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 harmonic (overtone) series — which arises spontaneously from any vibrating string or air column — the subharmonic series is something you have to build by deliberately adding lower octave-and-fifth-related tones below a fundamental. Hardware units like the DBX 120A Sub-Harmonic Synthesizer and the Waves MaxxBass plug-in implement a similar principle by generating sub-bass synthesis (or subharmonic synthesis) in a signal-processing chain; this simulator lets you explore the same f/n relationships directly in the browser without any hardware or plug-in required.
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 (the sub-fifth interval). 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 approximately −6 dB below the dry signal in a parallel synthesis or parallel processing chain and it sounds "fuller" without sounding "wrong". This technique underpins hardware "sub-harmonizer" units and the sub-octave generators found in vintage synthesizers like the Roland SH-101 and Korg MS-20. The "Octave reinforcement" preset demonstrates this technique. To check the frequency content you are adding in a mix context, the FFT frequency analyzer can confirm the relative levels of f and f/2 in your signal chain.
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 (also called virtual pitch or residue pitch): the auditory system finds the highest common divisor of the present partials and reports its pitch as the "fundamental", even though no acoustic energy exists there. For f = 600 Hz the three subharmonics are at 300, 200, 150 Hz — the perceived "phantom f0" is at 50 Hz. The difference-tone effect at the cochlear level contributes to this perception: nonlinear interactions between 300 Hz and 200 Hz generate a 100 Hz distortion product, and between 200 Hz and 150 Hz generate a 50 Hz product, reinforcing the phantom fundamental. 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. After experimenting here, use the sub-bass frequency detector with a microphone to verify whether your speakers are actually reproducing the low frequencies or the bass is purely psychoacoustic.
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.
How is this browser tool different from a hardware subharmonic synthesizer?
Hardware units like the DBX 120A Sub-Harmonic Synthesizer detect the dominant frequency in an input signal and add synthesized sub-octave content in real time — they work on any incoming audio. This tool is a standalone oscillator bank: you set the fundamental manually and hear the output directly. There is no input processing. The advantage is precise, clean tones with full control over each subharmonic's waveform and level, making it ideal for psychoacoustic testing, speaker evaluation, and understanding the theory before purchasing hardware.
Which waveform shape produces the most perceived sub-bass body?
For sub-bass fundamental reinforcement, a sine wave on f/2 or f/4 adds clean, pure low-frequency energy with no harmonic clutter — ideal for subwoofer testing and clean bass enhancement. A triangle wave is the next most pure; a sawtooth or square wave on a sub slot adds rich odd or mixed harmonics that make the sub-bass perceptible on speakers that roll off below 60 Hz (the harmonics carry the pitch information even when the fundamental can't be reproduced). For small speakers, sawtooth on f/4 exploits the missing-fundamental effect to imply bass that isn't physically there.
Can I use this tool to calibrate or tune a subwoofer crossover point?
Yes, as a reference signal source. Set the fundamental so that f/2 or f/4 falls at your target crossover frequency (e.g., 80 Hz), play the sub-bass driver test preset, and listen for the handoff between your main speakers and subwoofer. Because this tool generates known, stable sine-wave tones, it is a more controlled test signal than music. For measurement-based crossover alignment, combine it with an acoustic measurement tool that uses a calibrated microphone — this browser tool generates the signal; the measurement tool confirms what the room actually hears.