Low Pass Filter

In essence, a low pass filter, also known as LPF, is a filter that lessens or eradicates all the frequencies that are higher than the cutoff frequency. People also know it as a high-cut filter or treble-cut filter. Regardless of the name, it works by passing the low-frequency signals and blocking the high-frequency ones.

Low Pass Filter: the Basics

To tell the truth, this kind of filter is fairly common in equalizers. It is capable of rejecting high-frequencies, but leaving affect low-frequencies untouched. The amplitude response of the filter is the function of pushing the gain of a filter at every frequency. In order to obtain the output spectrum, this type of filter has to multiply the input spectrum by amplitude response of the filter. By doing this, the signal elements that are above the cutoff frequency are completely eliminated, and lower frequencies remain unaffected. 

Its Characteristics

LPFs tend to temper high harmonics and make sounds smoother and darker in timbre, especially in synthesizers. Producers and players use low pass filters as signal modifiers in synthesizers. This is due to the fact that this type of filter is capable of leaving the signal intact. In addition, it typically doesn’t affect the subjective part of the pitch of the signal. Low pass filters used in synths are usually two-pole or four-pole.

Moreover, low pass filters tend to remove unwanted counter-productive bandwidth. Some designs are able to self-oscillate when they reach a high resonance setting. Most of them also generate a waveform that is quite close to a pure sine wave in self-oscillation. 

In summary, low pass filters are fixed and modulated tone controls. In order to achieve drastic changes within a tone, you must alter the cutoff frequency of a low pass filter. An LPF involves both resistors and capacitors or inductors. For instance, an RC filter is a low pass filter with a resistor and a capacitor, and an RL filter is a low pass filter that contains a resistor as well as an inductor.