You are here

EQ: How & When To Use It

Tips & Techniques By David Mellor
Published March 1995

Equalisation is one of the most powerful tools in your sonic toolkit and can be your greatest enemy or your greatest ally in the battle for the perfect sound. David Mellor gives advice on how and when best to use it.

The next time you make a recording, as an experiment set all the EQ controls of your mixing console to their centre positions and leave them there until you have finished the final mix. Don't be satisfied with anything less than perfection, and don't give yourself the excuse that you can't get a good sound because you were not able to use the EQ.

EQ is a very powerful and effective item in your sonic toolkit, not unlike a circular saw in fact! But you wouldn't use your Bosch or Black & Decker for a fine carving, would you? No, you would use basic hand tools and, most importantly, your skill and judgement. As a recordist, it is your own abilities which are going to be most important to the degree of success of your recording, and you should always use the appropriate tool for the appropriate situation.

It is always best to ensure that you get as good a sound as possible from the microphone, synth or sampler coming into the mixing console. If you start off with good sounds, then a good result is almost inevitable. It is becoming increasingly popular to use microphones for recording, even when DI (direct injection) is possible, because of the wider variation of tonal qualities available. Even small variations in microphone position make vast differences to the sound picked up. It is a sign of an expert recording engineer that he or she will listen carefully to the sound from the mic and adjust its position and angle, and even try out several microphones, rather than pretend that it is always possible to get it right first time.

Graphics are great for EQing an entire mix so that you can shape the sound as a whole, even after you have processed the individual elements.

Once you have built up your skills in this area, then you can think about using EQ. I could spout all sorts of proverbs about the things you can't make silk purses out of and the things you ought not try to polish, and these proverbs apply especially to EQ. You should always aim to use EQ to improve an already wonderful sound. If the sound isn't good without EQ, then you will never end up with anything but second best. The only time you should ever use EQ to 'save' a sound is when you have been given a tape to work on that was recorded by a lazy engineer.

Just as there is an art to creating a brilliant sound, there is an art to bringing that sound to perfection, and also blending several sounds together to make the perfect mix. Van Gogh didn't learn to paint overnight, and no‑one is born with the inbuilt ability to EQ. It's a skill that is learned by experience and a good deal of careful listening.

What Is EQ?

As a first step (although I know 99% of you have used EQ already!), let's see what EQ is and what it does. Then I'll move on to looking at the machinery and techniques.

Figure 1 shows one of the parameters you would expect any item of sound equipment to aspire to — a flat frequency response. This, or at least a very close approximation, will be the frequency response of your mixing console with the EQ controls set to their centre positions, or with the EQ buttons switched off. Here, the balance of frequencies of the original signal is preserved in correct proportion at the output. In other words it is just as trebly, tinny, harsh, nasal, honky, bassy or boomy as it was when it left the microphone; or just as perfect perhaps.

Notice that the frequency response indicates what the EQ does to the sound. A cymbal will naturally have strong high frequencies, for example, and that emphasis towards HF will be preserved by a flat EQ setting. Likewise, a flat EQ will reproduce perfectly the boomy bottom end of an undamped bass drum.

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 around 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 it's hardly ever done, except on the test bench, it's 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, "Let's 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 (dB) 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, producing 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 underused resource on many consoles, but more on this later...

The third parameter is Q, which is only occasionally offered on mixing console EQ. As well as being the star of the last ever episode of Star Trek: The Next Generation (or so my crystal ball informs me), Q 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 chosen 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 isn't possible to say which type of curve is better, for it depends on what you want to achieve, but some consoles have a button to allow you to choose.

Outboard EQ

Mixing console EQ is getting better and better, particularly in the low‑to‑mid price range. There was a stage where I was sure that the designers were inventing their EQs with the aid of a pointed finger and a pocket calculator rather than a keen pair of ears and advice from practising recording engineers, but this is no longer true of most console EQs. Nevertheless, no matter how good the EQ on your mixing console, there will come a time when you need to use an external or 'outboard' unit. This might be because you need a facility not available from your console EQ, or you might prefer to use an EQ unit for some subtle characteristic sheen it gives to the overall sound.

