The subharmonic series at a fundamental f consists of the integer divisions: f, f/2, f/3, f/4, f/5, .... This tool exposes the first four. Unlike the natural overtone series (which arises spontaneously from any vibrating string or column), the subharmonic series is something you have to build by deliberately adding lower octave-and-fifth-related tones below a fundamental.
Music intervals each subharmonic produces
- f / 1 = f — the fundamental itself. No interval, just the reference pitch.
- f / 2 — one octave below f. Same note name, an octave lower (220 Hz → 110 Hz; A3 → A2).
- f / 3 — one octave + a perfect fifth below f. A different note than f, with a 2-cent deviation from equal temperament (just intonation 3:1 is slightly flat of ET's 19 semitones).
- f / 4 — two octaves below f. Same note name, two octaves lower (220 Hz → 55 Hz; A3 → A1).
Bass enhancement
Adding a small amount of f/2 below a fundamental thickens the bass without changing the perceived pitch (because the listener interprets the f and f/2 as the same note in different octaves). Recording engineers use this trick on kick drums, bass guitar, and synth bass — mix a sub-octave oscillator at ~ −6 dB below the dry signal and it sounds "fuller" without sounding "wrong". The "Octave reinforcement" preset demonstrates this.
Organ pedal stops
Pipe organs have "stops" that select which pipes sound when a key is pressed. 16-foot stops sound an octave below the key (f/2). 32-foot stops sound two octaves below (f/4) — the deepest pipes on the largest organs. A "quint" stop sounds f/3, an octave + fifth below: a perfect 5th below the 16-foot, which combines with the actual key to produce a perceived sub-octave fundamental through the difference-tone effect. Try the "Organ quint" preset.
Missing-fundamental demonstration
If you play just f/2, f/3, f/4 (with f muted), the brain often perceives a phantom pitch lower than any of the actual tones. This is the missing-fundamental phenomenon: the auditory system finds the highest common divisor of the present partials and reports its pitch as the "fundamental", even though no energy exists there. For f = 600 Hz the three subharmonics are at 300, 200, 150 Hz — the perceived "phantom f0" is at the GCD-derived frequency of 50 Hz. This is the same effect that lets you hear telephone bass response — the speaker can't reproduce 100 Hz but you "hear" it from its harmonics.
Sub-bass driver testing
Playing f/4 alone at a low fundamental gives a clean sine in the deep-bass range (60 Hz fundamental → 15 Hz sub). Useful for checking whether a subwoofer cleanly reproduces sub-audible content or whether it adds harmonic distortion. The "Sub-bass driver test" preset puts you in this configuration.
Is "subharmonic" the same as "undertone"?
Mathematically, yes — both terms refer to the series f/n where n = 1, 2, 3, .... Music theorists sometimes prefer "undertone series" while audio engineers prefer "subharmonic". The German theorist Hugo Riemann pioneered the undertone-series framing in the late 1800s as a symmetry argument with the overtone series; modern psychoacoustics doesn't fully support the undertone series as physically real (real vibrating bodies don't spontaneously produce undertones), but the construction is a useful tool for sound design and analysis.
Why does f/3 show a "−2c" cents deviation from the equal-tempered note?
Because f/3 in just intonation is a perfect 12th (octave + perfect 5th) below f, but the equal-tempered perfect 5th is approximately 2 cents sharper than the just 3:2 ratio. So just f/3 lands 2 cents flat of the nearest equal-tempered semitone. This is the classical "syntonic comma" deviation that explains why pure 5ths and 4ths can't all coexist with equal-tempered octaves.
Why does the "Missing fundamental" preset sometimes produce no perceptible phantom bass?
It depends on whether the implied fundamental falls in your audible range and on individual perception. For fundamental f, the implied phantom-f0 from {f/2, f/3, f/4} sits around f/12 (the largest common divisor). At a fundamental of 240 Hz the phantom is at 20 Hz — at the lower edge of audibility, and not all listeners hear it. Try a fundamental of 600 Hz or higher (phantom at 50+ Hz) for a more reliable demonstration.
How is this different from the Multi-Frequency Mixer?
The Multi-Frequency Mixer has four independent oscillators — you set each frequency directly. This tool ties all four oscillators to a single fundamental-frequency control via the f/n divisor relationships. Changing the fundamental moves all four together; you can't set f/2 to an arbitrary frequency relative to f. The mixer is more flexible; this tool is more focused on the specific musical structure of subharmonics.
Why does the "Wide bass spread" preset sound so much fuller than a single tone at the same volume?
Three reasons: (1) you're literally hearing more total energy because three oscillators are summed; (2) the f + f/2 + f/4 octave stack has perfect just-intonation ratios that fuse perceptually into a single rich tone; (3) the f/4 component sits in the chest-rumble region (below 100 Hz at typical fundamentals) and engages somatic feel as well as hearing. This is the same trick subwoofers, big speaker cabinets, and orchestral contrabass sections use.
Can I use this to build a synth bass patch?
It's a building block, not a complete patch. Add sub-octave + sub-twelfth content to your bass synth's output (route it through a parallel signal-flow with this tool's settings) and you get the spectral structure of subharmonic-rich bass. Real synth patches add envelopes, filtering, and modulation, which this tool doesn't provide — but the spectral character carries over.
Are the subharmonics actually below human hearing at low fundamentals?
For fundamental f = 60 Hz: f/2 = 30, f/3 = 20, f/4 = 15 Hz — the last is at the very edge of human hearing (most adults can't perceive 15 Hz as a tone, but feel it as vibration). Your speakers / headphones may also roll off below 60 Hz, so what you actually hear depends on your playback chain. The spectrum plot's lower-frequency floor is set at 10 Hz so these sub-audible tones are still visualised.
Will sub-bass content damage my speakers?
Small speakers can mechanically over-excurse on sub-bass at high volume — listen for distortion, port chuffing, or cone bottoming. Back off if you hear those. Properly-rated subwoofers and headphones handle moderate sub-bass routinely. The Sub-bass driver test preset is designed to be a clean reference; if you hear non-sine artefacts from it, your playback chain is adding distortion below its rated lower limit.