Resonance in plain English
A low-pass filter lets the low frequencies through and turns the high ones down. The point where it starts turning them down is the cutoff. With no resonance, the level just slides away above the cutoff, and the sound gets darker without changing character.
Resonance adds a bump right at the cutoff. The frequencies just under it come out louder than they went in, and then everything above falls away as before. A small bump makes the filter sound brighter and a little nasal. A big one is narrow and tall, and it picks out whatever is sitting at the cutoff so loudly that you hear it as a note.
In an analogue filter the bump comes from feedback. Part of the filter's output is fed back into its input, and the frequencies at the cutoff get reinforced on each trip round. More feedback means a taller, narrower peak, and past a certain point the loop keeps itself going with nothing coming in. Digital filters get the same shape from maths, which is how the lab on this page does it.
This page goes deep on that one control. For filters and EQ in general, start with the EQ guide.
What does resonance do to the sound?
It depends on what you put through it, which is why the lab starts on pink noise. Noise has energy at every frequency, so the filter has something to boost wherever the cutoff sits. Add resonance and a pitch appears out of nowhere, because the band at the cutoff is now much louder than its neighbours. At 800 Hz you hear a note close to G5.
On a sound with a pitch, like a chord or a sawtooth synth, the filter boosts whichever harmonics fall near the cutoff. The vowels of your voice are peaks like this in the spectrum, called formants, so a resonant filter can make a synth sound like it's saying "ee" or "aw", and moving the cutoff changes the vowel. Sweep the cutoff up over a note and you get the "wow" of a lot of synth basses. A wah pedal is a resonant filter you sweep with your foot.
On drums, resonance adds pitch to things that had none. A resonant filter rings after a sudden sound, so the click at the start of each hit turns into a short ping at the cutoff frequency. There's a section on that further down.
Reading the lab
This is the same EQ lab as the other lessons in the category, set up with two filters. Band 1 is a low-pass and band 2 is a high-pass, which starts switched off. The Q knob is the resonance control for whichever band is selected, and the big screen shows the result in two layers: the pink line is the filter curve, and the teal shape is the spectrum of the sound after the filter. On pink noise the teal shape follows the curve, so the resonant peak is easy to spot.
Drag the numbered point sideways to move the cutoff while the sound plays. That's the quickest way to hear what resonance does, because a peak that moves is much easier to pick out than one that sits still on a steady sound.
The Right now panel measures the curve as you go. When you change the Q it tells you how tall the peak is in decibels, and otherwise it reports how far down the sound is an octave past the cutoff. A/B switches the filter off and matches the loudness of the original, so a resonant peak can't win just by being louder.
The quiz under the lab puts a 12 dB per octave low-pass at 1 kHz on the sound and asks how much resonance it has: none (a Q of 0.7), some (3) or lots (10). The screens are covered and the level is matched, so you are listening for a whistle at the cutoff. Pink noise suits it best, because it has energy at the cutoff for the filter to boost. Your best score out of eight is saved in your browser.
Resonance and Q in numbers
Plug-in EQs measure resonance with Q, the same control that sets the width of a bell band. On a high-pass or low-pass filter it sets the height of the peak instead. The reference point is a Q of 0.71, a filter with no peak at all: it's 3 dB down at the cutoff, which is the usual definition of where a filter's cutoff is. That shape is called a Butterworth response, after Stephen Butterworth, who described it in 1930.
Raise the Q and the level at the cutoff comes up. At a Q of 1 the cutoff is back to 0 dB, with a slight 1 dB rise just below it. Above a Q of about 2, the peak in decibels is close to 20 × log₁₀(Q), so a Q of 2 gives about 6 dB, a Q of 4 gives 12 dB and a Q of 10 gives 20 dB. Every doubling of Q adds 6 dB.
| Q | Level at the cutoff | One octave above | What you hear |
|---|---|---|---|
| 0.71 | −3 dB | −12 dB | A smooth roll-off with no colour |
| 1 | 0 dB | −11 dB | Slightly firmer, still smooth |
| 2 | +6 dB | −10 dB | A little bite at the cutoff |
| 4 | +12 dB | −10 dB | Clearly nasal, with a vowel to it |
| 10 | +20 dB | −10 dB | A whistle at the cutoff |
| 12 | +22 dB | −10 dB | Close to a pure tone on noise |
The peak also gets narrower as it gets taller. Its width, measured 3 dB below the top, is roughly the cutoff divided by Q, so at 800 Hz and a Q of 10 the peak is only about 80 Hz wide. That's why high resonance sounds like a single note.
