A low-pass filter, also called a high-cut filter, reduces everything above a chosen cutoff frequency. How sharply it does so is described by its slope: filters on mixing desks and in DAWs commonly fall at 12 or 18 dB per octave, while synthesiser filters can reach 24 dB per octave. Set low, a low-pass filter makes a sound feel distant or heard through a wall; swept slowly, it produces the familiar opening and closing filter effect of electronic music.
Low-pass filtering is one of the defining moves of lo-fi hip hop. In a 2022 study in the journal Organised Sound, Adam Scott Neal describes the genre’s habit of rolling off high frequencies, for instance muffling a piano with a low-pass filter, which contributes to its soft, dusty sound. Switch a filtered loop back to full range and it suddenly sounds bright and close.