Recording For Guitarists, Part 5 – EQ
Chances are you already know what an equalizer does – it amplifies or attenuates certain frequencies in a signal (the sound of your guitar, for example) to make it sound better, reduce feedback (in a live situation) or both. Probably the most confusing thing about EQs is the terminology (a high-pass is the same thing as a low-cut, for example), so my focus here is going to be on what all the words mean that are associated with EQ. EQs are generally referred to as filters, because they filter out unwanted stuff (though they do other stuff, too). More…..
Let’s start with the most basic type of EQ, the kind you might find in your car stereo, for example. This is the kind that will have a knob for ‘bass’ another for ‘treble’. These are called shelving EQs, and what they do is to increase or reduce all of the frequencies above or below a certain frequency. The confusion comes in the names people use for shelving EQs, so let’s take a look:
Let’s say you want to get rid of all frequencies below 100Hz. There are three ways you can say this.
1 – Low cut – We can call this a ‘low cut filter’ because we are cutting all the lows below 100Hz.
2 – High pass – This is the same term looked at from the other side: rather than thinking of it in terms of cutting the lows, we think of it in terms of allowing all of the frequencies above 100Hz to pass through.
3 – Roll off – To roll of the lows below 100Hz is exactly the same thing as the two above terms, and it probably comes from the fact that most shelving EQs don’t just shut off everything below a certain frequency at the frequency, but more or less gently ‘roll them off’ so that you might have 100% of 100Hz, 75% of 90Hz, 50% of 80Hz and then a complete shut off of any frequencies below, say, 60Hz (I’m making up these numbers – each roll-off has its own curve).
Of course you can also use shelving EQs to boost all frequencies below or above a certain point. That’s what you’re doing if you turn up the bass or treble knobs.
A Bandpass filter is a combination of a low-cut and a high-cut filter. You can set a bandpass to roll off everything below a certain frequency and everything above another frequency, and you are left with everything in between, which can be very useful for live situations to avoid feeding back due to rumble or low end frequencies as well as avoiding the squealing that comes from higher frequencies feeding back.
In practice you can create your own bandpass on most mixers by simply turning down both the lows and the highs a little and allowing everything else to pass, since most simple mixers have shelving EQs for the low and highs.
A notch filter, also called a ‘band reject’ filter is a very useful simple EQ that takes out a certain frequency range (usually a pretty limited range) – this is particularly helpful in live situations when one frequency is causing feedback but you otherwise like your sound.
A peaking EQ boosts or cuts a range of frequencies around a ‘center frequency’. In practice this means that if a peaking EQ is set to, say, 2kHz (a kHz is 1000Hz, so above 999Hz we tend to speak in terms of 1.3kHz instead of 1300Hz, etc..) you’re boosting or cutting the frequencies around 2kHz. If you just raised a single frequency you probably wouldn’t notice anything, though, so a peaking EQ will raise the frequencies around the center frequency in what looks like a parabola (and the peak of that parabola is your center frequency – 2kHz in this case).
*see the section on parametric EQs to understand this a little better.
A graphic EQ is simply a box with a lot of peaking EQs, each representing a certain range of frequencies. The typical graphic EQ has 31 little sliders, and each slider is a peaking EQ (you can also use them as 31 little notch filters). This way, without knowing much about frequencies you can say to yourself ‘it sounds a little muddy’ and start cutting various frequencies or groups of frequencies on the lower spectrum until you find the offending range.
As a general rule, if things don’t sound bright enough you’re probably better off rolling off the lows than boosting the highs, and if it sounds too bright you should consider rolling off the highs before boosting the bass. The reasons are complicated, but this is a bit of conventional wisdom that really pans out in real life.
Parametric EQ is simply a type of bandpass EQ that gives you much more control over a set of parameters (hence parametric) than non-parametric. The three parameters are frequency, amount of boost/cut and Q (which stands for ‘quality factor’ and is also known as width).
With a parametric EQ you choose your center frequency and how much to cut or boost, but you also choose your Q, which is the width of the parabola, or the size of the range of frequencies around your center (on a peaking EQ the Q is fixed). This is immensely useful because you can not only decide that you want to boost the frequencies around 2kHz, but you can decide that you want a huge range of the frequencies surrounding 2kHz boosted, or only a very tight range to boost or cut only the frequencies very near to 2kHz. The higher the Q the tighter the parabola, and the lower the Q the wider the range of frequencies you affect.
There is also something called a semi-parametric EQ (which is found on most live mixing boards) which allows you to determine the center frequency and the amount of boost or cut, but semi-parametric EQs have a fixed Q, so you don’t have any say over the range of frequencies affected. These cans till be very useful, though. [Just last weekend at a gig my percussionist was really feeding back a lot and the sound guy couldn’t fix it. I downloaded a free frequency analyzer for my phone and saw that the frequency that was feeding back was 120Hz, so I went to the board and on the semi-parametric EQ there I set the frequency to 120Hz and cut it all the way back. Feedback was gone and the Udu sounded great.]
For live music EQ is essential for preventing feedback, but in the studio it can be used in very creative ways or as a way of ‘sculpting’ a sound. Often when mixing you’ll find that certain mixes occupy the same range of frequencies, making it hard to really bring out one or the other, and so EQ’ing them a little differently might help you there.
In the studio the use of EQ is really more of an art form than a science (though I’m sure someone will disagree), so I won’t get into what choices you should make, but hopefully this information will help you figure out how to use your EQ. I should mention that in the digital world of plug-ins you’ll find all sorts of great user interfaces for EQs that are much more transparent than the knobs on your mixer, so it’s easier than ever to play around, but a little knowledge of what you’re doing will still save you a lot of time.
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