Notch Filter

A notch filter is the inverse of a bandpass: instead of isolating a band, it cuts a narrow slice out of the spectrum and leaves everything above and below it intact. Because the removed band is usually thin, the effect can be subtle on its own, but sweeping the notch frequency creates a phaser-like sound as the missing band moves through the harmonics. In mixing, narrow notches are used surgically to remove problem frequencies — a resonant ring, mains hum, or a harsh peak — without disturbing the surrounding tone. In synthesis, modulated notches add hollow, sweeping movement, and stacking several notches together is the basis of the classic phaser effect.