Configure asubboost Cutoff and Decay in FFmpeg

The asubboost audio filter in FFmpeg is designed to generate and boost sub-bass frequencies from an existing audio track. This article explains how to specifically configure the cutoff frequency and decay parameters within the asubboost filter, detailing their functions, valid ranges, and practical command-line examples to help you shape your audio's low-end response.

Understanding Cutoff and Decay

The asubboost filter shapes low-end sound using several key settings, with cutoff and decay being essential for defining the filter's range and duration:

  • cutoff (or c): Specifies the cutoff frequency in Hertz (Hz). Only frequencies below this threshold are detected and used to generate the boosted sub-bass content.
    • Range: Typically 20 to 300 Hz.
    • Default: 100 Hz.
  • decay (or d): Controls the decay rate of the generated sub-bass energy. Higher values prolong the sub-bass rumble, while lower values keep the bass tighter and more immediate.
    • Range: 0.0 to 1.0.
    • Default: 0.0.

Filter Syntax

In FFmpeg, filters are applied using the -af (audio filter) flag. The parameters can be passed as key=value pairs separated by colons:

-af "asubboost=cutoff=<value>:decay=<value>"

Alternatively, you can use the short aliases:

-af "asubboost=c=<value>:d=<value>"

Practical Examples

1. Tight Sub-Bass Boost (Low Cutoff, Low Decay)

For acoustic, rock, or dynamic tracks where you want a clean sub-bass layer without muddiness or lingering sustain, set a lower cutoff and minimal decay:

ffmpeg -i input.mp3 -af "asubboost=cutoff=80:decay=0.1" -c:a libmp3lame -q:a 2 output.mp3
  • cutoff=80: Only processes audio below 80 Hz.
  • decay=0.1: Quickly cuts off the generated low frequencies to prevent booming.

2. Deep, Sustained Sub-Bass (Higher Decay)

For electronic music, hip-hop, or cinematic sound effects where you want sustained, resonant 808-style low-end:

ffmpeg -i input.wav -af "asubboost=cutoff=100:decay=0.6" output.wav
  • cutoff=100: Captures bass content up to 100 Hz.
  • decay=0.6: Lengthens the resonance of the generated sub-frequencies, creating a sustained tail.

3. Combining with Wet and Dry Mix

To balance the boosted sub-bass with the original signal, pair cutoff and decay with wet and dry gain controls:

ffmpeg -i input.wav -af "asubboost=cutoff=90:decay=0.3:dry=0.8:wet=0.5" output.wav
  • dry=0.8: Reduces the original audio level slightly to prevent clipping.
  • wet=0.5: Adds the boosted sub-bass signal at half its generated volume.