10 EQ Modes Explained:  High Pass and Low Pass EQ

Many people start recording their own music and learning the basics in mixing because they have an idea for a song, and others get involved because they have played an instrument for some time and want to record themselves in the context of a song with other instruments. Still others join a band, and once they sound decent they want to document their efforts, and some like to sing and write lyrics and try their hand at making their own tunes, either with MIDI or samples. Whatever road that gets them there, if we are talking about the self-taught beginning recordist, one thing that typically mystifies the early learner is the high pass/low pass filter or Equalizer. 

They might have an easy grasp on what an Equalizer is, and what makes for different Frequencies, Spectrum, Gain, among other vocabulary. But, high and low pass may not automatically make sense. Especially if they learn about “pass” at the same time as they learn about “cut” they may be even more thrown for an answer.

The truly simple explanation for high and low pass EQ is to take it for face value. A high pass Equalizer allows everything above a certain Frequency to “pass,” meaning that all of the signal above that setting will remain and will not be removed. For example, a high pass EQ set to 100Hz means that everything from 101Hz and up will remain. This also means that everything below 100Hz will be removed. 

The same concept applies to low pass EQ. In this case, the low signal spectrum is going to be allowed to pass. So, in the same situation, if a low pass EQ was set to 100Hz, we would no longer see any audio above 101Hz, thus we would only hear low Frequencies.

The “pass” EQ is designed to isolate only what is needed and to remove everything that is causing trouble at a fixed point. Because it can make a very strong statement and a very obvious change to the signal, the “pass” point is usually not set, but is usually given several fixed Frequency options. These are usually chosen at points that are known to have typical audio issues. One point might be set to remove low end rumble, and another may be set to reduce grounding noise or audio interference hum. Another setting may be set below our usual hearing level just to clear any noise that is not intended just in case it is there. Higher Frequency high pass filters, like those above 80Hz, sometimes as high as 150Hz or even 200Hz help to clarify a signal that does not rely heavily on low Frequency content. This could be anything from a mid-to-upper piano part or vocal, a guitar or cymbal, and numerous other items. A pass filter can be used on an effect channel like reverb or time-based effects like delay or chorus or flange, to separate out residual effects from their original signal, making the dimension of a song more intricate.

The low pass EQ can be used to get rid of buzz or hum sitting on top of a clean bass guitar line, or change a slap and tap string artifact to sound more like a purposeful tonal performance. High pass can be used to remove all upper Frequencies and hiss and noise that is recorded as part of a low Frequency signal that is not desired. It could also be used on effect tracks to change the way an effect sounds on content that otherwise has a lot of high Frequencies, but it can also be used at the highest Frequency option supplied on a low pass filter to create a special effect from a track that normally has more spectrum in the sound, like making a voice sound like it is coming from a telephone or an older recording device.

It is very rare that a pass EQ will be completely variable, meaning that it is usually provided with a few fixed Frequency options that make for a usable utility for removing trouble above or below a certain point, but this is usually not given complete flexibility. It is also not as common for there to be only a single fixed pass Frequency, since there can be different spectral issues that come up on a regular enough basis that call for using pass filtering options.