A notch filter reduces only a narrow band of frequencies and leaves as much of the surrounding sound as possible. It is also called a band-reject filter. You use it to tame a specific resonance, hum, or harsh component that got into a recording. Unlike high-pass and low-pass filters that remove wide ranges, its character is that you aim it narrowly.
What a notch reduces
A notch attenuates deeply around the frequency you set. The narrower the width, the more it acts on a specific component while keeping the surrounding tone. It cannot remove only the problem, since musically necessary content may sit close by, so adjust the depth and the width by ear.
When to use it
Use it to tame room or instrument resonances, electrical hum, a particular ringing tone, or a frequency prone to feedback. In a mix, it is an option when a narrow peak is getting in the way of other instruments. If the problem covers a wide range, another EQ shape may sound more natural.
How to find the frequency
One method is to boost a narrow band temporarily and sweep it to locate the problem. Once you find the spot, undo the boost and cut only as much as you need. Cutting deeply from the start can take out the core of the sound. Use a spectrum display as a clue, and make the final judgment by ear.
Balancing Q and the amount of cut
The width of a notch is related to Q. A higher Q means a narrower band you can treat pinpoint, but the effect weakens if the setting drifts off target. The deeper you cut, the more the tone changes, so keep it to the minimum. After processing, check how it sounds in the track and not only in solo.
Phase and how it sounds
A steep, narrow filter can reduce the resonance you aimed at, but it may also affect the surrounding tone and the phase. With several tracks or stereo material, check in mono as well. You can review the role of EQ, including notches, in the EQ article.
