Filters are everywhere in music production.
From the iconic low-pass filters of classic analog synths to modern digital EQs, filters play an important role in shaping the sounds you use in your music.
If you’ve used filters before, you’re likely familiar with the basic settings—filter type, cutoff frequency and resonance.
But there’s another key parameter called filter slope that’s often overlooked when it comes to sound design with filters.
In this article I’ll break down everything you need to know about the filter slope control on your plugins and explain how different settings affect your results.
Let’s get started.
What is filter slope?
Filter slope determines how aggressively a filter attenuates an incoming audio signal beyond its cutoff frequency.
Higher slope values create steeper, more dramatic slopes, while lower ones result in smoother, more gradual shapes.
Different filter types such as high-pass, low-pass and band-bass may all have variable slope settings, but the overall action is the same, even if the attenuation is oriented in a different direction.
In general, slope controls the severity of the filter’s attenuation effect by changing how drastically the material beyond cutoff is reduced in intensity.
What is dB/octave?
If you have a filter plugin with adjustable slope, you may have seen that the settings are expressed in dB (decibels) per octave.
This is used to measure the intensity of the filter's attenuation throughout the frequency range above or below the cutoff.
In this case, it’s important to remember that an octave refers to a doubling of frequency rather than a musical note, although the principle is the same.
Imagine a low-pass filter meant to subtly roll off unneeded high frequencies. With the cutoff set to 8 kHz, a 6 dB/octave slope means it will attenuate frequencies above 8 kHz by an additional 6 dB with each doubling in frequency.
6 dB may seem like a small amount, but consider that each 3dB decrease in signal intensity results in a significant reduction of apparent volume.
Compounded over multiple octaves, the filter will eventually attenuate nearly all of the energy left in the signal, "filtering" out the remaining material.
In other words, dB/octave expresses how quickly the filter reaches full attenuation of the incoming sound beyond the cutoff frequency.
What are the most common filter slopes?
With the basics out of the way, here are a few common settings you’ll see on audio tools with variable slope capability.
6-18 dB/octave
As in the example above, 6 dB/octave is usually the most gentle setting found in variable slope filters.
In high or low-pass applications, these can provide a smooth and transparent reduction in frequency energy that feels gradual all the way through.
12 dB/octave and even 18 dB/octave are still considered gentle, and while the former can be found on some vintage-style synthesizers, they’re almost always used in equalizers or other frequency shaping tools for less invasive sound.
Try these gentler slopes in any situation where you want the filter’s action to come on slowly, without creating synth-style filter effects.
24 dB/octave
24 dB/octave is the classic choice for synthesizer low-pass filters. With a bit of resonance and animation from an envelope or LFO, 24 dB/octave filters are responsible for some of the most recognizable sounds in synthesis.
This sound design potential is the inspiration behind Motion Filter, our all-purpose dynamic filter plugin.
While Motion Filter is capable of slopes that range from 12 dB/octave all the way to 96 db/octave, the familiar feel of a 24 dB/octave, synth-style filter is a great place to start.
Try this setting for squelchy envelope filtering that reacts to your sound’s dynamics. The effect similar to the results you’d get from routing your signal through a hardware synthesizer’s audio input—a trick that’s been used all over music production!
Of course, 24 dB/octave slopes also work great for classic LFO filter sweeps. That makes this slope setting a great place to start when using Motion Filter’s LFO mode for tempo-synced motion that evolves over time.
48 dB/octave and above
Motion Filter doesn’t stop there though. Its advanced architecture allows it to achieve slopes of up to 96 dB/octave without introducing artifacts.
At these nearly vertical drops, almost all the material beyond the cutoff frequency is attenuated right away.
This is perfect for sound design, especially in cases where you don’t need to worry too much about preserving the original characteristics of the sound.
That said, steep slopes are more challenging to implement and analog filter designs are only capable of so much before they become impractical.
That’s while you see most 48-96 dB/octave filters in DAW plugins and high end mixing tools.
How does slope affect filter behavior?
As I mentioned above, extreme slope settings make it harder to design a filter with even response.
After all, the perfect attenuation curve you see in a mathematical function isn’t always possible in practice with real audio tools.
Steeper slope values can cause the filter’s response to become erratic at high frequencies, introducing an unpleasant tonal quality.
This is especially true for material that approaches the Nyquist frequency, or half the sampling rate.
The challenges associated with correcting the response around the Nyquist are part of the reason why higher order filters are rare, even in plugins.
Even so, clever DSP engineers have devised ways to get around these issues and you’ll see clean, Nyquist-corrected filters in some premium plugins like Motion Filter.
It’s important to keep in mind that aggressive slope settings may come with tradeoffs in some cases, so make sure to choose plugin that corrects for those issues.
Slippery slope
Filters are capable of so much when it comes to sound design, but only if you understand how to use them.
If you’ve been neglecting the slope setting on your EQs and effects, it’s worth exploring what it can do when used creatively.
Now that you understand the basics of filter slope, get back to your DAW and keep creating new sounds.
