Building a Sub-Bass Generator
An octave divider cuts the frequency in half. Chain two together and you're two octaves down. Instant sub-bass.
Frequency Division
This oscillator plays at the frequency you pick. The divider below it plays at exactly half that frequency, one octave down. In analog circuits, this is a flip-flop that toggles on every other cycle of the input. Digital synths just set a second oscillator to half the frequency.
Chain a second divider and you're two octaves below the original. Three simple oscillators, and suddenly you have serious low end. This is the technique behind the sub-oscillator on a Minimoog, the SH-101, and dozens of other classic synths.
Mixing the Layers
The key is the mix. Too much sub and the sound gets muddy. Too little and you don't hear the added weight. Most patches work best with the original at full volume and the sub-octaves mixed in below it. The dividers always stay locked to the fundamental, so everything tracks together when you play different notes.
Try it! Press Play and drag the mix sliders. Start with just the original, then bring in the octave-down divider. Add the second divider for deep sub-bass. Play the keyboard and watch all three track together.
Why It Matters
Sub-bass generators show up everywhere in synthesis. The Moog Sub 37 is named after its sub-oscillator. The Roland SH-101's sub-osc is what gives it that fat bass sound. In a mix, sub-bass fills the 20-80 Hz range that you feel more than hear. Without it, bass lines can sound thin on big speaker systems. With it, the same patch shakes the room.