You should always aim to use EQ to improve an already wonderful sound. If the sound isn't good without EQ, then you will never end up with anything but second best.

Outboard EQs come in two basic flavours: graphic and parametric. A good graphic equaliser typically has 30 or so slider controls for frequency bands nominally covering a third of an octave each. You would use two for stereo. The basic idea of a graphic is that as you set the slider controls to achieve the sound you want, the levels of the sliders 'draw' the EQ curve, as if you had measured and plotted it the long way. Unfortunately, graphic equalisers are somewhat economical with the truth and only give a rough idea of the actual curve. This is because each band does not cover only a third of an octave; its effects are felt most there but the slider will actually affect frequencies belonging to two or three bands either side of it to a distinctly noticeable extent.

Whatever deficiencies graphic equalisers may have under the Trades Descriptions Act, they are still very useful tools to have around. Mixing consoles can handle basic EQ tasks better and more quickly, but there are certain applications where graphic EQs have the edge. More on this shortly.

The alternative to a graphic outboard EQ unit is the parametric EQ. This is so called because it offers control over all three EQ parameters I mentioned earlier — frequency, Q, and gain. A good parametric EQ unit may offer five bands, which cover the entire frequency range, or you might find three fully parametric bands with dedicated low and high frequency bands too.

Using EQ

Successful equalisation requires good equipment and a thoughtful approach from the engineer. Experienced engineers EQ by instinct and their fingers operate the controls as fluently as a jazz pianist tickles the ivories. But this fluency doesn't come automatically, it can only be won by experience. Anyone can grab the low frequency knob and wind up the bass to the maximum, but if you are serious about your recording then you will realise that it isn't just yourself you have to please; you have to consider what other listeners like and what systems they may be playing the recording on.

There is also a good technical reason why you should think before adding a lot of bass: for a given level of input, any small or medium size loudspeaker will produce much more sound at mid frequencies than at low, and if you boost the low frequencies too much then the overall level the speaker can achieve without significant distortion is less — sometimes much less. It's a matter of compromise: the more bass you add, the lower the overall level can be. This also applies to other frequencies in the mixing console itself.

Adding EQ adds level, and it is very easy to boost the signal so much in the EQ section of the console that you run into clipping and distortion. Since the fader comes after the EQ, lowering the fader will do nothing to solve this. The answer is to reduce the gain, to allow the signal a little more headroom if necessary. One further technical point: changing the EQ of a signal nearly always changes the level, so each time you adjust the EQ you will have to consider moving the fader to compensate. It's something that will come automatically after a time, but newcomers to recording often concentrate more on the change in the sound itself and don't notice that it has suddenly become more or less prominent in the mix.

Enough of the technical stuff, recording is an artistic occupation so let's consider the subjective facets of EQ. If we consider individual sounds first, let's assume that the signal coming from the microphone is already as perfect as can be, being the result of careful positioning and angling. Each instrument has certain bands of frequencies that are strong and some that are weaker. The human voice, for example, is very strong around the 3 to 4kHz region, no matter whether male or female, or what note is being sung. When using EQ, you will be considering which characteristics of the sound you want to accentuate, or which you want to reduce. One way to consider this might be to imagine an instrument which was an 'average' of all real instruments, where the characteristics of normal instruments were smoothed out into something that had a neutral sound. When EQing a real instrument, you will either want to exaggerate its individual characteristics and make it more distinctive, or reduce its individuality and make it more like this hypothetical 'average' instrument.

This is quite simple to do, and we can make use of the standard sweep mid range control that is found on most mixing consoles, with controls for frequency and gain. A fully parametric equaliser with a Q control can offer even more precision.

First set the gain control to a medium amount of boost — the three o'clock position of the knob is usually okay. Now sweep the frequency control up and down to the limits of its range and listen for the frequencies at which the effect is strongest. These are the frequencies in which the instrument is rich. Boosting the instrument's strong frequencies will enhance its individual characteristics and, for example, make a clarinet even more dissimilar to an oboe or any other instrument. In effect, you are making the clarinet even more clarinet‑like.

