Filters

Filter Sweep Visualiser

A filter sweep only works when you can hear exactly where the cutoff is sitting relative to the material that drives it. Guess and a sixteen bar build turns into a dull smear or a screaming resonance with no payoff. This visualiser drops a second order resonant section on screen so you can watch the peak form, see how much gain stacks up before it feeds back, and compare two views of the same move: the filter position you are actually hearing, and the same sweep redrawn as a gain curve travelling through time. It also checks the sweep against the fundamental of a kick at your tempo, because that number decides whether the sub survives the move or gets cut out from under the arrangement. Everything runs on an analogue prototype with the damping taken from Q, close enough to a ladder filter section to reason with and honest enough to show where it stops describing real plugin behaviour. Move the cutoff, push the Q past the point where plugins start to scream, and read the numbers before you commit them.

Filter type

The shape of the sweep you are planning

Cutoff frequency

Where the filter sits right now, not the end of the move

Resonance Q

0.130

Above Q 6 several analogue-modelled plugins begin to self-oscillate

Sweep start

Cutoff at the top of the build

Sweep end

Cutoff at the end of the build

Sweep length

bars

How long the filter takes to travel

Track BPM

BPM

Used to place the kick fundamental and the sweep in time

Result

How to use it

STEP 1

How to use this visualiser: pick the filter type first. Low pass shapes downwards, high pass clears space upwards, band pass isolates a moving window, and a notch parks a hole on a snare or a vocal. Set the cutoff to the frequency you want to hear right now, then work on Q. The curve shows how much gain stacks at that frequency, and the self-oscillation warning tells you whether the resonance will stay musical or start feeding back on its own.

STEP 2

Next set the sweep range in Hz and the length in bars. The tool measures the travel between the two endpoints, so a 200 Hz to 12 kHz move reads as nearly six octaves and sounds like a real build, while 2 kHz to 4 kHz reads as two thirds of an octave and will barely be noticed. The duration line converts bars into seconds at your BPM so the sweep can be matched to an arrangement section.

STEP 3

Set the BPM and read the kick fundamental row. A kick landing on every quarter note at 128 BPM has a fundamental near 32 Hz, so a cutoff at 800 Hz sits roughly twenty five times higher. When the cutoff is far above that number the sub passes through untouched and only the beater click moves. When it drops close to the fundamental, the sweep starts eating the sub itself and the kick disappears on small speakers.

STEP 4

Finally compare the two panels. The left panel is the filter as a position, one static curve at the cutoff you have set. The right panel is the filter as a curve, gain arriving over time at one kilohertz and at the kick fundamental. If the two panels disagree about where the energy is, you have found the part of the move that will sound wrong before you print the track.

Three common mistakes

Common mistake: typing the cutoff where you want the sweep to end instead of where the filter sits right now. The cutoff field describes the present position of the filter, not the destination of the move. Enter twelve kilohertz, then open the filter down to three hundred hertz while playing, and the tool was describing the wrong filter the whole time. Sweep start and sweep end are the move; cutoff is the moment you are in.
Watch out for the resonance peak sitting slightly below the cutoff you typed. A second order section does not peak exactly at its nominal centre, and the gap widens as Q drops, which is why the model reports a peak frequency of 794 Hz for an 800 Hz cutoff at Q 6. Chasing that last kilohertz is wasted effort, because the difference between a peak at 794 Hz and one at 800 Hz is inaudible on any system you will mix on.
Key point that gets missed: high Q does not add presence, it adds gain. A section at Q 6 stacks over fifteen decibels at the resonant frequency, which is plenty to drive a limiter, a saturator or the next stage in the chain straight into clipping. If the visualised peak gain is larger than the headroom you have left on the return, lower the Q before you lower the cutoff, otherwise the build turns into a distortion hit instead of a crescendo.
Easy to get wrong: assuming the sweep moves the kick. A filter on the drum bus does not move the kick, because the kick is a separate instrument sitting in that bus. It moves whatever shares the bus with it. Read the sweep against the kick fundamental as a reference point for how deep the move is, then decide whether the filter belongs on the kick, on the sub bus, or on the whole drum room.

How the response model works, and where it stops working

The curve comes from a second order analogue prototype with the damping ratio derived from Q, so the resonance peak, the rolloff slope and the passband all emerge from one set of numbers instead of from a drawn approximation. Light damping, which is what a high Q means, lets the response climb far above the passband before the section settles. That is why the tool reports the frequency where the response actually peaks and the gain of that peak relative to the passband, rather than only echoing the cutoff you typed.

Every curve here describes a single two pole section, not a complete filter emulation.
A ladder filter, a state variable filter and a biquad with a different gain compensation land on visibly different curves for identical numbers.
Analogue-modelled plugins add drive, oversampling and modulation, none of which are represented here.
The high pass curve is the low pass prototype mirrored around the cutoff, which is a reasonable stand-in for a real high pass but not exact.

