Low Pass Filter
Last Edited: Dec 23, 2023
What Is a Low Pass Filter
A low pass filter, known as LPF, is essentially a filter that reduces or eliminates the frequencies above the cutoff frequency. It passes low-frequency signals and blocks or impedes high-frequency signals. In audio applications, this filter is often called a high-cut or treble-cut filter. As his opposite high-cut filter, the low-cut filter is commonly an equalizer. A low pass filter does not affect the low frequencies and rejects high frequencies. The function of pushing the gain of a filter at every frequency is the amplitude response (or magnitude frequency response). One can obtain the output spectrum by multiplying the input spectrum by the amplitude response of the filter. This way, signal components are eliminated and "stopped" at all frequencies above the cutoff frequency. On the other hand, lower frequencies are not changing.
Attenuating High Frequencies
In synths, low-pass filters often attenuate high harmonics and make sounds "darker" or "smoother" in timbre. In synthesizers, the resonant low pass filter is the most common type. Artists and producers frequently use them as signal modifiers in synths in part because of the way the harmonic series works. The low pass filter leaves the fundamentals of the signal intact, so it doesn't (usually) affect 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).
Eliminating Unwanted Bandwidth
Some resonant LPF designs will self-oscillate at a sufficiently high resonance setting, so they sometimes control the oscillators' voltages. Most designs will produce a waveform fairly close to a pure sine wave in self-oscillation. Low-pass filters can also eliminate unwanted, counterproductive bandwidth. One common example is using an LPF to establish the limited bandwidth of a low-frequency transducer, like a 'sub' mic on a kick drum. Other examples include the entire universe of subtractive synthesis.
Low Pass Filter Construction
Low-pass filters consist of resistors with capacitors or inductors. Specifically, an RC filter is a low-pass filter with a resistor and a capacitor. In contrast, an RL filter is a low-pass filter with a resistor and an inductor. Low-pass filters are both fixed and modulated tone controls. The most transient elements of any waveform are the direct contribution of that waveform's highest harmonic content. Consequently, by changing the cutoff frequency of an LPF, one can achieve dramatic changes in tone.
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