Synths rarely show a Q number. Their resonance knob runs from 0 to 10 or 0 to 100%, and what each position means varies from one synth to the next, so go by ear and by the meters there. The Minimoog calls it Emphasis and the Korg MS-20 calls it Peak; the knob does the same job on both.
Resonance makes a filter ring
A resonant peak on the frequency curve has a twin in time. Any filter with a tall, narrow peak keeps vibrating at its peak frequency for a while after something hits it, the way a bell goes on sounding after the strike. The higher the Q, the longer it rings.
A rough rule for a filter like this: it rings for about 2.2 × Q cycles before it has faded by 60 dB. So at 500 Hz with a Q of 10, it rings for 22 cycles, or 44 ms. That's long enough to hear as a pitch. Drop the cutoff to 100 Hz and the same Q rings for about 220 ms, because each cycle takes five times as long.
That's what happens to drums in the lab. Each hat and snare hit is a sharp click, and the filter turns every click into a short ping at the cutoff frequency. You can do this on purpose to give a drum loop a pitch. The Roland TR-808 makes its kick and several other drums the same way, by hitting a circuit tuned to ring with a short pulse.
What is self-oscillation?
Push the feedback far enough and a filter rings forever. It makes a sine wave at its cutoff frequency even with no sound coming in, which is called self-oscillation. On the Minimoog, and on many synths since, full resonance does exactly this, and players use it as an extra oscillator: with the filter's keyboard tracking at full, the cutoff follows the keys and the sine plays roughly in tune.
Self-oscillation is loud. The sine can be much louder than the sound you were filtering, and it appears suddenly as you pass a certain point on the knob, so turn your monitors down before you go looking for it.
The filters in EQ plug-ins, and the ones in this lab, don't self-oscillate. However high you set the Q, they only ring when sound goes in, which is what you want from an EQ. At a Q of 12 on pink noise the lab gets close to the sound of an oscillating filter, but stop the noise and the tone stops with it.
Resonance and filter slope
The slope of a filter is how fast the level falls past the cutoff, in decibels per octave. A 12 dB per octave filter with a big peak still lets plenty of high end through: one octave above the cutoff it's only about 10 dB down. On a 24 dB per octave filter the same peak sits in front of a much steeper drop, about 23 dB down one octave above, so the peak stands out on its own and everything past it disappears.
That accounts for a lot of the difference between synth filters. The Minimoog uses a 24 dB per octave ladder filter, while the Oberheim SEM uses a 12 dB per octave filter that sounds more open. The Moog-style ladder also has a quirk that many plug-ins modelled on it copy: as you raise the resonance, the frequencies below the cutoff get quieter, so a resonant ladder filter makes a bass sound thinner.
Resonance on a high-pass filter
A high-pass filter can have resonance too. The peak sits just above its cutoff, and everything below the cutoff falls away as usual. On a synth that gives a thin, hollow sound that cuts through a mix.
In mixing, a small amount is useful on bass and kick. A plain 24 dB per octave high-pass at 40 Hz removes rumble, but it's also 3 dB down at 40 Hz and still a little down at 45. Raise its Q to 1.5 and that dip turns into a 4 dB bump at about 43 Hz, which still adds 2 dB at 55 Hz, the open A string of a bass guitar. The sub keeps its weight and the rumble still goes. Many parametric EQ plug-ins, FabFilter Pro-Q among them, let you set the Q of their cut filters for this.
Keep it gentle: a Q of 1 to 1.5 is a tone move, and much higher than that on a bass turns the bump into a boom on any note that lands near the cutoff.
How to use filter resonance, step by step
This order works for a synth bass, a lead or a filtered loop, in a synth or in an EQ plug-in.
- Set the cutoff with the resonance at zeroFind the brightness you want first. Resonance adds a peak where the cutoff is, so the cutoff has to be in the right place before the peak means anything.
- Bring the resonance up slowlyStop when the sound gains the edge or vowel you're after. For most basses and leads that's a peak of 6 to 12 dB, or a Q of 2 to 4.
- Move the cutoff againThe peak changes where the best cutoff is, so nudge it until the boosted harmonics sit where they help. Moving it by hand now also tells you whether a sweep would sound good.
- Add movement if the part needs itUse a filter envelope for a sweep on every note, an LFO for a repeating wobble, or automation for a slow build. The filter envelopes lesson covers the first of those.
- Check the loudest momentsWatch the meter as the cutoff passes the loudest notes. If it jumps, lower the resonance or put a limiter after the synth.
