Low-Pass Filter

A Low-Pass Filter is used to allow all frequencies below a cut-off frequency to pass through, removing all other frequencies in the process. It can also be known as a high-cut filter and complements the high-pass filter.

The slope of the filter is measured in poles, you can see these marked as 2-pole, 3-pole in synth filter specifications. Each pole defines the slope of a filter in 6dB/octave steps, so a 2-pole filter refers to a 12dB/octave slope. The higher the slope/decibels, the more severe the attenuation.

Low-pass filters can have drastic effects on complex sound waves. For example, a square wave is more complex than a sine wave as it contains higher frequencies and harmonics. If you used a low-pass filter on a square wave then it would start to sound and look more like a sine wave.

In the world of synth music, low-pass filters can have a major effect by giving your music a darker and deeper tone. A quick and easy way of fully understanding how low-pass filters work is to use the filter on the output signal (if your synth allows), giving it full control of the sound.

As you bring the cut-off frequency down you will notice the sound becomes a lot deeper and muddier, taking the brightness and life away. Take the cut-off back up and the sound starts to come back alive with all of the higher frequencies and harmonics being passed through again.

You can liken this effect to when you walk past a club and only hear the bass, then the door opens and the rest of the sounds can be heard. The door is the low-pass filter, blocking the higher frequencies when closed (filter on) and offering the full sound when open (filter off).