To Calibrate, Controlling standing waves

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

1Main menu, set volume -30 dB

2Main menu, select Bass Management: 80 Hz

3Main menu, select Setup menu: Calibrate level

4Select a task for the monitor

5Run CD low frequency sweeps; tracks 40-42

6If needed: Adjust level or crossover frequency

7Store 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).