what SMOOTH does
SMOOTH detects and suppresses problematic resonances in your audio: the harsh frequencies that make vocals pierce, synths bite, and mixes fatigue. it analyzes your signal across 40 psychoacoustic frequency bands, identifies which peaks are resonances versus musical character, and transparently reduces only the problems.
think of it as an intelligent, frequency-selective compressor that works on thousands of frequency bins simultaneously, 43 times per second.
what SMOOTH is not for
- i would not use SMOOTH to add resonances or boost specific frequencies. it is a resonance suppressor, not an EQ.
- i would not expect SMOOTH to fix distortion or clipping already recorded into the source.
- i would not use it as a de-esser on its own. SMOOTH can reduce sibilance as part of broader resonance control, but a dedicated de-esser targets that specific task.
- i would not use SMOOTH as my only hum or broadband noise tool. a narrow steady tone can still sit above the spectral envelope and get reduced, but SMOOTH has no fixed-frequency hum tracker.
quick start
first time? start here
- insert SMOOTH on the track and play your material
- load preset Vocal
- adjust DEPTH from the preset's 14 dB until the resonances ease
- toggle DELTA to hear what is being removed
- toggle A/B or bypass to compare with the dry source
"my vocal is too harsh"
- load preset: Vocal or De-Harsh Vocal
- adjust DEPTH to taste (start at 6–8 dB)
- toggle DELTA to hear what's being removed. should sound metallic, not musical
- if needed, drag band 1 node to the harsh frequency (usually 2–5 kHz) and boost it
"my drum bus has ring"
- load preset: Drum Bus
- set ATTACK to 1–2 ms (fast, preserves transients)
- toggle INTENSE to preview the effect, then disable
- use band 0 to target the specific ringing frequency
"my synth is piercing"
- load preset: Harsh Synth Tamer
- increase DEPTH if needed
- band 1 is pre-positioned at 2.5 kHz. drag it to the problem frequency
- tighten Q (mouse wheel) for surgical treatment
"my mix sounds harsh at loud parts"
- load preset: Full Mix or Mastering
- keep MIX at 50–80% for gentle parallel processing
- set SENS high (8–10 dB) so only the worst resonances are caught
- use DELTA to verify you're only removing problems
"i want tighter low end in M/S"
- switch to M/S mode (click L/R → M/S)
- load preset: M/S Low Tighten
- this targets low-mid mud in the center while leaving sides untouched
listen
hear the difference.
click play. choose original or with SMOOTH while listening. replay from start to hear the phrase again.
🎧 for best results, listen on headphones or studio monitors.
space: play/pause. left/right or a/b: switch.
loading audio
space: play/pause. left/right or a/b: switch.
loading audio
controls
SMOOTH has 4 knobs, 2 toggle buttons, 4 draggable sensitivity nodes, a mode selector, and an AUTO loudness matching toggle in the mode bar. that's it.
knobs
| control | range | default | what it does | what you hear |
|---|---|---|---|---|
DEPTH | 0–24 dB | 6 dB | how much resonance is removed. higher = more aggressive suppression. | turn left: less cut, more ringing can stay. turn right: more aggressive resonance removal. |
SENS | 0–12 dB | 4 dB | how prominent a resonance must be before SMOOTH catches it. lower = catches more. higher = only the worst offenders. | turn left: catches quieter resonances. turn right: only the loudest peaks get suppressed. |
ATTACK | 0.5–50 ms | 2 ms | how quickly SMOOTH reacts. fast (1–5 ms) for transients. slow (20–50 ms) for gentle, transparent processing. | turn left: faster, catches resonances sooner. turn right: slower, lets more of each hit through before it grabs. |
MIX | 0–100% | 100% | wet/dry blend. use 50–80% for parallel processing. | turn left: more dry, less suppression. turn right: full processed signal. |
toggle buttons
| button | what it does | what you hear |
|---|---|---|
DELTA | hear what SMOOTH is removing. if you hear music, dial back DEPTH or raise SENS. if you hear only metallic resonance, you're dialed in. | off: normal mix. on: hear only what SMOOTH removes. |
INTENSE | 3x depth multiplier for exaggerated processing preview. use to quickly hear what SMOOTH is targeting, then turn off for mixing. orange when active. | off: true depth. on: 3× depth preview (orange readout). |
mode selector
| mode | best for | what you hear |
|---|---|---|
Resonance | default. spectral resonance suppression via the 5-stage pipeline. works on everything. | full spectral suppressor on sustained ringing. |
Transient | time-domain transient shaping. controls attack punch and sustain character. best on drums and percussive material. | time-domain shaping; more punch/sustain edit on drums. |
M/S toggle
the L/R ↔ M/S button in the bottom bar switches between stereo and mid/side processing:
- L/R (default): left and right channels processed independently. what you hear: resonances on each channel are treated on that channel.
