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.) is a great option for equalizing a signal. That’s because it allows you to boost or attenuate either the high end or the low end of the frequency spectrum. Whether you realize it or not, you’ve probably used a shelving filter before. That’s because these are the types of filters that are used in home Hi-Fi equipment and car stereos. But their use is not however just limited to consumer electronics. They can be immensely useful tools when mixing and mastering music too.

A shelving filter which boosts or attenuates the high end of the frequency spectrum is known as a ‘high shelf’. 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 breakdown what each of these filters do, how they are controlled and the benefits of using them.

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

Shelving filters differ from other types of filter. That’s because they apply an equal amount of gain to all frequencies between the filter’s frequency setting and the outer limit of the frequency spectrum. So as you can see in the example above, an equal amount of gain is being applied from the target frequency of 100Hz, 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 an 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

Much like the low shelf, the high shelf filter also applies an equal amount of boost or attenuation to all of the filtered frequencies. In the case of a high shelf, these are the frequencies between the filter’s frequency setting and the very top end of the frequency spectrum.

Shelving filter controls:

Shelving filters have three main controls. These include the filter’s gain setting, its frequency setting, and the steepness of its slope. Some filters will allow you to manipulate all three, whilst others may have fixed values or may provide predetermined values to choose from. 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 to the filtered frequencies.

Frequency setting:

The frequency setting defines the point at which the transition from unfiltered to filtered occurs. Depending on the way that your filter is designed, you may see this referred to as the corner frequency or center frequency, or more general terms like ‘target frequency’ or ‘reference frequency’ may be used. In the case of a high shelf filter, everything above the frequency setting will be controlled by the gain setting. Everything beneath it will remain unaltered. In the case of the low shelf filter, everything below the frequency setting will be controlled by the gain setting. Everything above it will remain unaltered.

Slope:

As you may have noticed from the diagrams, it’s not possible for the filter to apply the entire gain amount at once. It transitions gradually between unfiltered and filtered, creating a slope. Some filters allow you to control the steepness of this slope.

Here, you can see an example of a wide slope:

Wide slope

Whereas here, you can see an example of a steeper slope:

Steep slope

The benefits of using a shelving filter:

The biggest benefit that you will find when using a shelving filter is its ease of use. Shelving filters make it really easy to get the results 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 their characteristically natural sound. Due to the way that they apply a constant amount of gain boost or attenuation to a part of the frequency spectrum, they often sound more natural than other filters. This is especially true when applying large amounts of gain.

Do you use shelving filters on your mixes? If not, can you think of a way that you might benefit from using them in the future? Leave your thoughts in the comment section below.

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