Eq Filters and Slopes/Responses

Which filters to select when using equalisation, how to match filter slopes and responses and how best to utilise them.

Dynamic Processors


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When it comes to using equalisation in your productions it is imperative to understand how the slopes/gradients of the filters impact the equalisation process. Simply selecting an equaliser and boosting/cutting a sound is not enough. Understanding the importance of filter slopes and responses is as important as what type of equaliser you choose.

Slope or Gradient

Filters do not have a no-effect at a frequency and then instantly jump and suddenly reappear at the next frequency. They have to get there somehow. The way, and by how much, they get there is called the gradient or slope. In the case of the shelving filter, the most common slope is 6 dB gain change per octave (doubling of the frequency). It takes time for the filter to attenuate frequencies, in proportion to the distance from the cut-off point. This is the slope.

When cleaning, the slope of the eq/filter is as important as to where you set your cut-off. They can be designed with varying degrees of steepness expressed in dBs/octave where the more dBs, per octave, the steeper the filter. I always extend an eq’s display to show a much broader range as this will invariably cover all the slopes. This then allows me to shape and match the eq slope to the response of the sound. This makes for a much smoother and accurate cut.

Let us take vocals as an example:

A drastic slope can be unnatural when filtering vocals whereas a smooth slope can help to get the vocals to sit more naturally in the mix. Again, this comes down to taste and subjectivity but there does come a time whereby the integrity of the signal will suffer and this is usually the time to back off. If a 24dB/octave slope is too drastic on vocals then back off and select a smoother slope, maybe 12dB/octave.

The trick with all cleaning chores is to find the ‘biting point’ (the cut-off) and then to select the right slopes leading to and away from the biting point. A very steep slope is perfect for brick-wall filtering but not very good for sounds that need a lead in/out. A short static sound, like a kick drum, can be brick-wall filtered but a vocal, for example, would require a gentler slope as ‘lead in’ frequencies are critical to how a sound is perceived and vocals do not like instant/abrupt cuts as they tend to sound gated and unnatural. Certain frequencies, even if not needed, are important to include as they form part of the sound and how it behaves. For example, we don’t need sub-harmonics if we are using 50Hz for our kick drum. In fact, we could brick-wall cut everything below 50Hz but we might want to include the lead in from 45Hz to 50Hz so the kick comes in smoother. It is here where we would simply allow for a gentler slope instead of moving the bite point.

Lead In/Out

The lead in and out are governed by the slope/gradient. Quite often we will allow some sounds to have a slow progression to and away from the cut-off. Lead in allows frequencies below the cut-off to gently fade in based on what you select for the slope. The same process is applied but this time above the cut-off and we term this as lead out. Let me give you a simple example: if I am working on a vocal take I might want to cut everything below 200 Hz. If I place the eq node at 200 Hz and select a brickwall cut then almost everything is cut instantly below the cut-off value of 200 Hz. However, this type of aggressive cut using a very steep slope can be detrimental to the vocal take and a better approach might be to place the cut-off further up the spectrum, say 250 Hz, and use a gentler slope for the filter.

Matching Slopes

A great technique for achieving smooth transitions from filter to filter is to match the filter’s slope to the response (shape) of the sound.

In the video I show you how to use an equaliser’s spectrum analyser to ascertain critical information about the sound being processed. I explain how to select and match filter slopes to the sound’s response (shape). I explain how to use band-pass equalisation to ‘clean’ a sound. Shelving filters are explored in detail and the video ends with running an audio example through two different types of equalisers.

Plugin used in this video:

FabFilter Pro Q2

SSL X-EQ

Topics covered in this video are:

  • Which filters to select, how to match slopes, band passing
  • Slopes and Responses
  • Type and Use
  • Band-Pass and Shelving
  • Complimentary Responses
  • Sympathetic EQ
  • Banding
  • Understanding Crossovers
  • Tips and Tricks
  • Splitting Bands
  • Asymmetry
  • Panning Frequencies in Split Mode