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In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband.
Narrow notch filters are used in Raman spectroscopy, live sound reproduction and in instrument amplifiers to reduce or prevent audio feedback, while having little noticeable effect on the rest of the frequency spectrum. Other names include "band limit filter", "T-notch filter", "band-elimination filter", and "band-reject filter".
Typically, the width of the stopband is 1 to 2 decades. However, in the audio band, a notch filter has high and low frequencies that may be only semitones apart.