lowpass

A biquad low-pass filter. Passes content below cutoff_hz and attenuates content above.

Signature

signal.lowpass(cutoff_hz, q) -> Signal

• cutoff_hz — the −3 dB knee of the filter. Content well below this passes essentially unchanged; well above it attenuates at 12 dB/octave.
• q — resonance at the cutoff knee. 0.707 is the standard Butterworth value (no audible peak; the cleanest response). Higher values (2–10) create an emphasis right at the cutoff, often used for the “synth lead” tone. Beyond ~10 the filter self-resonates and may ring after the input ends.

The biquad uses the RBJ Audio EQ Cookbook coefficients, the same formulas in nearly every audio plugin.

Examples

// Cut everything above 5 kHz — kills breathy high-frequency noise
// from a noisy source. Butterworth Q for a clean roll-off.
noise("white", 1.0).lowpass(5000.0, 0.707)

// Subtle "muffle" of an existing signal — drops the top end without
// killing presence.
signal.lowpass(8000.0, 0.707)

// Resonant low-pass at 800 Hz — emphasises that frequency while
// rolling off everything above.
signal.lowpass(800.0, 6.0)

When to use it

• Tame a noise source. White noise is bright by default; a lowpass at a few kHz turns it from hiss into rumble.
• Simulate distance / absorption. High frequencies attenuate faster over distance in water and air. A lowpass models that.
• After bandpass synthesis. When you’ve carved a tonal shape from noise with bandpass, a lowpass on the sum can tidy up any residual high-frequency content that leaked through.

Notes

• 12 dB/octave roll-off. A biquad is 2nd-order, so the attenuation rate is 12 dB per octave above the cutoff. For sharper cuts, apply multiple lowpasses in series (each adds another 12 dB).
• Transient response. The biquad needs a few sample periods (~3 ms) to settle from cold; the first samples of the output are a brief ramp. Usually masked by env(...).
• Don’t use to “remove highs from the spectrum view.” If the FFT is showing energy above the cutoff, you’re seeing residual content after a single biquad. Either run it through two lowpasses or pick a tighter cutoff.