks. hold two at once.
press a key. nothing here is a metaphor — the numbers are the note.
one harmonic is a pure sine — a sound that basically doesn't exist in nature. everything real is a stack of them. that stack is why a violin and a flute playing the same note aren't the same sound.
pressure over time — the “wave” everyone draws
the harmonics inside it
so what is sound
a speaker cone pushes out. the air right in front of it has nowhere to go, so it bunches up. that crowded patch shoves the air next to it, which shoves the air next to that. nothing flies across the room — each molecule moves maybe a millionth of a metre and returns. the crowding is what travels, at 343 metres a second.
your eardrum is just a membrane waiting for those pressure changes to arrive. how often they arrive is pitch. how hard they shove is volume. what else rides along on top of the fundamental is timbre — why you can tell your mother's voice from a stranger's saying the same word.
the wavy line above is not a picture of sound. it's a graph — pressure on one axis, time on the other. the dark panel at the top is closer to the truth.
beats & interference
two notes almost in tune. they drift in and out of step, and where they oppose each other the sound cancels itself. nothing is turning the volume down — the waves are just disagreeing.
the last 1.4 seconds of loudness. each bulge is one beat.
tone A
440.0 Hz
tone B
443.0 Hz
beat rate
3.0 / sec
the classic wobble — this is how you tune an instrument by ear
the beat rate is just the difference between the two frequencies. no other maths. piano tuners listen for it to hit zero.