The Graphic EQ is ubiquitous, and almost essential in any PA system.
What it is
Physically, 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 ISO (International Standards Organisation) 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 PA system in one of two ways: on inserts, or in-line.
Using the main (left & 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 speaker 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 PA 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 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 speakers are on tables at waist height, and EQ will be a
much less useful solution to this than speaker stands. Try changing the
type and position of speakers
and mics 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):
1. 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 speakers.
2. 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 (& 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
speakers.
3. Feedback control.
With most of the mics you will be using in place, set up with
appropriate gain, & 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.
3. 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. You can also improve your frequency recognition
by downloading 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.
- Source (astralsound.com)
