What it is and how it works
A filter lets one part of the spectrum through and attenuates the rest progressively. The cutoff point is usually the frequency where the signal has already dropped by 3 dB. Beyond the cutoff the attenuation grows with a slope expressed in dB per octave: each “pole” of the filter is worth 6 dB/oct, so 1 pole = 6, 2 poles = 12, 4 poles = 24, 8 poles = 48 dB/oct.
Main types:
- High-pass (HP, low-cut): lets the highs through, removes the lows.
- Low-pass (LP, high-cut): lets the lows through, removes the highs.
- Band-pass (BP): lets a band around a frequency through, removes what is above and below.
- Notch (band-stop): removes a narrow band.
- All-pass: doesn't change the amplitude, only rotates the phase (the basis of the phaser).
- Comb: regular peaks and notches (the basis of flangers and resonators).
- Formant: several band-pass filters imitating vowels (Effector VOX, Fruity Love Philter).
Parameters one by one
- Cutoff: where the cut begins. In the mix it is chosen by ear relative to the lowest useful fundamental of the instrument.
- Slope: 6 dB/oct gentle, almost a shelf; 12 dB/oct natural; 24 dB/oct decisive (standard in synths, Moog ladder); 36–48 dB/oct surgical. The steeper it is, the more phase rotation and the possible peak near the cutoff. In Fruity Parametric EQ 2: 12, 24, 36, 48 dB/oct in Steep or Gentle versions.
- Resonance (Q, emphasis): raises the zone near the cutoff. At high values the filter “whistles” and in analog synths it self-oscillates, generating a sine wave at the cutoff frequency. In the mix, for cleanup, minimal resonance (Q 0.7 = Butterworth, no peak); for effect, high resonance.
- Characteristic: Butterworth (flat in the passband, no peak), Bessel (more linear phase, softer cut), Chebyshev/elliptic (steeper cut with ripple), Linkwitz-Riley (two Butterworths in series, used in crossovers because the bands sum flat).
- Drive: many synth filters (Fruity Love Philter, FabFilter Volcano filters, Serum) saturate before the filter and change the character.
- Modulation: envelope, LFO, sidechain, key tracking: they move the cutoff over time.
When to use them and on what
- Cleanup HP on everything that doesn't carry sub (see the table in the EQ guide). Reduces rumble, useless energy and the work of compressors and limiters.
- Cleanup LP on distorted guitars (8–12 kHz), synth basses (5–8 kHz to remove hiss), reverb returns (6–10 kHz), pads (8–12 kHz) to leave air for vocals and cymbals.
- Creative band-pass: telephone/radio effect on vocals (HP 300–500 Hz + LP 3–4 kHz, 24 dB/oct, plus saturation), filtered intros, transitions.
- Sweeps in arrangement (risers, build-ups): HP on the mix or on the music bus from 20 Hz up to 300–800 Hz over the last 4–8 bars before the drop, then reset on the drop hit.
- “DJ” filters: a single control that does LP to the left and HP to the right (Effector FILTER; Fruity Delay 3 Tone on the wet signal).
- Notch: mains hum at 50 Hz (Europe) or 60 Hz (USA) and harmonics (100, 150, 200 Hz…), single whistles. Q 20–40, −20/−40 dB.
- Formant: synthetic vowels on pads and basses (talking bass), “voice” on leads.
Starting settings
| Use | Type | Cutoff | Slope | Resonance |
|---|---|---|---|---|
| Male vocal cleanup | HP | 70–100 Hz | 12–24 dB/oct | minimal |
| Trap pad cleanup | HP | 150–250 Hz | 24 dB/oct | minimal |
| Vocal reverb return | HP + LP | 200–400 Hz / 6–10 kHz | 12–24 dB/oct | minimal |
| Telephone effect | BP | 400 Hz–3.5 kHz | 24 dB/oct | 0.7–1.5 |
| House build-up | HP sweep | 20 Hz → 400 Hz in 8 bars | 24 dB/oct | light (Q 1–2) for emphasis |
| Acid bass (TB-303) | LP | 300 Hz–3 kHz modulated | 18–24 dB/oct | high |
| 50 Hz hum | Notch | 50, 100, 150 Hz | — | Q 30 |
| Distorted guitar | LP | 9–12 kHz | 12 dB/oct | minimal |
By genre
- House, French house, nu-disco: automated LP filters on sample loops and on the whole groove are a compositional element (filter house).
