AIYIMA Full range Speaker Adjustable Notch Filter LCR Filter Stopper Full Range Speaker 4 8 Ohm 8 Inch DIY Sound Improvement|Speaker Accessories| - AliExpress
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Notch filter is insensitive to component tolerances - EDN
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Audio Remastering: The Notch Filter – Whine / Tone Removal – Martin C. Winer
Notch filter autotunes for audio applications - EDN