EQ

Shelving filter explained: understanding high-shelf and low-shelf equalizers

Shelving filter explained: understanding high-shelf and low-shelf equalizers

A shelving filter, also referred to as a shelf filter, shelf EQ, shelving EQ etc. allows you to boost or attenuate either the high end or the low end of the frequency spectrum. A shelving filter which boosts or attenuates the high end of the frequency spectrum is known as a ‘high shelf’. Whereas a shelving filter which boosts or attenuates the low end of the frequency spectrum is known as a ‘low shelf’. In this article, I’ll break down what each of these filters do, how they are controlled, and the benefits of using them.

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What is a Low shelf?

A low-shelf is used to boost or attenuate the frequency spectrum’s low end. You can see this in the following examples. Here, the low end of the frequency spectrum is being boosted:

Low shelf boost

Whereas here, the low end of the frequency spectrum is being attenuated:

Low shelf attenuation

You will notice that there is a transition band over which the filter applies the gain change. In a low shelf, once the full amount of gain is reached, it remains constant down to the very bottom end of the frequency spectrum.

What is a High-shelf?

A high-shelf is used to create a boost or attenuation in the frequency spectrum’s high end.

Here, the high end of the frequency spectrum is being attenuated:

High shelf attenuation

Whereas here, the high end of the frequency spectrum is being boosted:

High shelf boost

As with the low shelf, there is a transition band over which the filter applies the gain change. Once the full amount of gain is reached, the gain change remains constant all the way to the top of the frequency spectrum.

Shelving filter controls:

Shelving filters have three main controls. They are the filter’s gain setting, its frequency setting, and the steepness of its slope. Let’s look at each control in more detail…

Gain:

A shelving filter’s gain setting is used to control the amount of boost or attenuation that is applied.

Frequency setting:

The frequency setting defines the point at which frequencies above (high shelf) or frequencies below (low shelf) are cut or boosted.

Depending on your filter’s design, this frequency setting could refer to one of three different points on the transition band. It could be the point at which 3db of gain change occurs. This is known as the cut-off frequency. Alternatively, it could be the point at the very centre of the transition band. This is known as the centre frequency. Or it could be the point at which the full amount of gain change is reached. This is known as the corner frequency.

Slope:

Controlling the filter’s slope allows you to control the size of the transition band over which the gain change is applied.

Here, you can see an example of a gradual slope. The gain change is applied over a wide transition band:

Wide slope

Whereas here, you can see an example of a steeper slope. The gain change is applied over a smaller transition band:

Steep slope

Controlling a shelving filter:

Some plugins allow you to control the gain, frequency and slope of the shelving filter. There are however some plugins which offer control over the gain and frequency only, and not the slope. These are commonly referred to as ‘sweepable’ or ‘swept’ filters. There are also filters which offer control over the gain only. The frequency and slope cannot be adjusted. These are referred to as ‘fixed band’ or ‘fixed frequency’ filters. (It’s worth noting that the terms fixed frequency, sweepable, swept etc. are not exclusive to shelving filters. They can also be used to describe bell EQs when they exhibit the same level of control.)

The benefits of using a shelving filter:

Shelving filters make it really easy to achieve the tone that you’re looking for. If you want things to sound brighter, you boost with a high shelf. If you want things to sound less bass heavy, you attenuate with a low shelf. They’re a great way to make broad overall changes to the sound of a signal. Another benefit to shelving filters is the way that they tend to achieve natural sounding results. Due to the way that they apply a constant amount of gain boost or attenuation to the high or low end of the frequency spectrum, they often sound more natural than other filters.

Do you use shelving filters in your mixes? If so, what do you use them for? Leave your thoughts in the comment section below.

Want to know more about EQ? Learn about pass filters here. Learn about parametric equalizers here. Download your free EQ settings cheat sheet here.

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