Filters and Cutoff

Filters shape the frequency content of sounds. In dark electronic music, they’re essential for creating movement, building energy, and separating elements.

What Filters Do

Filters remove frequencies from a sound:

• Low-pass filter (LPF): Removes high frequencies (makes sound darker/muffled)
• High-pass filter (HPF): Removes low frequencies (makes sound thinner)
• Band-pass filter (BPF): Keeps only a range of frequencies

The Cutoff Parameter

Most Sonic Pi synths have a cutoff parameter — this controls the low-pass filter:

use_synth :tb303

play :d2, cutoff: 60   # Dark, muffled
sleep 1
play :d2, cutoff: 80   # Balanced
sleep 1
play :d2, cutoff: 100  # Bright
sleep 1
play :d2, cutoff: 120  # Very bright, almost harsh

Cutoff Values Guide

Filter as Effect

You can also apply filters as effects to any sound:

Low-Pass Filter (LPF)

with_fx :lpf, cutoff: 70 do
  sample :loop_amen
end

High-Pass Filter (HPF)

with_fx :hpf, cutoff: 80 do
  sample :loop_amen  # Bass removed
end

Combining Both (Band-Pass)

with_fx :hpf, cutoff: 60 do
  with_fx :lpf, cutoff: 100 do
    sample :loop_amen  # Only mid frequencies
  end
end

Filter Movement

Static filters are useful, but moving filters create energy.

Cutoff Slide (On Synths)

use_synth :tb303
play :d2, cutoff: 60, cutoff_slide: 4
sleep 0.1
control cutoff: 100  # Slides from 60 to 100 over 4 beats

Filter Automation (On Effects)

with_fx :lpf, cutoff: 50, cutoff_slide: 16 do |fx|
  control fx, cutoff: 110  # Opens over 16 beats
  
  16.times do
    bass :d2
    sleep 1
  end
end

This is the classic “filter opening” build.

Resonance

The res parameter adds emphasis at the cutoff frequency:

use_synth :tb303
play :d2, cutoff: 75, res: 0.1  # Subtle
sleep 1
play :d2, cutoff: 75, res: 0.4  # Classic acid
sleep 1
play :d2, cutoff: 75, res: 0.7  # Screaming (careful!)

Resonance Values

Filter Patterns in the Album

Intro Pattern

Start filtered, gradually open:

# INTRO
with_fx :lpf, cutoff: 50, cutoff_slide: 32 do |fx|
  control fx, cutoff: 80  # Slow open
  8.times { bassline 0.6 }
end

Build Pattern

Open filter as energy builds:

# BUILD
with_fx :lpf, cutoff: 60, cutoff_slide: 24 do |fx|
  control fx, cutoff: 100  # Opens during build
  6.times { |i| bassline (0.7 + i*0.05) }
end

Main Section

Filter at “home” position:

# MAIN
bass :d2, v, 80  # Cutoff 80 = balanced default

Peak Section

Filter fully open for brightness:

# PEAK
bass :d2, v, 90  # Brighter!
bassline 1.1, 92  # Even brighter at maximum energy

Break Pattern

High-pass to remove bass, create tension:

# BREAK
with_fx :hpf, cutoff: 90 do
  with_fx :reverb, room: 0.9 do
    melody 0.5  # No bass frequencies, floating
  end
end

Outro Pattern

Filter closing as energy fades:

# OUTRO
with_fx :lpf, cutoff: 80, cutoff_slide: 24 do |fx|
  control fx, cutoff: 50  # Closing
  6.times { |i| bassline (0.8 - i*0.1) }
end

Cutoff Variation in Patterns

Don’t use static cutoff — vary it within patterns:

define :bassline do |v=1, c=80|
  bass :d2, v, c        # Root cutoff
  sleep 0.5
  bass :d2, v*0.7, c-10 # Darker ghost note
  sleep 0.5
  bass :f2, v*0.9, c    # Back to root cutoff
  sleep 0.5
  bass :d2, v*0.8, c-5  # Slightly darker
  sleep 0.5
  bass :a2, v, c+5      # Brighter for emphasis
  sleep 0.5
  bass :d2, v, c        # Return to root
  sleep 1.5
end

The cutoff variations (c-10, c-5, c+5) add subtle movement.

Filter Per Section

Typical cutoff progression through a track:

Quick Reference

# Synth cutoff
use_synth :tb303
play :d2, cutoff: 80, res: 0.3

# LPF effect (removes highs)
with_fx :lpf, cutoff: 70 do
  # dark sound
end

# HPF effect (removes lows)
with_fx :hpf, cutoff: 80 do
  # thin sound
end

# Filter automation
with_fx :lpf, cutoff: 50, cutoff_slide: 16 do |fx|
  control fx, cutoff: 110  # Opens over 16 beats
end

# Cutoff in patterns
bass :d2, v, c      # Main note
bass :d2, v, c-10   # Ghost note (darker)
bass :a2, v, c+5    # Accent (brighter)

Tips

1. Start dark, open up — Classic build technique
2. Close for outros — Signals ending
3. Vary within patterns — Static cutoff is boring
4. Higher cutoff = more energy — Use for peaks
5. HPF for tension — Remove bass before drops
6. Resonance with caution — A little goes a long way