- M/S: mid and side channels processed independently. allows surgical treatment of the center image vs. the sides. what you hear: suppress problems in the centre without touching side reverb, or the reverse.
sensitivity nodes
the 4 colored nodes on the spectral display let you shape where SMOOTH is most and least aggressive.
these are not an EQ. they look like one, and that trips people up, so to be plain about it: moving a node does not add or remove gain from your track. it changes how hard the suppression works in that region. boost = more reduction, cut = less. that is the opposite of what the shape suggests, and it is the same convention soothe uses. if you want to see it rather than trust it, turn on DELTA and drag a node: the delta gets louder where you boosted, while your actual signal is untouched.
how they work
- drag horizontally: move the node's center frequency (20 Hz to 20 kHz)
- drag vertically: adjust sensitivity (-12 to +12 dB). boost (up) = more aggressive processing. cut (down) = less processing.
- mouse wheel on node: adjust Q (bandwidth). narrow Q = surgical. wide Q = broad.
- shift + drag: fine adjustment (0.2x sensitivity)
- double-click: toggle node on/off
- right-click: choose channel routing (stereo, mid, side) and shape (bell or shelf for bands 0 and 3)
shelf mode (bands 0 and 3)
the outermost nodes (band 0 and band 3) can switch from parametric bell to shelving. right-click the node and choose "shelf" under the shape section. band 0 becomes a low shelf (affects everything below its frequency). band 3 becomes a high shelf (affects everything above). the Q control adjusts how steep the shelf transition is.
node colors
| node | default frequency | color |
|---|---|---|
| band 0 | 250 Hz | teal |
| band 1 | 2500 Hz | gray |
| band 2 | 800 Hz (off) | orange |
| band 3 | 6000 Hz (off) | purple |
bands 2 and 3 are disabled by default. double-click to activate them.
shelf mode
hover over any node to see inline controls. click the enable dot (left of node) to toggle it on/off. for bands 0 and 3, click the type badge to cycle between BELL, LS, and HS. low shelf on band 0 shapes everything below, high shelf on band 3 shapes everything above. double-click the label text to type exact values (e.g. "2.5k +3.0 dB Q 0.7"). right-click any band for the full menu as a fallback.
per-band M/S routing
when in M/S mode, each node can be routed independently:
- stereo (default): applies to both mid and side engines
- mid: only affects the center image (vocals, kick, bass)
- side: only affects the stereo field (reverb, room, width)
right-click a node to change its routing. mid-routed nodes show a green "M" badge. side-routed nodes show a purple "S" badge.
spectral display
the real-time spectral display shows:
- cyan curve: your input signal's frequency spectrum
- orange overlay: gain reduction being applied (taller = more reduction)
- colored fill behind spectrum: sensitivity curve shape from your EQ nodes
- range handles: two draggable handles at the edges let you restrict suppression to a specific frequency range. everything outside passes through untouched.
