Expander Audio Filter

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.

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.

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