The cutoff frequency is the frequency that marks the boundary between the band a filter passes and the band it attenuates. It is used with high-pass, low-pass, band-pass, and other filters. In practice the sound does not vanish suddenly at that frequency; it changes gradually along the slope. Do not decide by the number alone, but listen to how it sounds with your source and inside the song. It is the basic starting point when you move a filter. Judge the amount you need within the track.
High-pass and low-pass
A high-pass filter reduces what is below the cutoff, and a low-pass reduces what is above it. Moving the setting changes the weight of the low end and the brightness of the high end. It is used not only to clean up unwanted content but also to change the tone as an effect.
How it relates to band-pass
With a band-pass filter, the cutoff relates to both edges of the range that passes. The center frequency, the width, and the slope together decide what remains. Narrower settings are more noticeable, but they tend to sound less natural.
Choosing a value
Move the frequency up and down to find the point where unwanted rumble and noise are reduced while the core you need remains. Something that sounds extreme in solo can be right inside the song. Deciding in solo alone, on the other hand, makes you miss overlaps with the other instruments.
Resonance and slope
Resonance, which emphasizes the area around the cutoff, and the slope, which shows how steeply the level drops, also affect the tone. Even when you think you moved only the cutoff, some settings create a noticeable peak. Be careful especially in the low end or with high resonance.
How to check your work
Compare on and off, and check whether the sound has simply become quieter, or whether something important in mono has been lost. For the basics of filtering, see the article on EQ.
