EQ (part 2)
If Figure 1 shows a flat response between about 20Hz and 20kHz, Figure 2 and Figure 3 show two of the curves you might expect to get from a mixing console EQ. Oddly enough, measuring the EQ and plotting the curve is something that only 0.001% of recording and sound engineers ever get round to doing at any stage in their creative careers, and only 0.0001% have their own equipment to do it to any reasonable accuracy. Even if its hardly ever done except on the test bench, its a useful concept which you can carry around in your head without ever bringing to the forefront of your mind. So if a producer ever says to you, Lets have a little more presence in the vocal, your subconscious mind will retrieve the bell shaped curve of Figure 2 from your memory bank while your conscious mind adjusts the controls and judges the sound. In Figure 2 we are adding an EQ boost, and there are three parameters that we would like to be able to control (if the EQ has knobs for all three). First and foremost is the frequency: this boost could be centred on any frequency according to the instrument and according to which characteristics you want to accentuate. Second is the gain, which is the degree of boost and can be measured in decibels at the centre frequency. Some mixing consoles even calibrate this control in dB, and a good thing too! You might like to have a range of up to 12 or 15dB as a maximum. Gain can also be negative to produce an EQ cut, which would be written as a gain of -6dB, or whatever, at the centre frequency, so the curve would dip downwards. EQ cut, by the way, is a vastly underutilised resource on many consoles, but more on this later. The third parameter which is only occasionally offered on a console EQ is Q. Q, as well as being the star of the last ever episode of Star Trek The Next Generation, or so my crystal ball informs me, is a measure of the width of the bell shaped curve - the bandwidth as some might say. A low Q - 0.3 is low - will allow the EQ to cover a wide range of frequencies while a higher Q - 5 is high - will allow you to home in on a particular feature of the sound.
The bell shaped curve of Figure 2 is often referred to as peaking EQ, and applies to all mid frequency range EQ sections and a good proportion of high and low frequency EQ sections too. Figure 3 shows a shelving EQ where the boost - or cut - extends from the set EQ frequency all the way to the extreme end of the range. I have shown a low frequency shelving EQ in boost mode, but it could have been a high frequency cut with a similarly shaped but differently orientated curve. It isnt possible to say which type of curve is better, it depends on what you want to achieve and some consoles have a button to allow you to choose.