Cut Subsonic and Ultrasonic Audio with FFmpeg

Audio files often contain subsonic and ultrasonic frequencies that are inaudible to human ears but can negatively impact overall sound quality, headroom, and compression efficiency. This article provides a straightforward guide on how to use the asupercut filter in FFmpeg to eliminate these unwanted low- and high-frequency extremes, helping you clean up your audio tracks using simple command-line examples.

Understanding the asupercut Filter

The asupercut filter is a specialized FFmpeg audio filter designed to cut off frequencies at both ends of the human hearing spectrum. By default, human hearing ranges roughly from 20 Hz to 20,000 Hz (20 kHz). Frequencies below 20 Hz (subsonic) and above 20,000 Hz (ultrasonic) cannot be heard but still consume digital headroom and energy. Removing them allows your audible audio to sound cleaner and compress more efficiently.

Basic Syntax and Parameters

The basic syntax for applying the filter in an FFmpeg command is:

ffmpeg -i input.wav -af asupercut output.wav

The filter accepts several parameters to customize the cutoff thresholds:

• cutoff_f (or f): Sets the subsonic cutoff frequency in Hz. The default is 20 Hz. Frequencies below this value will be filtered out.
• cutoff_f2 (or f2): Sets the ultrasonic cutoff frequency in Hz. The default is 20000 Hz. Frequencies above this value will be filtered out.
• order: Controls the filter order (steepness of the cutoff slope). The default is 10. Higher values create a steeper, more aggressive cut.
• level: Sets the input level (gain) multiplier. The default is 1.0.

Practical Command Examples

1. Using Default Settings

To quickly clean up an audio file using the standard human hearing limits (20 Hz to 20,000 Hz), run the following command:

ffmpeg -i input.mp3 -af asupercut output.mp3

2. Customizing Cutoff Frequencies

If you want to apply a more aggressive cut—for example, removing everything below 40 Hz (common for voice recordings to remove low-end rumble) and everything above 18,000 Hz:

ffmpeg -i input.wav -af "asupercut=cutoff_f=40:cutoff_f2=18000" output.wav

3. Adjusting the Filter Order (Slope)

If you want a sharper, steeper cutoff at the specified frequency thresholds, increase the order parameter (e.g., to 16):

ffmpeg -i input.wav -af "asupercut=cutoff_f=30:cutoff_f2=19000:order=16" output.wav

Using these configurations, you can easily optimize your audio files for streaming, broadcasting, or archiving by ensuring no digital bandwidth is wasted on inaudible frequencies.