(Acronym LPF) A filter that reduces or eliminates frequencies above the cutoff frequency. Low-pass filters are used in synths to attenuate high harmonics and make sounds “darker” or “smoother” in timbre. The resonant low pass filter is the type of filter most often seen in synthesizers. They are used frequently as signal modifiers in synths in part because of the way the harmonic series works; the low pass filter leaves the fundamental of the signal intact, and so it doesn’t (usually) effect the subjective pitch of the signal. Typical low pass filters used in synths are two-pole (slope 12 dB/octave) or four-pole (24 dB/octave). Some resonant LPF designs will self-oscillate at a sufficiently high resonance setting, and these are sometimes used as voltage controlled oscillators; most designs will produce a waveform that is fairly close to a pure sine wave in self-oscillation.
Low pass filter designs used in synthesizers are typically chosen for their musical character and their capabilities for extreme filtering, which are considered more important than having perfectly stable frequency and phase behavior in the passband. Because of this, typical audio EQ filter types such as Butterworth, etc., are not used. The most commonly used designs for analog types are transistor ladder, diode ladder, and OTA-based.