When you have found the instrument's strongest frequency band, set the amount of boost according to taste and always compare what you are doing with the flat setting. If you have EQ sections to spare, you may be able to cut down on frequencies which don't enhance the sound of the instrument. Some instruments which are not known as bassy instruments nevertheless have a high low frequency content; cymbals for instance. On many occasions it will be well worth cutting down on frequencies which you don't consider to be any use to the instrument, freeing up a space in the frequency spectrum for another instrument to use.

Enhancing the sounds of individual instruments in this way is useful, but watch out when mixing that you are not boosting the same frequencies on each instrument. It is a trap for the unwary to boost every instrument at around 3kHz to help it cut through at a frequency where the ears are very sensitive. This will produce a mix that is very tiring to listen to.

The opposite of the enhancement technique is where you lessen the individuality of each instrument and make it more like our hypothetical 'average' instrument. To do this, find the instrument's strong frequencies with the mid EQ set to boost as before, but then cut these frequencies, by as much as you feel appropriate. This won't make the instrument sound better in isolation, but it will help it blend in with the other instruments in the mix.

Many aspiring engineers do not appreciate how useful EQ cut can be, but the expert will skilfully share the frequency spectrum among all the instruments so that each has its own space and doesn't have to fight with the others for attention. Using EQ in this way can result in a powerful and full sound from a small number of tracks.

Mixing consoles differ in the usefulness of their high and low frequency EQs, and it is often necessary to bring in an outboard EQ that can do the job better. I would say that it is the purpose of the low frequency control to add 'weight' to the sound without making it 'boomy'. These are subjective terms I know, but I think we can all appreciate the difference between a sound which is firm and solid in the bottom end, and one which has plenty of bass but gives the impression of being out of control. In the other direction, the low frequency control should cut low frequencies that are not contributing anything useful to the sound, while retaining the depth and body of the low mid. At the high frequency end, you should be able to cut any 'fizz' from the sound while still leaving it clear and incisive, and you should be able to make the sound brighter without the extreme top becoming aggressive. If you can't achieve all this with your console's EQ, you may have to spend a thousand pounds or more on an outboard unit that can.

When you have explored all the possibilities your console's EQ can afford and you have visited your local hire company for outboard units that perform the same function only better, you'll be keen to get your hands on a graphic equaliser. This is a rather different animal which appears at first to offer the ultimate in flexibility: just raise or lower the frequency bands you are interested in for quick and precise control. Unfortunately, you will find that precision is lacking because each individual band alters frequencies over quite a wide range on either side of its nominal centre frequency.

This doesn't mean that graphics are useless — far from it. Graphics are great for EQing an entire mix so that you can shape the sound as a whole, even after you have processed the individual elements. If you know your way around, you can do this by taking a couple of outputs from the mixing console back into two channels and using the console's EQ again, but you'll only be applying more of the same, and doing it the graphic way really is much more satisfying. Graphics are also great for adding bite to a sound: just raise one or two sliders somewhere in the upper frequency region and you will make the sound more cutting without lifting the whole of the high frequency range. Experiment at your leisure.

Problem Solving

If you are working on a tape made by another engineer who isn't quite as fastidious as you, then you may find yourself faced with problems that EQ can help rectify. Unwanted sounds have a knack of finding their way onto recordings, particularly live recordings. If you have a 50Hz mains hum, for example, then a graphic will be able to help at only a little loss to the musical sounds on the recording. You can also use a parametric equaliser set to a high Q to home in on the unwanted frequency. Some equalisers have special notch filters to cope with precisely these situations. 50Hz hum may be removed to a reasonable extent, but the buzz caused by lighting dimmers may be impossible to get rid of. If the buzz isn't too harsh then you can try cutting the 50Hz fundamental and its harmonics at 100Hz, 150Hz, 200Hz etc. I can't promise anything, but it may make the recording just listenable.

Apart from hum or dimmer noise, if a recording is too noisy then very often the noise is most noticeable at high frequencies. Here you can use your EQ to strike the best compromise between cutting as much of the offending component of the noise as possible while still retaining some brightness in the sound. You may be able to apply a little boost at high mid frequencies, although the result will remain a compromise.

