A low-pass filter (LPF) passes content below a frequency you set and reduces content above it. It is also called a high cut. You use it to tame piercing highs, noise, and unwanted brightness, or to make a sound seem more distant and darker. Cutting too much high end also costs you definition and presence, so decide on a purpose before you adjust.
What passes and what is reduced
A low-pass passes what is below the cutoff frequency and gradually attenuates what is above it. The lower the setting, the darker the sound, and the steeper the slope, the bigger the change. Some filters also offer resonance, which emphasizes the area around the cutoff.
Using it to clean up the high end
Use it on hiss, air conditioning noise, harsh components from amps and distortion, and highs that clash with other instruments. Putting a low-pass on reverb and delay returns creates a space that sits further back than the direct sound. Check within the song, not just with the source soloed.
It works for sound design too
Moving a synth filter to change the brightness, cutting the highs DJ-style, and creating a texture like a voice over the telephone are all production effects. When you automate the cutoff, watch out for sudden level changes and for resonance getting too high.
Choosing between high-pass and low-pass
A high-pass reduces the low end, and a low-pass reduces the high end. Using both lets you keep only the midrange you need. Both are tools for removing unwanted bands, but think separately about the range you cut and the role of the part within the song.
Checking with EQ
After adding a filter, toggle it on and off and check not only that the harshness is gone but also that you have not lost the vocal consonants, the definition of the drums, and the air. Go back to a gentler slope if you need to. You can review the basic thinking behind EQ in the article on EQ.