- Compare in the mixSoloed, resonance can sound great. Play the part with everything else and listen for whether the peak is fighting the vocal or the lead.
Starting settings for resonance
These are places to begin. The peak heights are for a filter like the one in the lab.
| Use | Filter | Q | Peak |
|---|---|---|---|
| Darkening a pad or a loop | Low-pass | 0.7 – 1 | 0 – 1 dB |
| Synth bass with bite | Low-pass, 24 dB/oct | 2 – 4 | 6 – 12 dB |
| Acid line or squelchy lead | Low-pass with a filter envelope | 6 – 12 | 15 – 22 dB |
| Filter sweep on a loop | Low-pass or high-pass, automated | 2 – 6 | 6 – 15 dB |
| Tightening the low end of a bass or kick | High-pass, 24 dB/oct | 1 – 1.5 | 1 – 4 dB |
| Pitched drums | Low-pass on the loop | 8 – 12 | 18 – 22 dB |
Resonances in a recording
In mixing, "resonance" also names a different problem: one frequency in a recording that rings louder than the rest. Snare drums and small rooms both have them. Those are resonances in the physical sense, the same ringing as a resonant filter but coming from a real object.
You fix those with a narrow cut. The usual way to find one is to boost a narrow bell band a long way and sweep it until the problem jumps out, then turn the boost into a cut. The Q and bandwidth lesson covers how narrow to go, and the notch filter lesson covers the very narrow cuts used on hum and single ringing notes.
Common resonance mistakes
Ignoring the level
At a Q of 10 the filter turns whatever sits at the cutoff up by 20 dB. When a sweep lands on a loud note, the level can jump by most of that, which is enough to clip a mix bus that was fine a second earlier. Watch the meters during the whole sweep. A limiter after a heavily resonant synth is a sensible safety net.
Setting resonance before cutoff
Resonance boosts whatever sits at the cutoff, so it only sounds right once the cutoff is right. Turn it up first and then hunt for a cutoff, and you tend to end up with a peak in the wrong place that you've got used to hearing.
A peak fighting the vocal
A resonant peak parked between 1 and 4 kHz sits right where the voice carries its words. Soloed, the synth sounds exciting, but in the mix it makes the vocal harder to follow, because the two are fighting over the same few kilohertz. Move the cutoff away from that range, or save the big resonant moments for when nobody is singing.
Losing the bass on a ladder filter
If your synth bass shrinks as you add resonance, the filter is doing what Moog-style ladders do. Back the resonance off a little, or add a low shelf after the filter to put the weight back.
Too much resonance on a high-pass in the mix
A Q of 1.5 on a bass high-pass adds weight. A Q of 4 adds a 12 dB boom that comes and goes with the notes, so if the bass sounds uneven after a high-pass, check the Q before anything else.
Questions people ask
What does the resonance knob do on a synth?
It boosts the harmonics right at the filter's cutoff frequency. A little makes the sound brighter and more nasal; a lot makes the filter whistle at the cutoff, and moving the cutoff then sweeps that whistle up and down. On many synths the highest settings make the filter produce a tone of its own.
Is resonance the same as Q?
On a filter, yes. Most EQ plug-ins label it Q, synths usually say resonance or Res, and the Minimoog calls it Emphasis. In every case it controls how tall and narrow the peak at the cutoff is. On a bell band, Q sets the width of the bell instead.
What is self-oscillation?
It happens when a filter feeds back so much of its own output that it keeps ringing with no input at all. You hear a sine wave at the cutoff frequency, and with keyboard tracking at full you can play it like an oscillator. Plenty of analogue synth filters do it at full resonance, but no EQ filter is built to.
Why does my bass get quieter when I turn up the resonance?
Some filters, including the Moog-style ladder filter and plug-ins modelled on it, lose low end as the resonance goes up. The peak at the cutoff gets louder while the frequencies below it drop. Bring the bass back with a low shelf after the filter, or use less resonance.
How much resonance should I use?
A smooth filter on a pad needs none, or a Q of about 1. A synth bass with some bite usually wants a peak of 6 to 12 dB, which is a Q of 2 to 4. Acid lines and obvious sweeps go higher still, and then a limiter after the synth stops the peak clipping.
Does a resonant filter boost the volume?
Only around the cutoff. With a Q of 10 the filter turns the cutoff frequency up by 20 dB, so when the cutoff lands on a loud note or harmonic the level can jump. Watch your meters while you sweep, and judge the result at matched loudness.