Owner's manual

EQ231G User Manual7
6. Techniques
6.1. Room measurement and equalisation
There are several ways of measuring a room's frequency response, and these are
usually all similar in technique, although differing in hardware. Usually a known quantity
of an audio signal is generated and transmitted through the sound system. The result is
then read through a calibrated microphone and a graph of the frequency response is
indicated to the user usually in 1/3rd octave intervals.
The actual method may employ "pink noise" and an led display device, or a series of
swept sine waves measured on an audio voltmeter or plotted on an audio graphic
recorder. Other methods involving pulses and time delay are also available.
The microphone is located in the listening area and moved around to check for
variations in response at different locations. These variations are then averaged to make
a "raw house curve". This is used as the starting point for corrective equalisation, and
where there are dips in this curve, the appropriate frequencies are boosted, and where
there are peaks, those frequencies are cut etc. The room is then checked again for the
combining effects of adjacent filters and so on until the house curve is flat.
It should be noted that there are many problems associated with multiple speaker
arrangements and quite often re-positioning of speakers and re-balancing will correct
many of the problems before equalisation, thus reducing the possibility of unwanted
effects of extreme filter settings. The major problem is usually one of phase, and using
such extreme filter settings to overcome the problem will do no more than worsen the
situation, since further phase shifts may be introduced.
Other problems are usually speaker distortion at higher frequencies and an imbalance
between direct and reverberant fields. All of these possible causes should be checked
before using the graphic equaliser, which, although a powerful tool, cannot cure
everything.
6.2. Feedback suppression
In many auditoriums, a graphic equaliser may be used to increase the overall sound
level without feedback. The usual method is to set the graphic equaliser to a "flat"
position, (i.e. all sliders at 0dB) and to increase the overall level in the auditorium, until
feedback commences. Then reducing the level to just under the feedback point, each
frequency band in turn is boosted until the feedback frequencies have been identified. By
notching the offending bands down slightly, the overall level may be increased. This
may be repeated until the overall gain reaches the required level.
6.3. Graphic equalisers and limiter's
Generally, an equaliser should come before a limiter or compressor in an audio chain.
This is because an equaliser changes the frequency balance and harmonic
relationships of the signal. Therefore to obtain the desired effect of limiting, it is best to
limit the program material as you want it, i.e. equalised.
A graphic equaliser can be used in conjunction with a limiter/compressor to retain a
larger dynamic range by reducing unwanted frequencies which would have otherwise
triggered the limiter earlier.
Distortion can occur when low frequencies are limited too fast, this can be alleviated by
reducing the low frequency content.
By placing the equaliser first, if a range of frequencies are being attenuated, a higher
threshold would be attained before limiting commenced, as proportionately these