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CALIBRATING THE SUBWOOFER - with the main monitors
Bass Management
Bass management (sometimes called bass redirection), is
a very important and useful tool - and it is in general an
absolute necessity if you want to make a 5-channel setup
in a small room! There are several reasons for this.
The bass management system is designed to subtract the
bass contents of all main channels and reproduce this by
the use of a subwoofer. The AIR-series provides a number
of possible cross-over frequencies that can be determined
by the user.
Checking summing phase cancellation
In a basic 2-channel setup or 5-channel setup (like the ITU-
775) a sub-woofer is not specified and is not required. But
making audio production to be played back on consumer
equipment, one has to ensure that the mix will sound right
in this situation. Many consumer setups containing a
subwoofer. Hence it is very important to check what the
summing of the bass is like. If the program material
basically is out of phase, the bass will be cancelled when
summed electrically. But this is not the case if the bass
signals are reproduced by the main monitors and summed
acoustically. A dramatic difference may exist!
Controlling standing waves
It can be very difficult to get two monitors in the right
position in order to prevent the excitation of the standing
waves. But placing five monitors are even more difficult!
Each monitor should play under the same conditions as the
others but that is seldom the case.
Three front monitors can normally be placed away from
disturbing boundaries to prohibit the excitation of the
standing waves. But in small rooms the rear monitors very
often are placed in a corner or on a wall. Having the full
frequency range reproduced in a monitor like that, there
may occur serious problems which results in a poor low
frequency sound distribution in the room.
So this is another reason for using bass management.
Simply: Only one monitor - the subwoofer - has to be
placed in a safe position. The cross over frequency can be
selected to a value just above the problematic frequency
range.
To Calibrate
1 Main menu, set volume -30 dB
2 Main menu, select Bass Management: 80 Hz
3 Main menu, select Setup menu: Calibrate level
4 Select a task for the monitor
5 Run CD low frequency sweeps; tracks 40-42
6 If needed: Adjust level or crossover frequency
7 Store and Exit / get ready for music!
Bass management on
The volume is set to a reasonable level (first item in main
menu) ex -30 dB.
In the main menu Bass Management is selected. If no
other preferences for a crossover frequency 80 Hz is
selected.
If measurements or listening to the main monitors have
indicated uneven level distribution or uneven frequency
response at frequencies above 80 Hz, a higher crossover
frequency should be selected.
Checking the Crosspoint
In order to confirm the selected crossover frequency, the
CD is used. To be sure to listen to one monitor channel at
a time one of the main channels are selected in the Setup
menu/Calibrate monitor menu. For a start Master Left can
be selected.
Playback the CD track containing the low frequency
sweeps (trax 31 to 42). Listen to the response or perform
sound level measurement if you have a meter connected.
If you have a spectrum analyzer you can instead play the
CD-tracks containing full bandwidth pink noise.
Any uneven frequency response across the crossover
frequency should be avoided.
You may go back to the setting of the crossover frequency
in the bass management setup and select another
frequency if appropriate.
After doing the adjustment on one channel, the other
channels should be monitored one by on and finally all
channels at one time.
Now your system is calibrated.
Read more in the section: Phase Adjustment on the
following page.
Three other parameters to consider in relation to the
subwoofer:
LFE Low Pass Filter
Switches On or Off a very steep (7th order) low pass filter
at 120 Hz. When switched On, the LFE signal is filtered for
content above 120 Hz. Switch this filter On when a media
encoder is not in use and switch it Off when your signal
have been through a media encoder / decoder (Most media
encoders - DTS, Dolby etc. - employees an anti-aliasing
low-pass filter. Therefore in a production environment it is
important use this filter when monitoring the LFE channel in
order to access how the signal will sound after encoding /
decoding (in a domestic or cinema environment).