Introduction to Synthesizers part 2: Filters

Bob Moog

Just as important as the oscillator is the filter. This is where the audio goes to next after it is generated by the oscillator. A filter in a synth works the same way as a filter in any situation which is to say it allows certain things to pass while stopping others. There are 4 types of filters that see widespread use. Any others are simply variations of or different combinations of the 4 main filter types.

The 4 main types of filters are high pass, low pass, band pass, and band reject or band stop (commonly referred to as “notch” filters). Below is a useful image that can help visualize what exactly these filters are doing.

In these images, the X axis represents frequency while the Y axis represents amplitude (commonly referred to as level, gain or volume)
In these images, the X axis represents frequency while the Y axis represents amplitude (commonly referred to as level, gain, or volume)

High pass filters are extremely useful. Every sound in a song that isn’t contributing to the bass is filtered with a high pass filter to some extent. This removes any presence the sound might have in the low end leaving room for the bass. Ableton Live’s EQ plugin has a nice graphical display, clearly demonstrating the effect of a high pass filter. You can see the boom of the low frequencies on the left hand side and how the orange high pass filter is taming some of that boom.


The band pass filter has another great sound. Band pass filters allow only the frequencies within a certain range to pass, stopping all others. Wah pedals are a great example of band pass filters in action. Think of the opening riff of Voodoo child and how it sounds like the guitar is almost saying “wah wah”.

The opposite of a band pass filter is a band reject filter (notch filter). Notch filters are most commonly utilized in a very useful effect called a phaser. Phasers use multiple notch filters moving up and down through the frequency of a sound. The best way to get familiar with this is to listen and train your ears (really thats the best way to master any musical tool). In the beginning of the track Ages by Mmmmaven’s own Nick Garcia (Twitter), the piano has a very nice phaser moving through it. This creates rich and luscious motion in the sound.

Low pass filters are also often used, allowing the low frequencies to pass while attenuating or stopping the frequencies above the cutoff.

Cutoff is a parameter found on every filter and it refers to the frequency which the filter is centered around. A great example of a low pass filter with a cutoff slowly moving up is in the first minute of this unreleased track from Deadmau5 (Twitter) titled All I Have. Notice how the main chord synth starts out fairly bright. Then the cutoff moves down very quickly stifling the synth, and slowly moves upward allowing the bright higher frequencies to come through.

Another important parameter on filters is resonance (also referred to as “Q”). The simplest explanation of resonance is that resonance amplifies those frequencies just before the cutoff, creating a more pronounced cutoff curve. The more resonance is increased, the easier it is to hear the filter cutoff sweeping through the sound. A great example of very pronounced resonance in a synth is the bass at 5:10 of this Claude Vonstroke (Twitter) remix (although I recommend listening to the whole song). Note the vowel like qualities of the synth and the high pitched peak generated when the resonance is turned up (in this case way up).

All of these filters are great, but these examples wouldn’t sound so good if the filter cutoff wasn’t moving or being changed overtime. Next we will take a look at Envelopes and LFOs and see how they are used to create motion in otherwise static sounds, adding to their character and making them evolve over time.

These articles only scratch the surface of music production, discover much more in our Ableton Live classes!