Graphic EQ

Graphic Equalisers in Live Sound

The Graphic EQ is ubiquitous, and almost essential in any PA system.

What it is

Physically (except on most digital mixers, which typically use channel faders for graphic EQ control), it is usually a 19” rack-mounted box with vertical faders, each controlling a limited frequency range. Generally it will be between 1U and 3U in height. It will usually have two identical channels (although some single-channel graphics are available, and have their uses). Each channel will have either ten, fifteen, or thirty-one (sometimes only thirty) frequency bands. Usually, the centre frequency of each band will be an International Standards Organisation (commonly referenced just by its initial letters: ISO) standard frequency. For reference, over 31 bands these are:

20Hz, 25Hz, 31.5Hz, 40Hz, 50Hz, 63Hz, 80Hz, 100Hz, 125Hz 160Hz, 200Hz, 250Hz, 315Hz, 400Hz, 500Hz, 630Hz, 800Hz, 1kHz, 1.25kHz, 1.6kHz, 2kHz, 2.5kHz, 3.15kHz, 4kHz, 5kHz, 6.3kHz, 8kHz, 10kHz, 12.5kHz, 16kHz, 20kHz.

On a thirty-one band graphic equaliser, each band covers one third of an octave (you can work this out from the fact that one octave represents a doubling - or, going the other way, halving - of frequency, and there are ten octaves between 20Hz and 20kHz: on a 31-band graphic there are three steps between each doubling of frequency). Ten-band (octave) and fifteen-band (2/3 octave) graphics are not generally adequate for live applications, as each frequency band is too broad for anything more than approximate tone shaping.

What it does

It boosts or cuts a signal in one or more narrow parts of its frequency range. A line taken across the faders gives a graph-like view of the approximate overall effect, which is why this kind of equaliser is called a graphic EQ.

How it works

Each fader controls the level of an individual bandpass filter circuit, dealing with its own specific frequency range. Moving the fader up boosts that range, and moving the fader down reduces it. The combined effect of the filters is to change the overall balance of frequencies.

How do you use it?

If all else fails, read the manual! You can also find general guidelines on many manufacturer websites.

A graphic EQ can be connected to a mixer in one of two ways: on inserts, or in-line. Using the main (left and right or group) mixer inserts will mean that any changes to the graphic settings will be seen on the channel meters, and heard on headphones or listen wedge. This is considered an advantage by many sound-engineers. If a graphic EQ is connected in-line (i.e. between the mixer outputs and the crossover or power amp inputs), changes will only be heard through the main or monitor loudspeaker system, and the mixer's meters may not accurately represent the signal strength at the controller or amplifier inputs.

The main use of a graphic EQ in live sound systems is to correct anomalies in the overall sound, and (to a limited extent) control feedback. Overall tone shaping (largely a matter of individual taste) is another common application.

As a corrective measure, cutting a particular frequency is generally more effective than boosting other frequencies. There are several technical reasons for this, but a simple thing to bear in mind is that the peaks stand out, and are therefore more noticeable (imagine a level floor - the theoretical ideal - and think of the difference between stepping on a nail and stepping on a nail-shaped dent in it). Taking out the peaks will have more useful effect (and is easier) than trying to fill the holes. Boosting is the equivalent of creating a more spiky floor, while cutting is the equivalent of creating a more dented one.

The anomalies EQ was designed to address arise from peaks and dips in overall frequency response.

Generally, peaks are caused by resonance. Where resonances arise from instruments or within the PA system itself, EQ can limit the damage, but it cannot eliminate them, or remove room resonances (often a major culprit). Also, resonance is a design feature of most musical instruments, and while reducing the most obvious ‘honk’ from a harmonica will help it sit more comfortably in the mix, trying to remove it altogether will rob it of what makes it sound like a harmonica.

Dips in response often result from phase cancellation (over which EQ is completely powerless), masking by obstacles (pillars and walls, over which EQ is relatively powerless), or inefficiency of the sound system in that frequency range (microphones and speakers are the most likely contributors here). The higher frequencies will not reach listeners at the back if the loudspeakers are on tables at waist height, and EQ will be a much less useful solution to this than loudspeaker stands. Try changing the type and position of speakers and microphones first.

As a rule of thumb, use any EQ as little as possible. Only resort to EQ if no other remedy is available, and apply it sparingly to the most obvious problem frequencies. A thirty-one band graphic gives you reasonably precise control. If you apply drastic cut to most of the mid-band (the novice's ‘smile’ EQ), you are wasting its precision.

