Article Preview - What's The Frequency?

Manipulating the frequency spectrum is one of the most important skills in recording and mixing. We explain the different types of EQ you can use in your mix and share some tips on how to get the best from them.

Paul White

It's always worth thinking about where you place EQ in relation to other processors. It makes sense to perform corrective EQ before any compression, so that the compressor isn't triggered by and doesn't boost any unwanted frequencies. This means that if your sequencer has a built-in channel EQ, you need to think whether it is more appropriate to use an insert — or possibly both, as shown here.

Equalisation, more commonly abbreviated to 'EQ', is one of the key elements of the recording, mix or mastering engineer's toolkit, and you'll hear engineers talking at great length about the sound characteristics of specific makes and models of EQ — such is the importance of EQ to a modern recording. But simply knowing that engineer Bloggs uses a Pultec on his kick drums teaches you very little about how and why he uses it. In this article, then, we'll take you through the different types of EQ and explain their applications, as well as offering tips and tricks about which frequency ranges you might find most useful for common instruments.

What Is EQ?

Using high- and low-pass filters (top), you can 'bracket' the frequency band you want to let through, cutting out unwanted frequencies on either side — a helpful way of making space for other elements in a mix. High and low shelf filters level out ('shelve'), so are a better option when you want to apply a gentle boost to the top or bottom end.

The term 'equalisation' comes from the pioneering days of the telephone, when it described the process of correcting for — or 'equalising' — tonal changes caused by losses in the long telephone lines, but today the term is more generally used to cover all types of audio 'tone' controls. To put it very plainly, an equaliser is a frequency-selective filter that's able to cut or boost the level of specified parts of the audio spectrum. The simplest equaliser consists of just one capacitor and one resistor. With the resistor in series and the capacitor linking the output to ground, you get a high-cut (alternatively, 'top-cut' or 'low-pass': they all mean the same thing) filter that's just like the tone control you find on an electric guitar — that is to say, one that filters out the higher frequencies. Putting the capacitor in series and the resistor to ground gives you a low-cut (or 'high-pass') filter, that cuts out lower frequencies.

As long as no additional electrical load is applied to such circuits, the response is 6dB/octave (which means that the signal level drops by 6dB for every octave below the filter's 'turnover' frequency), or 'first order'. These simple, passive circuits cannot be used to boost frequencies, they can only cut them. To achieve an EQ boost, you have to combine the filters with active circuitry, which is what Peter Baxandall did when he developed his bass and treble equaliser, which was capable of cutting and boosting both low and high frequencies using two independent controls. Baxandall's basic circuit still forms the basis of many mixing console high and low equaliser sections and is mimicked in the form of presets on a good number of EQ plug-ins.

The Highs & Lows

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Article Preview - What's The Frequency?

A Guide To Effective EQ