Even if the recording has no hum, buzz or noise, it may previously have been over‑EQ'd. It is quite difficult to ameliorate the results of over‑zealous EQing, particularly if some frequencies have been cut to a large extent. Trying to boost these frequencies back up again may result in an unacceptable amount of noise becoming apparent. Once again, compromise is necessary, although if you were dealing with one instrument from a multitrack mix you may be able to patch in a noise gate to help in this instance.

The Future Of EQ

There is no doubt that the designers of EQ both in mixing consoles and outboard units are going to pay far more attention to the sound of the EQ rather than the technical specs. Some manufacturers have started to drop the conventional 'low', 'mid' and 'high' labels and describe their controls with names such as 'bottom', 'sheen' and 'glow'. I don't think this is a bad idea, since it will focus our energies less on the technicalities and more on the sound the EQ produces. I wouldn't be at all surprised to see EQ being combined — not just in series within the same box, but fully integrated — with compression or distortion far more often than it has been up to now.

Whatever the future may offer, EQ will always be one of the most powerful tools in your recording toolkit, so make the most of it.

EQ With Boldness

When adjusting the amount of EQ to apply (ie. the EQ gain), it's tempting to adjust it very carefully and change the setting in small increments. The problems with this method are: (a) that if the EQ setting isn't right then it is wrong and thus needs total reconsideration; (b) that the ear quickly grows used to changes in the frequency balance of a sound.

It may not always be appropriate, but the next time you want to change the EQ level of a sound, grab the control firmly, twist it all the way up and all the way down and quickly settle on a new position which will hopefully be just right.

EQ Terminology

CUTOFF FREQUENCY The frequency at which a high or low frequency EQ section starts to take effect. Also referred to as turnover frequency.

SLOPE The rate at which a high or low frequency EQ section reduces the level above or below the cutoff frequency. Usually 6, 12, 18 or 24dB/octave.

PASS BAND The frequency range that is allowed through.

STOP BAND The frequency range that is attenuated.

FILTER An EQ section of the following types:

HIGPASS FILTER A filter section that reduces low frequencies.

LOW PASS FILTER A filter section that reduces high frequencies.

BAND PASS FILTER A filter section that reduces both high and low frequencies.

NOTCFILTER A filter that cuts out a very narrow range of frequencies.

GAIN The amount of boost or cut applied by the equaliser.

Q How broad or narrow the range of frequencies that is affected.

SWEEP MID A middle frequency EQ section with controls for frequency and gain.

PARAMETRIC EQ An EQ section with controls for frequency, gain and Q.

GRAPHIC EQ An equaliser with a number of slider controls set on octave or third octave frequency centres.

BELL An EQ with a peak in its response.

SHELF A high or low frequency EQ where the response extends from the set or selected frequency to the highest or lowest frequency in the audio range.

HF High frequencies

LF Low frequencies

MID Midrange frequencies

TREBLE Hi‑fi enthusiasts' word for HF.

EQ OFF BUTTON The sign of a good mixing console!

EQ Hints & Tips

  • If your mix sounds 'muddy', boost the main frequency range of each of the principal instruments. Boost 'decorative' sounds even more and pull the faders right down.
  • If you can't get your tracks to blend together in the mix, cut the main frequency range of the principal instruments.
  • To make vocals stand out in the mix, boost at around 3kHz.
  • For extra clarity, cut the bass element of instruments which are not meant to be bass instruments.
  • Adding EQ boost often adds noise. Listen carefully to arrive at the best compromise.
  • Changing the EQ changes the level. Always consider re‑adjusting the level after you EQ.
  • If you add a lot of EQ boost, you may run into clipping and distortion. Reduce the channel's gain to eliminate this.
  • If you use EQ to reduce feedback in live work, take care not to take too much level out over too wide a range of important frequencies, particularly the vocal 'presence' range around 3kHz.
  • If your mixing console has an EQ Off button, use it frequently to check that you really are improving the sound.