What is a high pass filter?


In order to put it simply, we could say that the high pass filter is just a filter (low-cut) that attenuates frequencies below the certain cutoff frequency and allows the frequencies above to pass. There are many kinds of high pass filters that can appear at various stages in the signal path. For example,  the mic, the preamp, the equalizer/EQ plug-in, etc. They can have many different qualities. The slope of filter attenuation is usually quantified in decibels per octave. For example, a 12dB per octave HPF located at 100Hz would accomplish 12dB of relative attenuation at 50Hz, and 24dB at 25Hz. This slope would continue to extend into very low frequencies, effectively attenuating the signal to an indiscernible amplitude.


Attenuating low frequencies


High pass filters, often named as HPS, are commonly used in studio recordings.  They often attenuate extraneous low-frequency content like mechanical rumble or vocal plosives. By choosing a filter with a cutoff frequency below the fundamental frequency range of the program, an HPF can differentiate between program signal and low-frequency noise.


High Pass Filter applications


High-pass filters have many applications. As part of an audio crossover, they can direct high frequencies to a tweeter while attenuating bass signals which could interfere with, or damage, the speaker. When such a filter is built into a loudspeaker cabinet it is normally a passive filter. It also includes a low-pass filter for the woofer and so often employs both a capacitor and inductor. However, very simple high-pass filters for tweeters can consist of a series capacitor and nothing else.


Mixing consoles and High Pass Filters


Mixing consoles often include high-pass filtering at each channel strip. Some models have fixed-slope, fixed-frequency high-pass filters at 80 or 100 Hz. Other models have sweepable high-pass filters, filters of a fixed slope that can be set within a specified frequency range. Such as, from 20 to 400 Hz on the Midas Heritage 3000, or 20 to 20,000 Hz on the Yamaha M7CL digital mixing console. Veteran systems engineer and live sound mixer Bruce Main recommends engaging high-pass filters for most mixer input sources. Nevertheless,  kick drum, bass guitar and piano are sources which have useful low-frequency sounds.


DI units and Microphones


Main writes that DI unit inputs (as opposed to microphone inputs) do not need high-pass filtering. This is because they are not subject to modulation by low-frequency stage wash. These are low-frequency sounds coming from the subwoofers or the public address system and wrapping around to the stage. Main indicates that high-pass filters are applicable for directional microphones which have a proximity effect. A low-frequency boost for very close sources. This low-frequency boost commonly causes problems up to 200 or 300 Hz, but Main notes that he has seen microphones that benefit from a 500 Hz high-pass filter setting on the console.


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