What Are Filters and How to Use Them

Filters are tools that increase or reduce the amplitude of a certain group of frequencies (in the case for equalizers) or even cut them out (not completely, but almost). This definition can vary depending on which field we are working on (it’s not the same, for example, in the amplifier circuit world) but this applies especially for music (mostly, electronic music) and sound design.

Every sound consists of a series of harmonics, and the importance of audio filters lies in the shaping of those, especially in subtractive synthesis.

Digital filters can be found either included in the synthesizer hardware or software (almost every synthesizer has filters because it’s one of the bases of sound design) or by loading an equalizer in the mixer of your DAW (almost every equalizer has an option to set up filters).

There are a lot of different filter types. Each of them differs in the way in which they cut out or boost the selected frequencies.

A good example of a filter is your own mouth; when you talk, you just generate a repeated waveform (that’s your voice) that is shaped by your mouth, which filters the harmonics in order to create different sounds. The most obvious example is the pronunciation of the letter m, where almost all harmonics are cut out. This same effect occurs when, for example, you listen to a sound that comes from another room (it sounds “dark” because it doesn’t have high frequencies).

In this article, we’ll be covering some of the filter types that you’ll most likely find in any plugin, along with examples of each of them, and some practical tips for using them in your music production.

But first, let’s explain the most important parameter of every filter: the cut-off frequency.

Cutoff Control

The cutoff point or frequency determines where the filter will start working (either attenuating or boosting). To be precise, the cutoff is defined as the point where the signal is attenuated by 3 decibels, as you can see in the images below.

For example, if you set the cutoff frequency at 2000 Hz, everything below that will be attenuated or cut if it’s a high-pass filter (which I’ll explain in a minute) and everything above that will be attenuated or cut if it’s a low-pass filter.

In synthesis and sound design, this parameter is almost always modulated by an LFO or envelope. You’ll listen to how this modulations sound with an example for every filter type below.

Now let’s go ahead and explain the most common filter types, which you’re most likely to find in your synthesizer.

High Pass Filter

High pass filter in ableton EQ

High pass filter in Ableton’s stock equalizer. The circle with the number 1 is the cutoff point. 

A high pass (HP) or low cut filter works by cutting out all the low frequencies (where the bass is) and leaves all the high frequencies clean (it “passes” the high frequencies).

It’s usually used in every instrument that does not have lower frequencies, to get a much cleaner mix. That way you cut away all that muddy stuff that is probably unnecessary.

Now, take a listen to this supersaw. First completely clean, and then high-passed with a cutoff point of 700 Hz (I recommend listening to it on headphones or decent speakers):

Apart from improving the overall mixing of the track, this filter is also very often used in sound design, especially if you apply some resonance (which I’ll explain later).

For example, let’s listen to the same sound, but this time modulating the cutoff point of the high pass filter. First, with an envelope, and then with an LFO:

Low Pass Filter

Low pass filter in Ableton EQ

The low pass (LP or LPF) or high-cut filter cuts out all higher frequencies than the cutoff point and passes the lower ones.

This is the typical effect that happens, for example, when you hear music coming from another room. Also, as we said earlier, what happens when you talk (and, for example, pronounce the letter m): the sound just doesn’t have higher frequencies.

Cutting the high frequencies helps to make sounds that are “far away”, mainly in secondary elements (you usually won’t cut the highs of the lead instrument).

Let’s listen to the same sound as before, first clean and then with a high-cut and a cutoff point off 2,000 Hz:

When modulated, this filter type can create really interesting effects, such as dubstep wobbles, especially if you apply some resonance. But it’s also useful for creating certain textures for leads and other types of sounds.

This is an example of a low pass filter while modulating the cutoff: first with an envelope, and then with an LFO:

Band Pass Filter

Band pass filter in Ableton EQ

The band pass (BP) filters cuts out every frequency except for the ones around the cutoff point. It’s like combining a low pass with a high pass filter, with the difference that there’s only one frequency that we can “choose” (only one band: hence its name).

Here is an example of a supersaw; first clean and then with a band pass filter with a cutoff point of 1500 Hz:

Notice how the sound losses low frequencies as well as higher frequencies. I encourage you to compare this sound to those of the previous filter’s, so you know exactly the difference between them.

You will probably like to modulate this filter with an envelope or LFO, probably more than you would like to using the previous ones. This is because it only leaves a very small amount of frequencies, therefore the resulting sound will be very weak (although that may be exactly what you want to achieve some times).

When modulated, it produces a “vowelly” sound that mimics the human voice. It is the classic sound of the Wah-Wah guitar pedal (which now is also used on a lot of instruments apart from the guitar). Also, it is a very used filter for creating dubstep wobbles (which is a bass that goes like “wub”).

