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LINE-UP & CALIBRATION - BACKGROUND INFO
Calibration of 5.1 in an ITU-775 arrangement
In a 5.1 system based on the ITU arrangement, all main channels have in principle the same conditions: there is one
monitor per channel and each is placed the same distance from the listener. Internationally, there is however neither
agreement on the level nor on the bandwidth for the noise signals that is used for acoustic calibration. Pink noise is good
since it includes all frequencies, however it is impractical due to its "unsettled" character, which makes it difficult to
measure at low frequencies.
Surround Sound Forum
Surround Sound Forum is a German interest group composed of the VDT (sound technicians), the IRT and the SRT. The
SSF has prepared guidelines, which are generally accepted in Europe. Three test signals are specified in it, which are
recorded at -18 dBFS (RMS).
Signal
(only in one channel)
1 kHz sine
Pink noise 20 Hz-20 kHz
Pink noise 200 Hz-20 kHz
PPM level
t <0,1 ms
[dB]
-18
-9
-11
PPM level
t <10 ms
[dB]
-18
-13
-15
RMS level
[dB]
-18
-18
-20
Sound pressure
level
SLOW
[dB]
82
80
Sound pressure
level
SLOW
[dB(A)]
78
78
Measurement Signals Listening Level
SMPTE
The corresponding standard from SMPTE (RP155) uses a
standard of -20 dBFS for the reference level. Here, the C-
weighted sound pressure level of a 500 to 2 kHz noise
signal ends up at 83 dB.
Calibration of cinema systems
In a cinema, the listeners are sitting far from the monitors.
Presumably, the majority sits in the diffuse sound field. In
any event, attempts are made to establish a diffuse sound
field from the surround monitors. Hence when the sound
pressure is measured inside the cinema or in a mixing
theatre it must be averaged over many different
measurement locations. The typical basis for the majority of
the standards is at least four locations. If there are different
areas for the audience, for example main floor and balcony,
measurement should be made in at least four locations in
each. Before this measurement can be performed, the
system's frequency response must also be in order.
How to measure the sound
pressure?
Sound level meter or microphone
If you are used to perform acoustic measurements you
probably already own a sound level meter. This can be
used for the level calibration of the monitors.
If you do not have a sound level meter, there still is another
possibility:
Take an omni-directional microphone. (This is also called a
pressure microphone). A studio condenser microphone is
normally preferred but the microphone must not be too big.
Then a simple clip microphone can be used instead as it
does not influence the sound field. The microphone should
have a flat frequency response.
Place the microphone on a stand positioned at the listening
seat. Leave the microphone pointing upwards in order to
avoid sound pressure build-up in front of the diaphragm
and to provide equal conditions to monitors in all directions.
Connect the microphone to a mixing desk or a recording
machine that has a meter built in.
Play back a track containing pink noise on the CD you got
with the monitors. (or use one of the other tracks as
instructed to during setup and calibration.
Adjust the gain in the microphone channel until the meter
deflection reaches a point 10 dB below full scale. This is
now your reference for your measurements. Be sure that
you have marks on the scale that makes it possible to read
steps of 1 or 2 dB on either side of the reference
deflection. If 10 dB below full scale is not an adequate
point on your meter you may choose another one, just be
sure to have a reading on a adequate resolution.
If you use this microphone setup as described, it can be
used for the level comparison between the individual
monitors.
Absolute sound pressure levels can be measured if you:
a) are using a well calibrated microphone channel
b) know the sensitivity of the microphone.