Subsonic Techniques for Electronic Music Producers

Exploring Subsonic Frequencies in Sound Design

What “subsonic” means

Subsonic usually refers to frequencies below the standard audible range — typically under ~20 Hz. While humans rarely hear these tones as distinct pitches, subsonic content affects perception, physical sensation, and the behavior of audible harmonics.

Why designers use subsonics

• Tactile impact: Low-frequency energy produces vibrations felt in the body or speakers, adding weight and immersion.
• Psychoacoustics: Subsonics alter perception of loudness and presence; they can make mixes feel fuller even when not consciously heard.
• Harmonic generation: Nonlinear playback systems (speakers, room modes, speaker cones) can convert subsonic energy into audible harmonics or interact with other frequencies.
• Atmosphere and tension: In soundtracks and game audio, subsonics create unease, rumble, or a sense of vastness.

Practical techniques

• Layering: Combine an audible low bass (40–80 Hz) with a subsonic sine or filtered rumble under 20 Hz to add depth without muddying the midrange.
• High-pass where needed: Use gentle high-pass filters on non-bass elements (around 30–40 Hz) to prevent build-up and preserve headroom.
• Controlled filtering: Use a low-pass or band-pass (e.g., 5–18 Hz) on sub elements to prevent them from overlapping audible bass harmonics.
• Saturation/distortion: Apply mild saturation to sub layers to generate audible harmonics if playback systems can’t reproduce very low frequencies.
• Sidechain and ducking: Sidechain sub layers to kick drums or key hits to keep low-end clear and punchy.
• Transient shaping: Slow attacks and long releases on sub elements help avoid abrupt clicks and make rumbles feel natural.
• Automation & modulation: Vary sub levels and filter cutoff to create movement and avoid listener fatigue.
• Monitoring checks: Reference mixes on systems that can reproduce low frequencies (subwoofer-equipped monitors, club systems) and also on small speakers/headphones—use analysis tools (spectrum, correlation) to ensure mono compatibility and phase coherence.

Tools and processing

• Generators: Sine-oscillators, low-frequency noise, or recorded rumbles.
• Filters: Steep low-pass and band-pass filters tuned to sub ranges.
• Analyzers: Spectrum analyzers, correlation meters, and RTA for visualizing energy under 20 Hz.
• Dynamics: Multiband compressors or limiters focused on sub band; lookahead limiters for transients.
• Distortion: Tube, tape, or clipper emulations to create harmonics.
• Spatial: Sub-bass is typically mono; use mid/side cautiously to avoid phase cancellation.

Common pitfalls and fixes

• Muddy mix: Remove sub from non-essential tracks with high-pass filters; narrow EQ boosts.
• Overloading speakers: Limit sub peak levels; check phase and use limiters.
• Translation issues: If subs disappear on small systems, add harmonic-rich layers or use pitch-shifted copies an octave up subtly.
• Phase cancellation: Keep sub mono and check phase correlation; invert phase to test.

Creative uses

• Cinematic tension: Low drones that slowly rise in amplitude or cutoff to build dread.
• Rhythmic sub pulses: Sidechained sub patterns synced to tempo for groove.
• Environmental realism: Layer in real-world rumbles (engines, distant thunder) for authenticity.

Quick starter settings (examples)

• Sub sine: 10–20 Hz, level -12 to -6 dB relative to mix, low-pass 18 Hz.
• High-pass non-bass tracks: 30–40 Hz, slope 12–24 dB/oct.
• Saturation drive for sub layer: subtle, 1–3 dB of harmonic enhancement.
• Sidechain ratio: 2:1–4:1, attack 10–30 ms, release 100–300 ms (to taste).

Summary

Subsonic frequencies are a powerful sound-design tool for adding physicality, depth, and emotional weight. Use filtering, saturation, and careful level management to harness their benefits while avoiding mix problems. Monitor across systems and focus on how subs interact with audible harmonics to ensure consistent translation.