- Techno and acid: resonance and cutoff as melodic voices; long automations of 16–64 bars.
- Trap: high HPs on melodies and samples (even 300 Hz) to leave all the low end to the 808.
- Lo-fi: LP 4–8 kHz on the music bus or on the master for the “cassette” character.
- Rock: HP and LP on guitars to define their “corridor” between bass and cymbals.
Phase and filters
Every IIR filter rotates the phase near the cutoff: the steeper the slope, the greater the rotation and a possible peak of a few tenths of a dB. On a single instrument it is inaudible; on two microphones on the same source filtered differently it can change the sum. In mastering and in crossovers, linear phase filters or Linkwitz-Riley are preferred. Fruity Parametric EQ 2 shows the phase rotation in IIR mode.
Common mistakes
- High 48 dB/oct HPs on everything out of habit: thin sound and strange phase behavior.
- HP on the master to “clean up” instead of solving it track by track.
- High resonance on cleanup filters: a peak that emphasizes exactly the zone you wanted to control.
- HP sweep on the master that also cuts the kick of the next drop (forgotten reset).
- LP too low on returns: reverb that becomes “muddy” instead of distant.
How to hear it
- On a pad, raise the HP until the vocal is no longer covered; then back off until the pad loses its warmth.
- Compare 12 and 48 dB/oct at the same cutoff on the bass: listen to the “punch” near the cutoff.
- Raise the resonance of a 24 dB/oct LP on a synth and sweep: hear the peak turn into a whistle.
Starting settings
| Parameter | Value | Why |
|---|---|---|
| Cleanup slope | 12–24 dB/oct | Natural, little phase rotation. |
| Surgical slope | 36–48 dB/oct | Only when a clean cut is needed (808 sub, rumble). |
| Resonance for cleanup | Q 0.7 (Butterworth) | No peak near the cutoff. |
| Creative resonance | Q 2–10, up to self-oscillation | Expressive peak, voice of the filter (acid, sweeps). |
| Build-up sweep | 20 → 300–800 Hz over 4–8 bars | Tension before the drop. |
| Anti-hum notch | 50/100/150 Hz, Q 30, −30 dB | Removes the hum without touching the rest. |
Frequencies to watch
| Range | What to do |
|---|---|
| 20–40 Hz | Universal cleanup HP (except sub/808/kick). |
| 50 / 60 Hz and harmonics | Notch Q 20–40 against mains hum (50 Hz in Europe). |
| 100–300 Hz | HP on pads, backing vocals, guitars, leads in dense mixes; even higher in trap. |
| 300 Hz–3.5 kHz | Band of the telephone/radio effect. |
| 6–12 kHz | LP on distorted guitars, reverb returns, synth basses. |
Example chain
| Step | Tool | Settings | Why |
|---|---|---|---|
| Cleanup filters (first slot) | Fruity Parametric EQ 2 / Pro-Q | HP/LP 12–24 dB/oct | Before dynamics and saturation. |
| Creative filter | Fruity Love Philter / Fruity Filter / Effector FILTER / Volcano 3 | Automated cutoff and resonance | Tonal movement. |
| Saturation before/after the filter | Fruity Blood Overdrive / Distructor | Moderate drive | Before: the filter tames the harmonics; after: the resonant filter saturates and becomes aggressive. |
| Filter on the effects return | Fruity Reeverb 2 (L.CUT, H.CUT) / Fruity Delay 3 (filter in the feedback, Tone) | HP 200–400 Hz, LP 6–10 kHz | Effects that don't dirty the mix. |
| Cutoff automation | Automation clip / Fruity Peak Controller / Fruity Formula Controller | Tempo-synced curves | Build-ups and rhythmic movement. |
How to check the result
- Track spectrum: below the cutoff the energy falls with the chosen slope, with no peak of more than 1 dB near the cut (cleanup filter).
- Mix sum below 40 Hz made up only of kick and bass/808 (analyzer on the solo buses).
- After a filtered build-up, on the first downbeat of the drop the filter is fully open (check the automation).
- Telephone effect: the energetic band lies between about 300 Hz and 4 kHz in the spectrum.