Filters: Sculpting the Sound
Every sound you've made so far contains frequencies. A sine wave has one. A sawtooth has dozens. Up to now, you've been adding things - modulation, arpeggiation, new waveforms. This lesson is about subtraction.
What is a filter?
A filter removes some frequencies from a sound and keeps others. Think of a kitchen sieve: water passes through, but chunks of pasta stay behind. A filter does this with frequencies.
The most common type is the low-pass filter. It lets low frequencies through and removes everything above a point called the cutoff frequency. Turn the cutoff down, and the sound gets darker. Duller. Muffled. Like hearing music through a wall.
Types of filter
Filters come in different shapes. Each one decides which frequencies survive and which get removed. The curves below show what each type does - frequency runs left (low) to right (high), and the line shows how much volume gets through.
Everything below the cutoff frequency passes through. Everything above gets removed. Turn the cutoff down and the sound gets darker - like hearing music through a wall. This is the most common filter in synthesis. The machine below uses it.
The opposite. Everything above the cutoff passes, everything below gets cut. Removes bass and rumble. Used in mixing to clean up muddy recordings - vocals, guitars, and hi-hats often have a high-pass filter removing frequencies they do not need.
Only lets through a narrow band of frequencies around the cutoff. Removes both lows and highs. Creates thin, hollow, telephone-like sounds. Wah pedals on guitars are band-pass filters with a cutoff that sweeps up and down as you rock the pedal.
Before and after
The two animations below show what filtering looks like. The top wave is a sawtooth - bright, buzzy, packed with harmonics. The bottom wave shows the same signal after a low-pass filter has removed the high frequencies. Notice how the sharp edges soften into rounder curves.
Try it: choose the right waveform
Switch to a sawtooth wave if you haven't already. Sine waves are too pure - they have only one frequency, so the filter has almost nothing to remove. Sawtooth waves are packed with harmonics: dozens of frequencies stacked on top of each other. Perfect filter food.
Cutoff: the brightness knob
Turn the filter on. Drag the cutoff slider slowly downward.
Listen to the brightness drain away. You are watching harmonics vanish in real time. The oscilloscope shows it - the jagged sawtooth edges soften and round off as the high-frequency content gets stripped out.
At very low cutoff, only the fundamental frequency remains. A sawtooth becomes a sine wave through sheer deletion.
- High cutoff - most frequencies pass through. Bright, buzzy sound.
- Low cutoff - only the lowest frequencies survive. Dark, muffled sound.
- Sweeping it - move cutoff slowly and hear the brightness change in real time.
Resonance: the feedback knob
Resonance boosts the frequencies right at the cutoff point. The filter feeds back into itself, amplifying its own edge.
- Low - a subtle emphasis. A gentle bump at the cutoff point.
- Medium - a vocal, almost vowel-like quality. The filter starts to "speak".
- High - a ringing, whistling tone that threatens to spiral out of control.
Filter + arpeggiator = low-pass gate
Turn on the arpeggiator with the filter. Each arp note triggers the filter to open wide (bright) then close back down to the resting cutoff (dark).
- Short release - each note gets a sharp "pluck" of brightness.
- Long release - the filter barely closes between notes, creating a wash.
This is a low-pass gate. It controls both volume and brightness together, the way acoustic sounds naturally work. When you hit a guitar string hard, it is loud and bright. As it fades, it gets quiet and dark. The low-pass gate recreates this coupling.
© ectoplasma.org
ectoplasma.org