Cutoff Filters

To understand what cutoff filters are, one must first learn what a filter is. In physics, a filter is a porous device for removing impurities or solid particles from a liquid or gas thet passes through it. In the audio universe, filters operate in a very similar way. A cutoff filter is a processor that can attenuate or completely remove certain partials out of any given frequency spectrum. In particular, this occurs when the signal passes through it. In this case, the user can customize both the upper and the lower frequency limit.

Basic Terminology 

Firstly, a passband is a band of frequencies, or shelf frequencies, that pass through a filter. If the signal travels through a low pass filter and the cutoff’s set is 700Hz, then the frequencies below 700Hz will pass through the filter.

Secondly, a stopband is a band of frequencies that get attenuated or removed from the spectrum. In other words, the filter impedes the band of frequencies to pass through. If you send the signal through a low pass filter and the cutoff’s set is 700Hz, the frequencies above 700Hz minimize or disappear completely.

Thirdly, in cutoff frequency, the filter by one-half, relative to the unattenuated passband frequencies, attenuates the frequency. The cutoff frequency is the particular frequency in the stopband diminished by 3dB. Sometimes, depending on the filter, you dilute the frequencies above or below the cutoff frequency.

Fourthly, the slope of a cutoff filter, measured in decibels per octave, represents progressively greater attenuation of frequencies above and/or below the cutoff, while a longer slope represents smoother attenuation. On the other hand, a pole is the increments of 6dB attenuation. The more poles, the steeper the slope.

Types of Cutoff Filters

There are four main types of cutoff filters: high pass filters (HPF), low pass filters (LPF), bandpass filters, and band-reject filters.

To begin with, a high pass filter is one that progressively attenuates frequencies below the cutoff frequency. These type of filter produces a thinner sound because it cuts off the low end. In addition, a low pass filter is a one that progressively attenuates frequencies above the cutoff frequency. Continuing with these types, bandpass filter involves two cutoff frequencies which allow the frequencies in between them to pass, while progressively attenuating frequencies outside the passband. Finally, a band reject filter is one that involves two cutoff frequencies. This filter attenuates the frequencies in between the two cutoffs while passing those outside the stop band.