The sine wave is the atom of sound, a single frequency with no harmonics, the clean whistle of a tuning fork that every additive patch starts from.
In the world
A flute's lowest register and a clean whistle are nearly sinusoidal: one frequency, almost no overtones.
A sine wave has a single frequency and nothing else. On the scope it is a smooth curve with no corners - the smoothest, roundest waveform you will encounter. Your ear hears it as a hollow, slightly synthetic hum because nature almost never produces a tone this pure.
Frequency is counted in Hertz, cycles per second. A4 is 440 Hz, which means 440 full sine cycles pass your eardrum every second. Double the frequency and pitch rises by an octave. Play the VCO at a low octave and the sine booms with body; play it high and it becomes delicate, thin, easily drowned out - because there are no harmonics to give it weight.
Did you know?
The international standard pitch A = 440 Hz was only agreed upon in 1955. Before that, orchestras across Europe tuned to anything from 415 to 460 Hz. Mozart's A sounded closer to our G sharp.
Explore
The ripple pool on the right is one pure sine wave made visible. Move your cursor up/down to change the frequency - watch how the ripple density changes. Move left/right to control amplitude. Click anywhere on the water to drop a stone and hear a pluck at that frequency.