Reading the sweep against the kick

The kick reference is deliberately simple: a kick that lands on every quarter note of a bar repeats once per beat, so its fundamental sits at BPM divided by four. At 128 BPM that is 32 Hz, and a filter cutoff at 800 Hz sits about 4.6 octaves above it. This is the number that tells you whether a sweep will survive translation.

Cutoff well above the fundamental, roughly eight times or more, and the sub passes untouched. Only the beater click is shaped, which is what you want for a bright build that keeps its weight.
Cutoff inside the body of the kick, roughly two to eight times the fundamental, and the sweep carves into the kick itself. That is the classic riser sound, and it disappears on small speakers unless you protect the sub.
Cutoff at or below the fundamental and the kick is gone. Useful as a deliberate drop, never as a surprise.
A half time pattern or a broken beat moves the real fundamental somewhere else entirely, so treat this reference as the straight four on the floor case.

Cutoff, Q and the self-oscillation wall

Q sets the width of the resonance band, and the band width is the cutoff divided by Q, so Q 6 at 800 Hz is a narrow band around the cutoff and Q 0.7 is a broad shelf. The gain that builds inside that band is what you hear as the sweep passes through, and it grows with Q. Below Q 2 the section is basically a clean slope. Between Q 2 and Q 6 the peak is obvious but controlled. From Q 6 upward, analogue-modelled plugins start feeding back, because the internal gain of the emulation exceeds what the hardware could sustain.

Practical range for a riser on a drum bus is Q 3 to Q 7, with a limiter on the return so the peak does not clip.
For a vocal chop sweep keep Q under 4, because vocal content is dense and high Q turns the peak into sibilance.
For a sustained pad or a reese, Q 6 to Q 9 is where the movement becomes musical rather than a whistle.
If the plugin self-oscillates, ride the cutoff rather than fighting it, and check whether the plugin has a compensation or limiter stage.

FAQ

Why does the peak frequency sit slightly below the cutoff I entered?

Because a second order section does not reach its maximum magnitude at exactly the nominal centre frequency. The damping that creates the resonance also pulls the peak a little lower, and the amount of the shift depends on Q. At an 800 Hz cutoff and Q 6 the model puts the peak near 794 Hz, and at Q 1 the shift is only a couple of hertz. This is not an error in the tool and it is not something you need to correct for. The audible consequence is that the sweep sounds slightly brighter than the number on the knob suggests, because the real peak arrives a moment before the cutoff reaches it.

Will this match exactly what my plugin shows on screen?

No, and it would be dishonest to claim otherwise. This is an approximate two pole analogue model, which is close to how a ladder or state variable filter behaves in a clean signal path. Your plugin may add oversampling, drive stages, a different Q compensation, a limiter, or modulation from an LFO. Two plugins with the same cutoff and Q label can sound completely different at high resonance. Use this tool to understand the shape of the move, the amount of gain stacking up and the risk of feedback. Then confirm with the plugin in front of you and your ears on the actual arrangement.

What Q value is safe for a vocal chop sweep?

Keep it under Q 4 and you will stay well clear of the self-oscillation band, which most emulated filters enter around Q 6 and above. Vocal material is dense and bright, so a high Q filter does not sound like a clean sweep, it sounds like the resonant peak is picking out sibilance and consonants. Somewhere between Q 2 and Q 3.5 gives you audible movement with a controllable peak. If you want more drama, keep the Q low and instead sweep further in frequency, or automate a resonant band pass across a vocal phrase where you have chosen the timing by hand.

Does this visualiser account for my room or my monitors?

No. It describes the filter itself and nothing else. Room modes, early reflections, speaker crossover points, headphone frequency response and the way a room colours a sweep are all outside the model. The tool also says nothing about whether the sweep sounds good in context. Treat it as a pre-check for the two things that most often ruin a filter move: a resonance that will self-oscillate, and a cutoff that is about to eat the sub. Everything else is your judgement and your monitoring.

Why is the kick fundamental calculated as BPM divided by four?

Because in a bar of four four time, a kick on every beat lands four times, and a kick repeating four times per bar produces a fundamental equal to the repetition rate. At 128 BPM each beat lasts 0.46875 seconds, so the fundamental is 32 Hz, which is where most tuned kicks sit. This is a useful reference because the fundamental is the part of the kick that a high pass sweep removes first. It is also an approximation of a straight pattern: a half time kick repeats once per two beats, a broken kick repeats at a different rate, and neither is described by this figure.

Limits of this tool

Limits: this is an approximate two pole analogue model, not a measurement and not a plugin emulation. Read the curve as a shape, not as a specification.
Not for: predicting the exact behaviour of oversampled, modulated or drive-stacked plugins, or of a specific hardware unit with a particular curve.
When this stops: the model holds up well from about Q 0.5 to Q 12. Above that, real filters saturate and self-oscillate, and the drawn curve no longer describes what you hear.
Scope: the kick reference assumes straight four on the floor in four four. Half time patterns, broken beats, three four arrangements and syncopated placements change the real fundamental.