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DELAYING MONITORS LINE-UP & CALIBRATION
42
ITU 775
When using a 5.x setup with monitors arranged on a circle
line (as with ITU 775) the distance from the listener to each
of the main monitors is the same.
In case of limited space the three front monitors can be
arranged on a straight line. To compensate the center
monitor being closer to the listener than the left and right
monitor, the center signal can be delayed. To obtain
coincident arrival of the sound from all front monitors the
delay ranges normally between 0 and 3ms. dependent on
the off-set needed.
The speed of sound is approximately 340 m/s or 1130 ft./s.
Approximately 1 ms. of delay will be used per 1 ft. or 30cm,
when the monitor has to be "moved" backwards.
Cinema setup
In the setup for the cinema the front monitors are always
arranged on a straight line. In this case the audience is
placed closer to the surround monitors than the front
monitor. In order to retain the directional information
towards the screen the surround monitors are delayed.
Normally the Haas-effect is taken into account. In the
listening position the sound from the surround monitors will
arrive about 15-20ms after the sound from the front
monitors. The delay times involved with cinema processing
for the surround channel is dependent on the size of the
mixing facility, the cinema, or the home theatre/living room.
The procedure is to calculate the difference in distance
between front monitors and the closest surround monitor.
Then the time difference is calculated. To this is added 15-
20ms.
Example: From a listening position there are 20 meters to
the front monitors and 5 meters to the surround monitors.
The difference is (20-5) 15 meters.
The time is 15/340 sec, which equals .044 sec or 44ms. To
this is added 15ms. The result is that the surround signal
has to be delayed 59ms (or the closest possible).
Acoustic summation of audio
signals
The total sound level in the acoustic summation of two
sound sources, for example of two monitor monitors,
depends upon both the signal and the acoustics. The
sound sources can be correlated or uncorrelated.
The listening position (or measurement position) can be
either in the direct sound field or in the diffuse sound field.
In the direct sound field, there is only one sound direction.
This direct field exists either in the open, in a reflection-free
room or close to the monitors. The diffuse sound field
occurs in a room when you are so far away from the
monitors that the portion of direct sound is less than the
sum of all the reflections. The distance from the monitors
where the direct sound field and the diffuse sound field are
equally large is called the critical distance. In a control
room, it can typically be 1-3 meters. The near field in front
of the monitors can be regarded as a direct field.
Based on all monitors in phase, the sum of the sound
pressure from two monitors each playing at the same level
is 3 dB, unless the signals are correlated (they are
identical) and the sum is measured in the direct field. In
this situation the resulting sound pressure level is 6dB
higher than the level of the individual monitors.
Acoustic calibration of multi-channel systems
Calibration of the acoustic sound levels has been a
requirement for many years when working with sound for
film, although it has not been particularly common in other
branches of the sound industry. However, with the
widespread use of multi-channel formats for all forms of
music and film presentation in the home, it has turned out
to be beneficial to also have the acoustic levels under
control here.
It is important to differentiate between production for the
cinema and production for 5.1 channel reproduction in the
home based upon ITU 775.