Expander Audio Filter

Documentation Documentation - English
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Reduces the level of audio signals that fall below a defined threshold, increasing the difference between quiet and loud parts.
Unlike a compressor, which reduces dynamic range, an expander increases dynamic range by attenuating low-level material.

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This filter operates in the amplitude domain and is evaluated continuously over time.
None of its parameters are keyframeable.

What this filter does

  • Low-level signals are reduced further
  • Loud signals remain largely unchanged
  • Background noise, room tone, or bleed can be suppressed
  • Transients and intentional sounds are preserved

The Expander is often used as a gentle noise-reduction or cleanup tool, not as a hard gate.

Parameters

RMS → Peak (0.0 - 100.0%)

Controls how the expander detects signal level.

  • 0.0% (RMS)
    Detection is based on average signal energy.
    Smoother, more natural behavior.

  • 100.0% (Peak)
    Detection reacts to instantaneous peaks.
    Faster, more aggressive response.

  • Intermediate values
    Blend between RMS and Peak detection.

Practical note:
RMS detection is usually preferred for dialogue and music.

Attack (2 - 400 ms)

Controls how quickly attenuation begins after the signal drops below the threshold.

  • Short attack
    Rapid suppression of low-level sounds.

  • Long attack
    More gradual reduction, preserving natural decay.

Release (2 - 800 ms)

Controls how quickly attenuation stops after the signal rises above the threshold.

  • Short release
    Fast recovery; can sound abrupt.

  • Long release
    Smoother transitions; more natural sound.

Threshold (-30 to 0 dB)

Defines the level below which expansion occurs.

  • Lower values
    Only very quiet sounds are affected.

  • Higher values
    More of the signal is treated as “low level”.

Ratio (1:1 to 1:20)

Controls how strongly low-level signals are attenuated once they fall below the threshold.

  • 1:1
    No expansion.

  • 1:2 to 1:4
    Gentle expansion; subtle noise reduction.

  • 1:8 to 1:20
    Strong expansion; approaches gate-like behavior.

Higher ratios result in more aggressive suppression.

Knee radius (1 - 10 dB)

Controls how smoothly the Expander transitions around the threshold.

  • Low values
    Hard knee: abrupt onset of attenuation.

  • High values
    Soft knee: gradual, less noticeable transition.

Soft knees generally sound more natural.

Attenuation (0.0 - 24.0 dB)

Defines the maximum amount of gain reduction applied to signals below the threshold.

  • Low values
    Subtle reduction of quiet material.

  • High values
    Strong suppression of background noise or bleed.

This sets a ceiling on how quiet low-level signals can become.

Channel selection (toggle buttons)

Determines which audio channels the Expander 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 processing per channel requires adding separate instances of the filter.

Auditory characteristics

  • Reduced background noise between sounds
  • Clearer separation between silence and signal
  • Can sound cleaner and more focused
  • Overuse can make audio feel unnatural or “choppy”

Recommended use cases

  • Reducing room noise in dialogue
  • Cleaning up microphone bleed
  • Suppressing low-level background sounds
  • Improving clarity without fully muting silence
  • Pre-processing before compression

Usage notes and tips

  • Start with low ratios and moderate attenuation.
  • Use soft knees and longer release times for natural results.
  • Avoid extreme settings unless a gate-like effect is desired.
  • Monitor quiet passages carefully to avoid unnatural dropouts.

Relation to the Compressor

The Expander is conceptually the inverse of the Compressor:

  • Compressor
    Reduces levels above a threshold.

  • Expander
    Reduces levels below a threshold.

They are often used together:

  1. Expander to reduce background noise
  2. Compressor to control loudness and dynamics

This combination improves clarity while maintaining consistent output levels.

Limitations

  • Not a true noise remover
  • No lookahead or sidechain
  • No keyframe support
  • Extreme settings can sound unnatural
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