AUTO loudness matching
the AUTO toggle in the mode bar enables K-weighted loudness compensation (ITU-R BS.1770). it automatically adjusts the output level to match the input, so your A/B comparison is fair. you hear the effect, not the volume difference.
what you hear: off: level follows depth and processing. on: output loudness tracks input so bypass comparisons stay level-matched.
display interactions
- scroll wheel: zoom frequency range (centers on cursor)
- double-click: reset zoom to full 20 Hz to 20 kHz range
- drag range handles: restrict processing to a frequency band
display range
- frequency: 20 Hz to 20 kHz (log scale)
- level: +12 dB to -60 dB
reading the display
the spectral display shows +12 dB to -60 dB. a curve flat along the top line is above the display range, not clipping in the audio. the display clamps the visualization at +12 dB to fit loud resonant peaks within the window. the audio itself is not clipped.
low-latency mode
toggle the LIVE button in the plugin UI to switch from standard mode (~93ms latency, full spectral resolution) to LIVE mode (~23ms latency, optimized for tracking and monitoring).
LIVE mode uses a smaller FFT window (1024 samples at 44.1 kHz vs 4096 in standard). the trade-off: slightly less spectral detail in the processing. on most material the difference is subtle. standard mode is better for critical mixing decisions. LIVE mode is better for hearing what the plugin does while you record.
right-click the spectrum display in LIVE mode to choose between LIVE analysis (matches the audio engine's lower resolution) or HQ analysis (runs the full-resolution analyzer alongside the LIVE audio path for a higher-detail visualization).
CPU stays under 3%. only one engine runs at a time. the inactive engine is fully gated. switching between modes uses a 50ms crossfade with no glitch or click.
LIVE mode state is preserved across preset loads and A/B comparisons.
presets
SMOOTH ships with 32 factory presets organized by use case.
resonance mode
| preset | use case |
|---|---|
| Gentle | subtle, transparent processing for sensitive material |
| Vocal | general vocal resonance taming |
| Drum Bus | balanced drum processing |
| Full Mix | safe mix bus treatment |
| Aggressive | heavy-handed resonance removal |
| Mastering | conservative, mix-bus safe |
| Dark Source | tames muddy low-mid buildup |
| Bright Source | controls harsh high-frequency content |
| De-Harsh Vocal | aggressive 2–5 kHz harshness removal |
| Harsh Synth Tamer | tames resonant synth patches |
| Surgical Strike | precise, deep cuts at specific frequencies |
| Nuke It | maximum processing. hear everything SMOOTH can do |
| Low-End Cleanup | room rumble and mud removal (80–400 Hz) |
| Kick & 808 | sub resonance taming, protects fundamental |
| Bass Clarity | boxiness removal (250–500 Hz) |
transient mode
| preset | use case |
|---|---|
| Drum Punch | enhance drum attack |
| Vocal Presence | add vocal clarity |
| Snare Crack | tighten snare attack |
| Subtle Shape | gentle transient control |
| Cymbal Tame | high-frequency harshness on cymbals (4–12 kHz) |
| Perc Click | control clicky percussion attacks (1.5–3.5 kHz) |
| Transient / Destroy | extreme transient removal |
M/S presets
| preset | use case |
|---|---|
| M/S Vocal Center | clean up the center image |
| M/S Side Tamer | reduce side-channel harshness |
| M/S Low Tighten | mono bass cleanup (common mastering technique) |
| M/S Master Glue | gentle M/S mastering. mid presence + side smoothing |
browsing
- use
< >arrows in the header to browse presets - the preset name and index are displayed between the arrows (e.g., "Vocal (2/32)")
- A/B button: compare two different settings instantly
tips
gain staging
SMOOTH runs in 32-bit float, so the STFT path does not clip internally. detection is relative: each bin's level minus the ERB spectral envelope sets prominence, and SENS is a dB threshold on that difference. ISO 226 loudness weighting and salience weighting are fixed curves, not level knobs (~110-168, ~785-791). turning the whole track up or down shifts both bin and envelope, so the same resonance can read differently if the noise floor or masking changes. i aim for a sensible recording level, then set DEPTH and SENS on what sticks up above the envelope.
