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Mix a pop song – 7 – Adding the Main Parts

Probably the first thing we need to get out of the way right now, is a more detailed examination of the role of EQ in mixing.
The Use of Equalisation

Equalisation – or EQ, to give it its common acronym – is fundamental to modern recording and mixing. However, its application is still very much hailed as a “black art” and many engineers don’t like discussing openly their use of EQ in case they give away their own “trade secrets”.

To confuse matters further, a lot of the common guidelines established over the years are touted around by educators as “rules” that can only be broken at your peril. Rules such as “get the sound right at source”, when overemphasised, can prevent a junior engineer from experimenting with the more subtle applications of equalisation, to the extent that they “play safe” too often – to the detriment of their mixes and their education.

This – coupled with the fact that poor EQ practice can ruin a good sound – can also lead modern record producers to be unnecessarily paranoid about the use of EQ, and cause them to slap the wrists of engineers who reach for the EQ controls when the sound is already “basically acceptable” without understanding that the engineer may have other important reasons to tweak the EQ.

EQ isn’t a necessary evil – it is instead, a highly versatile tool, which can be used for a wide variety of different applications which go far beyond an engineer simply “playing with the sound”. In fact, it may come as a surprise to you to discover that there are many, many different uses of EQ, and so to demonstrate this, I will go into some detail on seven of the major uses of EQ when mixing (and indeed recording):

  • Noise Elimination
  • Harmonic Damping
  • Sound Enhancement
  • Bong and Boff
  • Distance Placement
  • Creating Mix “Space”
  • Auto Mix “Levelling”

I’ll explain what I mean be these in a minute – but first:

Types of Equaliser

There are many different kinds of equaliser available to do these tasks. These are (in order of sophistication):


Simple filters are extremely useful on mixing and recording systems. They are more correctly known as “High (or Low) Frequency Roll Off Filters”, or – with confusingly opposite names – as “Low (or High) Pass Filters”. They either take the form of a single button to roll off the bottom end, or perhaps instead two rotary controls marked LF and HF. As you sweep these controls, the top and bottom end is “rolled away” from the signal. The frequency at which this happens depends on the position of the control. You can’t control how steeply the sound rolls away – that is always preset. The manual either for your software or mixing hardware will say what it is preset to. It will be described as something like “6dB per octave” or “12dB per octave”.

Not all mixing desks have simple, dedicated filters. On budget desks, it is assumed that the task can be done with by careful use of the main EQ, although this is a little restrictive – because having even just a single button for LF rolloff is extremely useful – especially when recording.

Shelf Equalisers

The filters described above don’t raise or lower the treble and bass in general terms – the sound is actually trimmed away to nothing the higher or lower the sound gets. This makes filters good for getting rid of undesirable sounds outside of the musical range of the instrument (see section on Noise Reduction Using Equalisation, below).

But, unfortunately, filters are not very good for shaping the actual musical sound itself. That’s where Shelf Equalisers come in. Just like filters, they come in two types – low frequency and high frequency. However, instead of having “Frequency” controls, they have “Gain” controls instead. The gain controls set how much boost or cut is applied to the signal. A high frequency shelf EQ will normally start working at around 8 to 12 kHz, and a low frequency shelf EQ works at around 80 to 150 Hz. The point about shelf equalisers, is that – unlike rolloff filters, they raise all frequencies above (in the case of high-frequency shelf EQ) or below (in the case of a low-frequency shelf EQ) by the same amount, which will give you much more “musical” results than rolloff filters. They are called “shelf” equalisers because of the shape of their frequency response when drawn on a graph.

The “Bass” and “Treble” controls on ordinary Hi-fi equipment are usually shelf equalisers with preset, unadjustable frequency points.

Some shelf equalisers allow you to set the frequencies at which they operate as well.

Sweep Equalisers

The problem with both filters and shelf equalisers, is that they are really only useful at either end of the audio spectrum.

So what about the middle?

Well, if you think about it, you can boost the middle by using a high and low shelf EQ to cut the bits either side, or – similarly – you can cut the middle by boosting the bits either side. But this is not very satisfactory, and difficult to control quickly and easily.

