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216
Using the GEQ
This 31-band, third octave graphic equaliser provides a high degree of accuracy and
control. Graphic equalisers may be used for corrective or creative purposes, depending
upon whether they are used live (MON or FOH) or in the studio (broadcast or
recording).
Studio and creative use
In the control room, a graphic equaliser may be used to remove problem frequencies
and improve deficiencies in room acoustics. This is commonly achieved with the use of
a real time analyser (RTA). As the frequency centres of the XL8 Control Centre conform
to ISO standards, corrections can be made by sight directly from the RTA to the
graphic.
However, it is important to mention that graphic equalisers can only compensate a
limited amount for a room with severe acoustic problems, in which case further
remedial treatment work may be required. Graphic equalisers can only help to reduce
the audible effects of standing waves and resonances and cannot overcome the loss of
clarity due to rooms with long reverberation times.
There are many creative and corrective uses for the GEQ. For example, by using the 31
equaliser bands and the high and low pass filters you can achieve the effect of someone
speaking on the telephone. When used in conjunction with a compressor, you can
create a de-esser. The GEQ can also be used for tonal correction of instruments or
vocals.
Live use (FOH)
It is often desirable to add equalisation to a venue to remove any frequency deficiencies
in the room before trying to engineer using the system. Again, it is common to use an
RTA and measurement microphone to set up the equaliser, using additive and
subtractive equalisation to correct problems in the room. It is recommended to
attenuate peaks in the room's response to the level of the surrounding frequencies
rather than boost the lower bands to meet the highest. This will help to retain
headroom in the equaliser and slight dips in frequency response are less noticeable
than large peaks. If an overall reduction in volume is observed, the gain make-up can
be used to return the output of the equaliser to the desired level.
However, please beware of excessive equalisation by, for example, using large amounts
of boost at lower frequencies to compensate for poorly performing bass enclosures.
This will use up large amounts of system headroom that could cause the system
amplifiers to clip, thus damaging loudspeaker HF components and introducing high
frequency harmonics, which may sound unpleasant. Using large amounts of low
frequency boost may also cause the over-excursion of bass drivers and result in their
mechanical and electrical breakdown.
Also, many sound reinforcement systems are only capable of adequately producing
frequencies up to around 18kHz. Providing high levels of boost at 20kHz to extend the
frequency response of the system is likely to result in a reduction in the life of high
frequency components in the loudspeaker, while not achieving a significant
improvement in the frequency response.
There are occasions where deficiencies in room acoustics cannot be corrected by
equalisation. For example, bass reduction due to phase cancellation or the cancellation
around the crossover point of a loudspeaker must be corrected before the use of
equalisation.
Chapter 11: Graphic Equaliser (GEQ)
XL8 Control Centre
Operator Manual
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