The mirror of the highpass filter, `lowpass` removes everything above a cutoff frequency. It is the right tool for taming sibilance, hiss, and high-frequency noise that survived the denoiser. It is also a quick way to make a recording sound 'warmer' without changing the overall level.

The command

ffmpeg -i in.wav -af "lowpass=f=12000:poles=2" out.wav

What it does

The filter passes everything below the cutoff and attenuates everything above. `f=12000` sets the cutoff to 12 kHz. A 2-pole filter is gentle; a 4-pole filter is steeper. For a voice recording with sibilance, start at 8 kHz; for a music track with harsh cymbals, 12 kHz; for a clean vocal, leave the default (off).

Requirements

Any audio input. The filter is in libavfilter. CPU-only, single-pass. The filter is mono by default; for stereo, pass `lowpass=f=12000:channels=stereo` or run two instances.

Things to avoid

  • Setting the cutoff below the sibilance range. Sibilance is between 5 kHz and 10 kHz; setting the cutoff to 4 kHz will muffle the entire recording.
  • Using a very steep slope. A 4-pole filter introduces ringing at the cutoff; start with `poles=2` and use a higher pole count only when the sibilance is severe.
  • Combining lowpass with a denoiser that already removed the high-frequency noise. The lowpass will further dull the recording without changing the perceived noise.
  • Replacing a parametric EQ. The lowpass is a single slope; a parametric EQ lets you cut a narrow band without affecting nearby frequencies.