A sweep equaliser is designed to solve this problem. With a sweep equaliser, only the parts of the signal surrounding the area selected by the frequency control are affected – by the amount set using the gain control. This allows you to boost or cut selected areas of sound quite easily. It isn’t possible to control the width of the sonic “area” affected by the EQ, and this width varies from manufacturer to manufacturer. Some manufacturers like to keep the area fairly broad, as this is more musical, but others prefer to keep it narrow as this is more useful for correcting harmonic problems like “ringing” on drums or other instruments.

Sweep equalisers normally come in pairs. A pair of sweep equalisers is often found on some of the better-quality “portastudio” devices, as it allows a great deal of sonic control for relatively little cost. Some manufacturers give completely different frequency ranges for each one of the pair, arguing that this allows for more precise control over the entire range of sound. This is acceptable providing that the preset “area” or “bandwidth” of the equaliser is extremely broad. If the bandwidth is smaller (“tighter”), then it is far more useful for a pair of sweep equalisers to have a generous overlap in their frequency ranges, so that you can use both of them at the same time in both the low and upper frequency ranges.

Semi Parametric

To make sweep equalisers more useful, they are sometimes fitted with a single button that changes the “bandwidth” or “area” over which they are effective.

When one or more sweep equalisers are teamed up with a couple of additional shelf equalisers for control of the very top and bottom end, the entire assembly of equalisers is referred to as a semi-parametric EQ.

Fully Parametric

On top-of-the-range mixing desks, you normally have fully parametric equalisation.

With a fully parametric equaliser, you can control the frequency, the gain, and – significantly – the bandwidth of the equaliser. Typically, four of these units are packed together, and a switch on both the first and last unit, allows them to be optionally used as shelf high and low frequency EQ’s respectively. Naturally, this is expensive.

For most instruments, you don’t need this level of control, and semi-parametric equalisers are fine. Even in a professional set up, you can often get by quite adequately using semi-parametric equalisers, provided that there are a couple of plug-in fully parametric equalisers available for troublesome instruments. The main problem however with a mixing desk with semi-parametric equalisers throughout, is that the manufacturers preset choice of bandwidth, can result in the “sound of the desk” colouring the mix overall, to a much greater degree than it does with fully parametric EQ.

But having said that – in almost all cases – the “sound” of the EQ is probably still the major distinguishing feature that separates one mixing console from another.

Graphic Equalisers

These are by and large an overkill They are best reserved for situations where a number of extremely subtle audio artefacts are already taking place – such as the equalisation of a recording studio control room’s main monitors, or the delicate final equalisation of a finished mix during the mastering process when preparing a CD or other release for listening by the public at large. Graphic equalisers are much less suited to situations where the actual “correction” required is more “general” such as EQing individual instruments as part of a mix – although it must be said, with some particularly difficult sounds, you may occasionally have to resort to using a graphic equaliser to solve the problem – but not generally.

Passive and Valve Equalisers

It’s worth mentioning in passing here, the subject of passive equalisers.

Most equalisers use circuitry that actively boosts or cuts the sound in the various bands using electronic feedback techniques which can (and often does) introduce audible “ringing” in the circuitry. Passive equalisers on the other hand work by already cutting the sound in all frequency bands to begin with, using simple, unpowered, passive electrical components like resistors, capacitors and inductors. A single, simply-designed amplifier stage after the equaliser usually makes up for the loss in signal level, so that the level is flat when all controls are in their “centre” position. Therefore, on such an equaliser, when you “boost” a frequency, you’re not really boosting it at all – you’re just allowing it to seep through unhindered by the passive circuitry. Valve equalisers often work in exactly this way.

This gives a much smoother sound. In fact a lot of the “smoothness” attributed to valve equalisers, often has little to do with the fact that they have valves in – it is instead due to the fact that the equaliser circuitry is passive rather than active.

So how do you use these all these different types of equalisers in practice?

Let me give some practical examples:

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