Notch filter

In signal processing, a notch filter (also band-stop filter or band-rejection filter), is a filter that passes most frequencies unaltered, but attenuates those in a narrow range to very low levels. It is the opposite of a band-pass filter.

Other names for the notch filter include 'T-notch filter', 'notched filter', 'band-elimination filter', and 'band-rejection filter'.

Typically, the width of the stopband is less than 1 to 2 decades (that is, the highest frequency attenuated is less than 10 to 20 times the lowest frequency attenuated). 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

References

• Federal Standard 1037C
• MIL-STD-188