Specifications

13
6. Equalization of a Sound Reinforcement System
It will be assumed that the necessary equalizer filters are in place in the amplifier chain,
and that the sound system is operational and ready for final adjustment. The procedure
for using the Model 140 as an aid to the equalization process may use either a single
microphone as the room sound level pickup, or up to three microphones with the
"MIC
PLEXOR"
accessory. Using a single microphone, it must either be a dynamic
microphone or else suitably buffered to work into a typical dynamic mic input. Usually
the mic will be one of the job units so that the mic characteristics are taken into account
in the equalization procedure.
The mic is suitably placed in the room and connected to the
"Microphone"
Input
through cable of in-place wiring.
SET THE MASTER GAIN CONTROL OF THE
REINFORCEMENT SYSTEM TO ZERO GAIN.
Then the pink noise output is fed
into the mixer or into a suitable input to energize the reinforcement system and the room
with pink noise. With the
"dB-SPL"
control set at 70 or 80, and with the pink noise
output control at either high or low level as required, begin raising the noise level pot and
the master gain control until the
SPL
column of the Model 140 is reading 80 to 90dB-
SPL, as experience may indicate. Beginning with the
"Display Range"
switch set at
20dB, adjust the
"Display"
control until the 1/3 octave display is suitably centered on the
raster. The maximum readings should be near the top of the
LED
raster.
The broadband
SPL
display can be interpreted as
dB-SPL
by adding the
"dB-SPL"
control setting to the dB indicated on the
LED
display, and making any correction for the
calibration of the mic if it differs from the reference value.
The readings of individual 1/3 octave channels will additionally require that the readings
of the
"Display"
control be
SUBTRACTED
from the sum of the other readings to
convert the indications to
dB-SPL.
However, in equalization it is the relative values of
the 1/3 octave display that are wanted and absolute readings are not usually needed.
If some of the channels are so low in output that they fall below the display, up to 20dB
may be added by the
"dB-SPL"
control to move the display up by the indicated amounts.
In this vay, lower level readings come into view and may be read off. The
"Display"
control, being calibrated, may also be used for this purpose if careful account is taken of
the changes in gain settings.