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SESSION NOTES 14 “Mastering” (part 4 “What Is Equalisation or EQ”)



Ahh yes, it’s time to look at the most powerful of all audio tools, the Equaliser. The equaliser, or EQ as it is commonly referred to is a TONE control. Most of us are familiar with the simple bass and treble controls on Hi-Fi equipment and other audio devices. These are a basic form of equalisation. The idea is to make the range of frequencies equal across the whole audible spectrum. The reality of this is not necessarily musical. Different genres of music require different EQ procedures. In mastering you have a ‘final’ mix that you then use EQ to achieve the optimum sound for that track. Using these devices we can make things sound bassy, trebly or middly. Yes “middly” is now a word. There are essentially three types of equaliser that can be used to shape the frequency response of our music and i shall now explain them thusly.


1. Graphic Equalisers -: These are very common on car stereo’s and PA (public address) systems. The outboard unit will consist of a row of fixed frequency sliders that are detented in the centre position. You raise the slider to boost or lower it to reduce the gain (or volume) of that particular frequency. The frequencies are always from left (bass) to right (treble). You can have two sets side by side for stereo. There can be as many as 30 bands for each channel with a fixed frequency usually spaced at ⅓ of an octave apart. The range or bandwidth is fixed. One of my first purchases as a fourteen year old was a 12 band per channel stereo graphic equaliser for my Hi-Fi to improve the sound of my albums. They are called graphic equalisers because they provide a visual graph of the settings you choose. It is very common to see a ‘smiley’ face setting on a lot of graphic EQ’s. People like loads of bass and treble in their music and boost these frequencies and cut those harsh middle frequencies. If you reverse these settings you will have a ‘sad’ face graph because it will sound like your music is coming out of the telephone … oh!! … wait a minute ….

2. Filters -: Filters do exactly what you expect them to do. Like a filter on a coffee machine that lets the coffee through but not the grounds. Audio filters allow only certain frequencies to pass. The three common types are the High Pass Filter (HPF) that cut bass frequencies and the Low Pass Filter (LPF) which cut high frequencies and finally ‘shelving’ filters. These first two were often found on cheap Hi-Fi’s and labelled “Rumble Filter” and “Scratch Filter. They are to be found on virtually every mixing desk. In the mastering studio i use them to reduce excessive low frequencies usually caused by wind noise on microphones and excessive high frequencies usually caused by pushing the cymbals too high in a mix. The last of these filters are known as ‘shelving’ filters. These are the bass and treble tone controls you find on most Hi-Fi’s and consumer playback systems. They will raise or lower the gain above or below a set frequency using a gentle shelf response. Finally there is the ‘notch’ filter. Used for cutting a very narrow range of frequencies.

3. Parametric Equalisers -: These as you can guess from the funky technical name are quite complex and VERY powerful tools. They are also known as ‘peak’ or ‘Bell’ EQ’s. With a Parametric EQ you have three controls. A Gain pot (rotary knob known as a potentiometer, hence the word “pot”) to boost or cut the volume. A Frequency pot, to select the preferred frequency and a Bandwidth pot which widens or narrows the range of frequencies to be worked on. Now, the bandwidth is also known or labelled as ‘Q’. This stands for Quality Factor and is derived from the world of physics and engineering. These types of EQ are extremely powerful, you can use the Q control to make a very gentle slope either side of the selected frequency or an extremely narrow precise ‘notch’ which will affect a very small range. I use a mixture of all the above EQ types in the mastering studio depending on the material i’m working on.

Next month I'm going to get stuck into the uses of these types of EQ and the reasons for my choices.

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