A notch filter, also called a band-stop filter, sometimes a narrow band-pass filter, or T-notch filter, is an electronic filter typically used when the high frequency and the low frequency are less than 1 to 2 decades apart (that is, the high frequency is less than 10 to 20 times the low frequency).

In the audio band, a notch filter uses high and low frequencies that may be only semitones apart.

Example 1: Anti-hum filter

Low Freq: 59 Hz

High Freq: 61 Hz

this means that the filter passes all frequencies, except for the range of 59-61Hz.

Example 2: Anti-presence filter

Low Freq: 1000 Hz

High Freq: 4000 Hz

See also: FM radio, Q factor