FFmpeg Low-Pass Filter Guide for Linux

Applying a low-pass filter to an audio stream using FFmpeg on Linux allows you to attenuate high-frequency sounds above a specified cutoff frequency while letting lower frequencies pass through. This article provides a straightforward guide on how to use FFmpeg’s built-in audio filtering tools, specifically the lowpass filter, detailing the essential commands for processing audio files and live streams, adjusting filter steepness, and optimizing your audio output directly from the Linux terminal.

Understanding the FFmpeg Low-Pass Filter

The primary tool for this task is the -af (audio filter) flag combined with the lowpass filter plugin. A low-pass filter is incredibly useful for removing high-pitched hiss, background noise, or preparing audio for subwoofers.

The two most important parameters you will configure are:

• f: The cutoff frequency in Hertz (Hz). Frequencies above this limit will be attenuated.
• poles: The number of poles (usually 1 or 2). A higher number creates a steeper, more aggressive cutoff slope. The default value is 2.

Basic Command Syntax

To apply a simple low-pass filter to an audio file, use the following command structure in your Linux terminal:

ffmpeg -i input.mp3 -af "lowpass=f=3000" output.mp3

In this example, f=3000 sets the cutoff frequency to 3000 Hz (3 kHz). Everything above this frequency will be significantly quieted down.

Advanced Filtering Options

Adjusting Filter Steepness with Poles

If you want a more gradual reduction of high frequencies, you can drop the poles down to 1. For the sharpest built-in cutoff, stick to the default of 2:

ffmpeg -i input.wav -af "lowpass=f=1500:poles=1" output.wav

Filtering Live Audio Streams

If you are dealing with a live audio stream (such as an RTMP stream or a pulse audio input device on Linux) and want to output it to a new stream or file, the syntax remains similar. You just need to specify the real-time input format:

ffmpeg -i rtmp://localhost/live/stream -af "lowpass=f=4000" -c:a aac output.mp4

Combining Low-Pass with Volume Normalization

Filters can sometimes lower the perceived overall volume of your stream. You can chain the low-pass filter with a volume filter using a comma:

ffmpeg -i input.ogg -af "lowpass=f=2500,volume=1.5" output.ogg

Verifying the Output

To ensure your filter has been applied correctly without fully exporting massive files, you can test the audio playback live in your Linux terminal using ffplay, which natively accepts the same filter arguments:

ffplay -i input.mp3 -af "lowpass=f=2000"