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Configure the Equalisers
Sensible correction of the in-room response requires the ability to measure
the acoustic frequency response of each speaker individually. The
measurement microphone does not need to be expensive and suitable
electret microphones are widely available. If the appropriate measuring
equipment is not available, we recommend that the parametric filters be
bypassed by setting switch S7 to 0. Setting of the low- and high-frequency
contour controls may be attempted by ear to the user's preference.
Correcting for room resonances requires thought, skill and patience. It is
not sensible to attempt to fully equalise response dips, as this may lead to
system overload. Fortunately, dips are less objectionable than peaks. For
this reason, each parametric filter allows just 3dB of boost, but 6dB of
cut.
It is also not advisable to make corrections to a single position
measurement. The response of the speaker/room combination can differ
considerably throughout the room and it is all too easy to worsen the
response in one position whilst making it better in another. An average of
several measurements taken at different positions throughout the room
should ideally be used to ensure an overall improvement in all listener
positions.
The Method
We suggest taking measurements at each seat position and at several
points outside the immediate listening area. Point the microphone towards
the speaker being measured.
The resonances change with height. Often seats will be at the same level,
so put the microphone at a typical ear height. If the home theatre room is
like a mini cinema with regular rows of raked seating, measure at each
seat at the likely ear height. Do not hand hold the microphone during
measurements. Use a tripod or other means of holding it still. It is advisable
to make the last measuring position n (the one used for subsequent
equalised measurements) the primary listening position.
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Procedure
To avoid errors from airbourne background noise and electrical
interference, switch off all sources in the room such as air conditioning,
refrigerators etc.
Laptop computer settings:
Run the laptop from its battery and disconnect its transformer power
supply when taking measurements. This will prevent noise being
generated by electrical grounding mismatches between the laptop,
sound card and CT8 XO. Make sure the battery is fully charged before
starting the measuring procedure.
Open the "Sounds and Audio Devices" window in "Control Panel".
Click on the "Sounds" tab and select the "No Sound" option in the
"Sound Scheme" drop-down menu.
Click on the "Volume" button in the "Sound Playback" section and
make sure the Microphone "Mute" box is checked to prevent
feedback.
Click on the "Audio" tab and then on the "Volume" button in the "Sound
Recording" section. Set the volume slider to maximum and make sure
the "Select" box is checked.
Select the test input by switching the input switch S1 to "Test".
Setting switch S7 to "None" to disable the parametric equalisers and set
the LF and HF contour controls, C1 and C2, set to zero.
Connect the output of the sound card to the front test input socket of the
CT8 XO and the microphone to the input of the sound card.
Open WinMLS2004
From the menu, select Setup...Load... and load the setup called "B&W
CT800" from the "Sound System" folder.
Click the button to open the "Volume and Input Levels" window.
Figure 22
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