Here is an example of the same sound, first with a band pass being modulated by an envelope and then by an LFO:

Notch Filter

Notch filter in Ableton EQ

The Notch or Band Cut filter is the opposite of a band pass filter: a band of frequencies is cut out while everything else is let through.

Let’s take a listen to how the notch filter sounds like on a supersaw:

As you can see (if you have read our article about audio effects), it sounds somewhat like a flanger, but most importantly, it sounds like a subtle phaser (at the end, a phaser is a combination of a bunch of notch filters moving around), especially when modulated.

This filter, as we’ll see later, sounds especially good when combined with other filters like the low pass or high pass.

This is what a Notch filter sounds like when modulated by an envelope, and then by an LFO:

Peak Filter

peak filter in Ableton EQ

Peak Filter. As you can see, at least visually speaking, it’s not as aggressive as the other filter types. Also, note that in this example the effect is exaggerated: most of the time you won’t use such a “high” peak.

The Peak filter is one of the very few filter types that, instead of cutting frequencies, it boosts a certain band. It is like the band pass, but instead of cutting frequencies, it just boosts a small amount of them.

The same as the notch filter, it sounds better when modulated and combined with other filters (at least for sound design; it’s also used statically for mixing when equalizing the different instruments).

This is what it sounds like:

As you can see, it creates some sort of little “resonance” in the mid-high frequencies.

This filter is often modulated to create laser sounds. Also, as I said earlier, it’s used in combination with other filters such as the low pass or high pass to create sounds like a dubstep growl.

This is what it sounds like when modulated:

Now that you know the most important filter types, let’s talk about the most important filter parameters apart from the cutoff frequency (which I’ve already covered). These are the resonance and the slope.

Resonance

filter resonance

High pass filter with a resonanse (Q) of 3.06.

The resonance (Q) is not a filter type, but a property that most filters have. It allows you to increase or lower the volume of the frequencies that are near the cutoff point. It is the “Q” parameter of the filter.

In other words, it’s like if you applied a peak filter exactly where the cutoff point is. By increasing the resonance, you increase the loudness of this peak.

This is what a high pass with very little resonance sounds like, versus the same sound with a resonance (Q) of 4:

And finally, this is what a high pass filter sounds like when modulated with an LFO; first with no resonance and then with a high one:

Slope

Most filters will let you control the slope of the high pass or low pass filter. This is basically how aggressive the effect acts. It’s usually measured in dB, being the most common values -6 db, -12 dB, -18 dB, -24 dB or -48 dB. This determines the amount of dB that will be reduced per octave.

For example, a filter labeled as hp24 indicates that it is a high pass filter with a slope of -24 dB, combined with a Peak filter (sometimes it can also mean that it’s just a high pass with this same slope).

Soft slopes, such as -6 dB, are often used for more subtle or “deep” sounds, mostly for the mixing stage of the production. Higher slopes are used for more aggressive and obvious results, mostly for sound design. Of course, these uses are not strict, you can use them however you want.

Serum‘s low pass filter with a slope of:

6 dB slope

-6 dB

12 dB slope

-12 dB

18 dB slope

-18 dB

24 dB slope

-24 dB

Combining filter types

Continuing with the example of Serum, some synthesizers like this one will let you combine different filter types. For instance, Serum has a “multi” section for selecting filters, which means that you can choose between different combinations of filters (you can combine up to 3 different types!).

Of course, at this point the possibilities for shaping your sound are endless. But for this example, I’ll pick the HP 12 Serum filter, which combines a high pass filter (with a slope of -12 dB) with a peak filter. You can modulate the cutoff point of each of them individually.

For this example, I’ll put a little bit of resonance and modulate these two filters’ cutoff inversely. This will create a ioi vowel sound, used a lot in genres like bass house and dubstep:

ioi filter demonstration

If you want to go further, you can even accentuate this vowel effect with some distortion or, in this case, bitcrushing:

So these are the basics of filters. As you can see (especially with this last example, where we started with a simple supersaw) the sound can be heavily altered by this tool. They are one of the fundaments of subtractive synthesis and knowing how to use them will let you shape the sound exactly how you want it.

I encourage you to experiment with these filter types and slowly identify them and understand how each of them sounds like. Take each parameter, such as the resonance, to the extreme (we talk about this kind of experimentation in our article about Improving at Music Production). If you can get a synth like Serum where you can modulate any parameter of the filter module, that’s even better.

If you haven’t already, you can read our article about the basics of synthesis and sound design where we go through the basic concepts of this sound design world, and we even have a dedicated article for each of them.