If your experience of using a graphic EQ is limited, try the following (start with all the graphic faders at their mid - 0dB of cut or boost - position):

Corrective. Using a CD player or similar source, play some material that has detail throughout the useful frequency range (i.e. 40Hz - 16kHz) through one channel (left, right, or one monitor channel) of the system. Boost each frequency range in turn on the graphic EQ. If the effect of boosting it simply makes that frequency stand out, return the fader to the mid position. If boosting it makes it boom, honk, squawk, shriek or whistle (or if boosting it makes it seem uncomfortably loud), move the fader below the mid position. How far below you move it is a judgment call, and depends on how badly it boomed, honked, squawked, shrieked or whistled. Either repeat this procedure for each channel individually, or copy the settings from the first channel to other channels using the same amplifiers and loudspeakers.
Corrective. Using a CD player or similar source (or the mixer's pink noise generator, if it has one) play some pink noise through one channel of the system. Use a calibrated microphone and spectrum analyser to view the output in the listening area (preferably, do this in more than one room position). Use the graphic EQ to reduce the level of any obvious peaks. It is usually unnecessary to get a flat reading (and it might sound a bit grim if you do), so after you have done this check the sound using some material that has detail throughout the useful frequency range, and if necessary reduce the biggest cuts by a few dB until it sounds OK. Either repeat this procedure for each channel individually, or copy the settings from the first channel to other channels using the same amplifiers and loudspeakers.
Feedback control. With most of the mics you will be using in place, set up with appropriate gain, and with all relevant input channels open, raise the fader of an output channel until it is on the verge of feeding back. Boost each frequency range in turn on the graphic EQ. If you can get it to the top without feeding back, return it to the mid position. If you can't get it to the top, move it as far below the mid position as it was below the top when it started feeding back. Check the sound using some material that has detail throughout the useful frequency range, and reduce the biggest cuts a little if it doesn't sound OK. If the system feeds back at a lot of frequencies (more than ten, say), you are pretty much at the limit of your usable headroom, and getting it any louder will only be achieved at the expense of noticeable colouration.
Overall tone shaping. Using a CD player or similar source, play some material that you know well (preferably with some detail throughout the useful frequency range), and have heard through a high quality sound system. Boost each frequency range in turn on the graphic EQ. Return the fader to the mid position unless boosting that frequency sounds horrible, in which case cut it a bit (in proportion to the horrible).

With any frequency alteration, bear in mind in mind that any change is relative: the effect of boosting or cutting one frequency range will be heard in relation to the overall sound. For example, substantially boosting bass frequencies will make the higher frequencies less noticeable in comparison (so it may sound ‘duller’ or ‘muddier’). Similarly, cuts in the lower frequencies may make the overall sound ‘clearer’ or ‘crisper’, as well as ‘thinner’.

Most graphic EQs have a master section, with controls that might typically include:

Input level. Some graphic EQs have meters or overload lights, and allow some attenuation or boosting of the input signal to bring it within the EQ's nominal operating range.
Output level. Even quite modest amounts of cut or boost in only a few frequency bands can be enough to cause a noticeable difference in the overall volume (both as it is heard, and as it appears on any subsequent meters). An output level control allows you to restore the overall volume, so that the graphic EQ affects only the tonal balance of the sound, not its apparent level.
High-pass and low-pass controls. Many graphic EQs include shelving EQ, ranging from fixed high-pass and/or low-pass filters (with an In or Out switch), to filters with variable frequency and variable gain. If you want to cut or boost the highest or lowest ranges, use these (rather than the faders).
EQ In/Out. Most graphic EQs allow you to bypass the EQ (on some, high-pass and low-pass controls may also be selected independently). This is useful for instant comparison: it is important that any changes you make actually improve the sound. In/Out comparison is made easier if the output level is adjusted so that switching the EQ in or out has no apparent effect on the overall volume. Sounds can apparently ‘improve’ (or get worse) from changes in volume, so comparison without level-matching may be misleading.

If you get the chance, play with a graphic EQ (using a variety of material) until you are familiar with the effect of cutting or boosting different frequencies. It may help you to get a grasp of frequency ranges if you learn to associate them with particular familiar sounds, or with vocal and instrumental pitch (for example, 1 kHz is the frequency of the BBC time-signal, and is just under two octaves above middle C on a piano, between the B and the C):

Table of Vocal and Instrumental Pitch

You can also improve your frequency recognition by downloading and using the (free) Simple Feedback Trainer from Sourceforge.

Do you need one?

Almost always.

What sort do you need?

1/3 octave is a must! There are 12 semitones in an octave, so even a 1/3 octave equaliser is relatively coarse when it comes to frequency control. Anything with less resolution - i.e.10-band (octave) or 15-band (2/3 octave) - is only useful for broad tone shaping.

The frequencies below 40Hz and above 16kHz are not vital, so a couple of the standard 31 bands are dispensable. In/Out switches and level controls are useful. High-pass and low-pass switches and/or frequency selectors are useful too. If you have enough rack space, the longer the faders the better.

Other factors (like whether the filters are constant-Q) are more open to debate, but each EQ has its own sound, so - if you can - listen before you buy.