auto comp measures K-weighted loudness of the input and output, then applies makeup gain to match them. it does not process or remove noise. if the source has noise and SMOOTH reduces the signal, auto comp will raise the gain back up, which also raises the noise. i turn auto comp off when comparing a noisy source, or i accept that the output will match the input loudness, noise included.
where it goes in the chain
i place SMOOTH before analog-style saturation or distortion. if i apply SMOOTH after saturation, and auto comp is on, the makeup gain can lift distortion already in the signal. i use SMOOTH to clean up resonances first, then add harmonic color after.
i also place SMOOTH before reverb when i want to suppress resonances in the dry signal that would otherwise excite the reverb tail.
general tips
- DELTA is your best friend. always check what you're removing. good delta = metallic resonance. bad delta = music.
- INTENSE is for previewing, not mixing. turn it on to hear what SMOOTH targets, then turn it off.
- less is more. a DEPTH of 4–8 dB handles most material. go higher only when you hear specific problems.
- SENS controls surgical precision. low SENS = catches everything. high SENS = only the loudest resonances.
- fast ATTACK preserves transients. for drums and percussive material, keep ATTACK at 1–5 ms.
- slow ATTACK for transparency. for vocals and mix bus, try ATTACK at 10–30 ms.
- use M/S for mastering. clean up the center (vocals, kick) without touching the stereo field.
- sensitivity nodes are optional. SMOOTH works with all nodes at 0 dB. use them when you want frequency-specific control.
troubleshooting
| problem | solution |
|---|---|
| "i can't hear any difference" | increase DEPTH. try INTENSE mode. lower SENS. |
| "it sounds duller" | reduce DEPTH. SMOOTH has auto-compensation, but extreme settings can still thin the sound. |
| "transients are smeared" | reduce ATTACK to 1–2 ms. the transient gate preserves attacks, but very slow attack settings override it. |
| "processing is too aggressive" | raise SENS (catches fewer resonances). reduce DEPTH. try MIX at 60–70%. |
| "i hear artifacts" | reduce DEPTH below 12 dB. at extreme settings, cepstral smoothing is disabled for maximum surgical precision, and artifacts may appear. |
| "CPU is too high" | SMOOTH targets <3% at 44.1 kHz on macOS (hardware-accelerated FFT). Windows CPU may be higher. at higher sample rates, CPU increases proportionally. |
| "plugin doesn't appear in DAW" | rescan plugins in your DAW. check that VST3/AU files exist in ~/Library/Audio/Plug-Ins/. |
keyboard / mouse reference
| action | control |
|---|---|
| adjust knob | click + drag vertically |
| fine adjust knob | shift + drag |
| reset knob to default | double-click knob |
| move EQ node frequency | drag horizontally |
| adjust EQ node gain | drag vertically |
| adjust EQ node Q | mouse wheel on node |
| fine adjust EQ node | shift + drag |
| toggle EQ node on/off | double-click node |
| change node routing | right-click node (M/S mode) |
| zoom frequency range | scroll wheel on display |
| reset zoom | double-click on display |
| browse presets | click < > arrows |
| toggle A/B comparison | click A/B button |
| toggle delta mode | click DELTA button |
| toggle intense mode | click INTENSE button |
| switch L/R ↔ M/S | click mode button in bottom bar |
for the curious
deeper signal-path detail, specs and install paths. skip this on a first pass.
how it works
SMOOTH uses a 5-stage spectral processing pipeline. no neural networks. no black boxes. just physics.
STFT analysis
your audio is windowed into overlapping frames and transformed to the frequency domain via FFT. at 44.1 kHz, this gives 2,049 frequency bins with 10.77 Hz resolution.
