Configure FFmpeg Lowpass Filter Cutoff and Poles

This guide explains how to configure the cutoff frequency and filter poles using the FFmpeg lowpass audio filter. You will learn the exact command-line syntax, the role of the frequency and poles parameters, and practical examples to precisely control high-frequency attenuation in your audio files.

The FFmpeg Lowpass Filter Syntax

The lowpass filter in FFmpeg passes frequencies below a specified cutoff frequency and attenuates frequencies above it. To configure this filter, you use the -af (audio filter) flag with the lowpass filter name and its parameters:

ffmpeg -i input.mp3 -af "lowpass=f=2000:p=2" output.mp3

Configuring the Cutoff Frequency (f)

The cutoff frequency determines the point at which the filter begins to reduce the audio signal’s volume.

• Parameter: frequency or f

• Value: A double-precision floating-point number representing frequency in Hz (Hertz).

• Default: 3000 Hz

• Example: To set the cutoff frequency to 1000 Hz:

  ffmpeg -i input.wav -af "lowpass=f=1000" output.wav

Configuring the Filter Poles (p)

The number of poles determines the steepness of the attenuation curve (the roll-off rate) past the cutoff frequency.

• Parameter: poles or p

• Value: An integer. Typically, the value is restricted to 1 or 2.

• Default: 2

• Behavior:

  • 1 Pole: Provides a shallower roll-off of 6 dB per octave. This results in a gentler, more natural reduction of high frequencies.
  • 2 Poles: Provides a steeper roll-off of 12 dB per octave. This blocks high frequencies more aggressively.

• Example: To set a steep 2-pole filter:

  ffmpeg -i input.wav -af "lowpass=p=2" output.wav

Combining Parameters in a Single Command

To customize both the frequency and the poles simultaneously, separate the arguments with a colon (:).

Example 1: Gentle High-Cut at 4 kHz

This command attenuates frequencies above 4000 Hz with a gentle 1-pole curve (6 dB/octave):

ffmpeg -i input.wav -af "lowpass=f=4000:p=1" output.wav

Example 2: Aggressive Bass-Pass at 500 Hz

This command cuts off everything above 500 Hz with a steep 2-pole curve (12 dB/octave), leaving mostly low-end frequencies:

ffmpeg -i input.wav -af "lowpass=f=500:p=2" output.wav