Audio Recording

You might encounter these terms when recording, mixing or mastering. For a beginner in music production, these things can be very confusing. So let’s end all the confusion once and for all with this guide. First, low shelf and low pass filter are not the same; so as with high shelf vs. high pass filter. For example in the post on complete EQ settings to start when doing audio mixing you may have encountered suggestions to apply a low shelf/high shelf filters. Another example application are also found on EQ settings for mastering. Thus, it is imperative that you understand how this works so that you will know how to use them properly during practice.

So what’s really the difference? The easiest way to understand is to show some response/output graphs that shows the behavior for each of these filters.

LOW SHELF FILTER

Definition: A low shelf filter will cut or boost signals of frequencies BELOW “fc” or cutoff frequency. Above fc, the frequency response will not be altered and will pass normally. Below is the screenshot of a low shelf filter used in cutting signals of frequencies below the cutoff “fc”.

Low shelf filter is commonly used during guitar EQ mixing and mixing vocals. The purpose is to cut the lower bass frequencies of each instruments so that it won’t conflict with the bass guitar and kick drum frequencies.

However, common misconception is using only the low shelf filter for cutting signals below the cut off frequency. This is not true in all cases. Sometimes, you will use a low shelf filter to boost the frequencies below the cutoff, such as the case below:

A common application is when you like to boost the bass frequencies below the cut-off “fc”. For example during mastering process when you observe that the bass is somewhat very weak.

LOW PASS FILTER

Definition: a filter that passes frequencies below the cutoff and drastically attenuates above the cutoff. Below is an example of a low pass filter response curve:

A low pass filter is only designed to cut or attenuate signals above the cutoff frequency but its never designed to boost signals. Common uses of a low pass filter includes cleaning noise or hiss signals above a certain frequency cutoff, for example 16Khz.

HIGH SHELF FILTER

Definition: A high shelf filter will cut or boost signals of frequencies ABOVE “fc”. Below the cutoff frequency, the input signal is unaffected. This operates opposite to a low shelf filter. Below is the response output of a high shelf filter used in cutting signals above “fc”.

A common implementation of high shelf filter reduction is similar to a low pass filter such as noise filtering. Unlike a low pass filter which is designed only for “attenuation” or “cutting signals”. A high shelf filter can also be used to boost signals above the cut off frequency, such as below:

A typical example is boosting high frequencies. For example in a digital audio music mastering process, the engineer might noticed that there is a significant lack of high end frequency response. To solve this problem, the mastering engineer can use a high shelf filter to slowly boost the high end frequencies.

HIGH PASS FILTER

Definition: A high pass filter will pass signals of frequencies above the cutoff frequency unaltered and drastically attenuate signals of frequencies below that value. Just like low pass filter, a high pass filter is only used for the attenuation of signals or cutting. It is not designed for boosting. Below is a typical response output of a high pass filter:

As you have noticed, high pass filter operates similarly to the “cutting” mode of a low shelf filter. But high pass filter drastically reduces the signals below the cutoff as compared to a low shelf filter.A common implementation is when mixing bass guitar vs kick drums. In rock music, bass guitar commonly occupies the bottom subwoofer frequencies while the kick drum is above the bass guitar. The mixing engineer can use a high pass filter on kick drums to cut its lower bass spectrum in favor of the bass guitar.

Summary

1.) A low shelf filter operates similarly with a high pass filter in cutting mode.
2.) A high shelf filter operates similarly with a low pass filter in cutting mode.
3.) A high pass filter is not the same as the low shelf filter because low shelf filters can be used in boosting signals while a high pass filter is limited only to attenuation.
4.) A low pass filter is not the same as a high shelf filter because low pass filter cannot be used to boost signals unlike a high shelf filter.

For best results, you might want to cut not more than -12dB on shelving filters. More than that, it is recommended to look into re-recording the affected track as over-EQ could introduce some artifacts. The maximum boost recommended would be around +3dB on high shelf boosting mode. You would also consider re-recording the track if you find to boost more than this amount. For example you find the cymbals and hi-hats sound lacking in treble definition, instead over-boosting it in mixing, it might be good that it would be re-recorded using proper microphone and placement.

Content last updated on March 1, 2012