ERB envelope estimation
the spectrum is analyzed across 40 psychoacoustic bands (ERB scale). this means SMOOTH is 7x more sensitive to resonances at 1 kHz than at 8 kHz, matching how your ear actually works.
spectral reassignment
sub-bin frequency precision identifies where each resonance sits, and determines whether each frequency component is tonal (resonance candidate) or noise (leave alone).
per-bin gain computation
for each of the 2,049 bins (that is the count at 44.1 and 48 kHz; the window scales to 8192 up to 96 kHz and 16384 above, so the bin count scales with it), SMOOTH computes how much to reduce based on: how prominent the resonance is, how tonal it is, how loud the surrounding spectrum is, and your sensitivity node settings. gains are smoothed over time using a zero-delay feedback integrator for tight, artifact-free tracking.
magnitude-only gain application
gains are applied to magnitude only. phase is never touched. this preserves stereo image, transient shape, and harmonic relationships. broadband energy compensation restores the body that resonance removal would otherwise take away.
technical specifications
| spec | value |
|---|---|
| formats | VST3, AU (AUv3), AAX, standalone |
| platform | macOS Universal Binary (Intel + Apple Silicon), Windows 10 + native |
| sample rates | 44.1 to 192 kHz (SFI: adapts automatically) |
| latency | ~93 ms at 44.1 kHz (4096 samples) |
| CPU target | < 3% at 44.1 kHz, 512-sample buffer (macOS with hardware-accelerated FFT) |
| FFT size | 4096 (44.1/48 kHz), 8192 (88.2/96 kHz), 16384 (176.4/192 kHz) |
| overlap | 75% (hop = FFT/4) |
| ERB bands | 40 (20 Hz to Nyquist) |
| sensitivity nodes | 4 parametric bells per instance |
| max reduction | 24 dB (36 dB with INTENSE mode) |
| phase modification | none. magnitude only |
| DRM | none. no iLok. no PACE. one-time activation, then offline. a monthly re-check runs only if you are online, and failing it does not deactivate. |
installation
- download the zip from kernaudio.io and unzip it
- copy the formats you use from the
macOSorWindowsfolder to the locations below (INSTALL.txt in the zip lists them) - open your DAW, scan for new plugins
- find "KERN SMOOTH" under manufacturer: "KERN Audio"
plugin locations
- VST3:
~/Library/Audio/Plug-Ins/VST3/KERN SMOOTH.vst3 - AU:
~/Library/Audio/Plug-Ins/Components/KERN SMOOTH.component - AAX:
/Library/Application Support/Avid/Audio/Plug-Ins/KERN SMOOTH.aaxplugin
activate once with your license key, then it runs offline. it re-checks about once a month if you happen to be online, and stays licensed if that check fails or never happens. no account, no subscription.
version history
v1.5.0 (september 25, 2026)
now available as aax for pro tools on macos and windows. hold shift while using the mouse wheel on a knob for fine steps. when intense is on, the depth readout turns orange and shows ×3, so you can see it at a glance. the frequency labels on the display are easier to read.
v1.4.4 (august 2, 2026)
the sensitivity curve overlay now draws the curve you shaped. it had been drawing a flat line in every previous version, because the frequency anchors it interpolates between were never filled in. the analyzer no longer pins at the top of the display on loud material, and the frequency labels at 20 hz and 20 khz are no longer clipped. one DSP change: below 3 db of depth the masking gate is now fully off, so a low depth setting sounds the same whether or not you had it set higher earlier in the session. also fixes a rare crash when opening the plugin window while audio is playing, and a buffer overrun in the shared auto-compensation code.
v1.4.3 (june 27, 2026)
drag to pan the spectral analyzer when zoomed in (a single click no longer resets the zoom; double-click resets it). the preset name shows a hover affordance so it reads as clickable. the activation window no longer reacts to non-license files dragged over the plugin. no DSP, preset, or parameter changes.
v1.4.1 (june 1, 2026)
offline activation for air-gapped machines: drop a .kernlicense file onto the activation window, no internet needed. once activated, the plugin stays active offline with no time limit (only a refund deactivates it). redesigned activation window. no DSP change.
