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If you've ever used the EQ or tone control in your car stereo, you've most likely experienced a shelving filter. Unlike a parametric filter, a shelving filter is designed to boost or cut the signal at the target frequency and continue that boost or cut into lower or higher frequencies past the target. A shelving filter gets its name from the distinct shelf shape it forms in the EQ's frequency graph. Shelving filters come in two distinct flavors: high shelf and low shelf.
With high-shelf filters, the frequencies above the target frequency are boosted or cut uniformly through the top end of the EQ. With low-shelf filters, the frequencies below the target frequency are boosted or cut uniformly through the low end of the EQ. Let's hear some shelving filters in action. First, listen to this parametric filter as I boost 6 dB at 6K. (music playing) Now listen as I switch the parametric filter to a high-shelf filter and perform the same 6 dB boost.
(music playing) Notice that the high-shelf filter affects more frequencies than the parametric, resulting in an overall brighter sound as the shelving filter continues to boost frequencies well beyond what the parametric filter covered using a modest Q setting. A shelving filter generally has two controls: frequency, which is used to set the target frequency or starting point of the shelf's cut or boost, and gain, which, like in a parametric EQ, determines the amount of amplitude change over the range of frequencies defined.
Many shelving filters feature an additional Q or quality control that determines how sharp the shape of the shelf's transition will be and whether or not it will have a resonant peak at the target frequency. This is used to tell the filter how quickly it will climb to the amount of gain you've set and if there will be a small bump or resonant peak at the target frequency before settling in for the rest of the shelf. Certain vintage EQs, specifically Neves, are well known and adored for their resonant shelves that peak a little more at the target frequency.
There's a reason that shelving filters are used in most consumer stereos for bass and treble controls, because the goal with the shelf is to shape the overall low end or top end tone of the signal, as opposed to just boosting one single target frequency, like a parametric filter. High-shelf filters work great for bringing out the sheen or air of a signal, especially in the ultra-high frequencies. Or alternatively, they can dull the signal's top end a bit and send it to the back of the mix, similar to how analog tape can soften the high frequencies.
Listen to this acoustic guitar with a high-shelf boost and then a high-shelf cut. (music playing) Notice that the picking is brought out when the shelf is boosting and pulled back when cutting.
Low-shelf filters are great at boosting and strengthening all bass content in a signal below a set frequency, as opposed to a parametric filter, which boosts the signal centered over a specific frequency in that specific pitch. Likewise, a low shelf is a great tool for tapering back some of the low end in a muddy recording, helping make room for other instruments in that range. Listen to this drum loop with a low- shelf boost and then a low-shelf cut.
(music playing) Notice the bump of the kick drum come and go. Shelves are an indispensable tonal shaping tool that work great for manipulating the low- and high-frequency content of an individual signal or a complete mix.
Because they paint with a broad brush across many frequencies, they're generally best to use with lower gain settings under 6 dB, but feel free to use your ears and experiment.
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