Low Pass Audio Filter

Documentation Documentation - English
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Attenuates high frequencies above a selected cutoff, allowing lower frequencies to pass through.
This filter is commonly used to soften harsh sounds, remove high-frequency noise, or deliberately reduce clarity for stylistic effects.

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Low Pass is a frequency-domain filter evaluated continuously over time.

Parameters

Cutoff frequency (5 - 21 600 Hz)

Defines the frequency above which audio is progressively attenuated.

  • High values (≈8 000 - 20 000 Hz)
    Very subtle effect; only extreme highs are reduced.

  • Mid values (≈3 000 - 8 000 Hz)
    Noticeable softening of clarity and brightness.

  • Low values (≈500 - 3 000 Hz)
    Strong filtering; audio becomes muffled or “covered”.

  • Very low values (<500 Hz)
    Extreme effect; only bass and low mids remain.

Note:
Lowering the cutoff removes more treble.

Rolloff rate (1 - 10)

Controls how steeply frequencies above the cutoff are reduced.

  • Low values (1 - 3)
    Gentle slope; natural-sounding attenuation.

  • Mid values (4 - 7)
    Clear cutoff with controlled transition.

  • High values (8 - 10)
    Aggressive filtering; high frequencies are sharply suppressed.

Behavior note:

  • High rolloff combined with low cutoff can sound unnatural or “boxed”.

Dry → Wet (0.0 - 100.0%)

Controls the mix between the original signal and the filtered signal.

  • 0.0%
    Original audio only.

  • 50.0%
    Partial filtering with retained clarity.

  • 100.0%
    Fully filtered audio.

This allows subtle softening without fully removing high frequencies.

Channel selection (toggle buttons)

Determines which audio channels the filter is applied to.
Buttons are toggle switches.

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Available buttons depend on the project’s audio channel configuration: Settings > Audio Channels

  • L — Front Left
  • R — Front Right
  • C — Center
  • LF — Low-Frequency Effects (LFE)
  • Ls — Surround Left
  • Rs — Surround Right

Note:

Independent filtering per channel requires adding separate instances of the filter.

Relation to the Low & High Pass filters

Low Pass and High Pass are complementary filters:

  • High Pass removes low frequencies and preserves highs.
  • Low Pass removes high frequencies and preserves lows.

Used together, they can:

  • Isolate a specific frequency band
  • Create band-limited or “telephone” effects
  • Clean both rumble and hiss from dialogue

Auditory characteristics

  • Reduced brightness and detail
  • Softer, darker sound
  • Suppressed hiss or sharp transients
  • Extreme settings can remove intelligibility

Recommended use cases

  • Removing hiss or high-frequency noise
  • Softening harsh dialogue or effects
  • Creating distance or obstruction effects
  • Stylized audio (radio, underwater, behind walls)
  • Preparing audio before further processing

Example settings

  • Subtle softening

    • Cutoff: 10 - 14 kHz
    • Rolloff: 2 - 4
    • Dry → Wet: 100%

  • Dialogue muffling / obstruction effect

    • Cutoff: 2 - 4 kHz
    • Rolloff: 4 - 6
    • Dry → Wet: 100%

  • Band-limited effect (with High Pass)

    • Low Pass cutoff: 3 - 4 kHz
    • High Pass cutoff: 300 - 500 Hz

Usage notes and tips

  • Start with high cutoff values and lower gradually.
  • Avoid extreme rolloff unless a strong effect is desired.
  • Combine with Dry → Wet for subtle results.
  • Always evaluate during playback.

Limitations

  • Cannot target specific high frequencies selectively
  • No resonance or Q control
  • Extreme settings reduce clarity significantly
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