What Are Subharmonics?
Harmonics go up (x2, x3, x4). Subharmonics go down (divide by 2, divide by 3, divide by 4). They add weight below the fundamental, but unlike harmonics, you have to create them yourself.
Above and Below
Play the tone. Those are its harmonics above: the overtones that make it sound bright. Every acoustic instrument produces them naturally. A vibrating string at 440 Hz also vibrates at 880, 1320, 1760, and so on. Those are the harmonic series.
Now toggle on the subharmonics below. Hear how the sound gets deeper and wider? Those frequencies sit at half, a third, a quarter of the fundamental. They don't appear in nature. No vibrating string spontaneously produces a tone at half its frequency. You have to build them yourself.
Why It Matters
Switch to a sawtooth. It already has harmonics built in. But there's nothing below the fundamental. Subharmonics don't appear naturally. You have to add them. That's why subharmonic generators exist in synths: they take whatever note you're playing and generate new tones below it, filling in the low end that the oscillator can't provide on its own.
Try it! Press Play and toggle individual harmonics and subharmonics on and off. Switch waveforms to see which already have harmonic content. Use the presets to hear the difference between harmonics only, subharmonics only, and the full stack.
Going Deeper
In the next part, we'll build a practical sub-bass generator that chains frequency dividers together. Two dividers in series give you two octaves of sub-bass from a single oscillator. That's the technique behind classic analog sub-oscillator circuits.