A sub oscillator one or two octaves below the main oscillator adds low-end weight without audibly changing the main timbre

A sub oscillator is a secondary simple-waveform generator (typically sine or square) set one or two octaves below the main oscillator pitch. Its role is to add low-frequency energy — weight, rumble, or punch — without contributing to the harmonic complexity of the mid/high range. Keeping the sub level low (barely audible on its own) is key: it should support rather than dominate. Common choices: sine for clean sub energy, square for a slightly harmonically richer sub. On basses, the sub provides the ‘felt’ low end on systems with subwoofers while the main oscillator handles the recognizable timbre.

Examples

Bass patch: main oscillator at -1 octave (saw). Sub oscillator at -2 octave (sine), level at ~30%. Result: the bass has both a defined timbre from the saw and felt low-end weight from the sub.

Assessment

Design a bass patch with and without a sub oscillator. Play the same note on a system with a subwoofer (or check in a spectrum analyser). Describe the difference in low-frequency content below 80 Hz.