Notch Filters
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 (high Q factor). Notch filters are used in live sound reproduction (Public Address systems, also known as PA systems)
and in instrument amplifier (especially amplifiers or preamplifiers for acoustic instruments such as acoustic guitar, mandolin, bass
instrument amplifier, etc.) to reduce or prevent 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'.