v1.4.0 (may 28, 2026)
LIVE mode (~23ms latency). preset recall fix for M/S routing. host-bypass latency reporting. shelf chevron on outer EQ nodes. expanded inline hover hit areas.
v1.3.3 (april 10, 2026)
- new: frequency-dependent spectrum smoothing: bass sits calmer, treble tracks transients more responsively (grounded in ERB critical-band integration time)
- new: peak hold ghost line: trails above the main curve when the signal stops, decays at 3 dB/sec, rendered as a solid Catmull-Rom trace in the industry-standard style (SPAN / Pro-Q)
- new: transient flash overlay: curve briefly brightens on kick, snare, or any percussive hit; baseline-subtracted so sustained music doesn't trigger it
- SMOOTH's reassignment spectrum display inherits all three upgrades via a shared base-class helper
v1.3.2 (april 7, 2026)
- refreshed visual identity: cabinet grotesk typeface, warm-tinted neutrals, redesigned arc knobs, background grain texture, spectral glow
- editable EQ values: double-click any node label to type exact frequency, gain, and Q
- improved inline controls: hover zones expanded, controls no longer disappear when moving toward them
- animated toggle button glow on INTENSE and DELTA buttons
- styled popup menus (preset browser, EQ routing, display options)
- smoother CPU meter display
- brighter range handle pills for easier visibility
- added stereo bus layout validation for broader DAW compatibility
v1.3.1 (april 2026)
- band limiting: two draggable range handles on the spectral display restrict suppression to a frequency range
- AUTO loudness matching: K-weighted compensation (ITU-R BS.1770) for fair A/B comparison
- redesigned EQ interaction: hover for inline enable dot and type badge (BELL/LS/HS), double-click to toggle
- per-sample mix smoothing (~5ms) eliminates automation clicks
- delta mode crossfade (~10ms ramp) eliminates toggle clicks
- CPU optimization: conditional skips for cepstral smoothing, group delay analysis, and masking gate
- 123 automated tests
v1.2.1 (march 2026)
- fixed: delta output silent at Depth=0% (STFT round-trip residual eliminated)
- fixed: bypass click artifact in Cubase and similar hosts
- improved: sensitivity curve precision at max SENS
- added: double-click outer EQ nodes for bell/shelf toggle
- added: shelf mode examples in factory presets
- fixed: GUI size persistence across sessions
- fixed: delta, bypass, and stereo mode persist across preset changes
v1.1.1 (march 2026)
- shelving EQ option: bands 0 and 3 can switch from parametric bell to low/high shelf via right-click menu
- spectral display +12 dB grid line and label
- 106 automated tests (26,755 assertions)
v1.1.0 (february 2026)
- perceptual salience weighting. suppression depth weighted by human hearing sensitivity (peak at 2–5 kHz), with tonality-aware backoff for lead clarity
- chirp group delay transient refinement. distinguishes genuine transients from vibrato/tremolo for more consistent suppression on tonal material
- asymmetric masking gate. psychoacoustic spreading function with direction-dependent slopes for transparency on dense mixes
- 5-stage spectral processing pipeline (STFT → ERB → reassignment → ZDF gain → OLA)
- 16 DSP optimizations (OPT-1 through OPT-11, OPT-A/B/C/D/E/F/G/H)
- ZDF envelope detection (Zavalishin 2012)
- auto-compensation (broadband energy restoration)
- sample-rate-independent processing (44.1–192 kHz)
- apple Accelerate vectorization
- 4 parametric sensitivity nodes with M/S per-band routing
- INTENSE mode (3x depth preview)
- catmull-Rom spline spectral display
- 32 factory presets
- 98 automated tests (26,743 assertions)
design & development: Jonas. algorithm based on published research by Auger & Flandrin (1995), Ephraim & Malah (1984), Griffin & Lim (1984), Glasberg & Moore (1990), Breithaupt et al. (2007), Zavalishin (2012), Schroter et al. (2022).