8 Subwoofers
SDDS Print Master Guidelines, Page
22
of
26
40
63
100
160
250 400 630
1K 1.6K 2.5K 4K 6.3K 10K 16K
Center channel loudspeaker response
40
63
100
160
250 400 630
1K 1.6K 2.5K 4K 6.3K 10K 16K
Digital subwoofer (LFE) channel response
10dB higher SPL
T he measurement is made acoustically, not electrically. T ypically, each band of the
analyser in the pass band of the subwoofer will then measure about 80 dB SPL. I f a
screen speaker and the LFE subwoofer loudspeaker are each measured with a
wide
band
SPL meter, the subwoofer will typically measure
approximately
5.5 dB higher.
8.4
W hy we do it this way.
T he reason for lowering the electrically recorded level of the subwoofer channel and
making it up by turning up the playback gain dates back to 70mm film. T he subwoofer
recording level was lowered to prevent saturation of the magnetic track and the
playback gain was increased to compensate. T he loss of signal to noise performance
was inconspicuous because the subwoofer signal was sent through a low pass filter
and did not reproduce hiss. T his level difference convention has been retained in the
digital world, where it serves to give additional effective headroom for the playback of
low frequency sound effects through the subwoofer loudspeaker.
8.5
W hat it means.
T he consequence of having 10dB more
acoustical
gain in the subwoofer monitoring
channel is that the dubbing engineer will tend to turn down the
electrical
recording
level of the subwoofer by about 10dB compared to what she would have done without
the increased monitor gain. However, unlike the screen speaker channels, the engineer
does not use a console meter to check the electrical recording level of the subwoofer
(except perhaps to guard against overload conditions).
8.6
Setting the digital screen speakers in the cinema.
I n the cinema, the screen speakers are adjusted in the almost same manner as they
were on the dubbing stage. T his calibration, which encompasses the cinema
processor’s main fader, equalizers, power amplifiers, crossovers, loudspeakers, screen,
and room response, is referred to as the B-chain alignment. Consider the digitally
driven loudspeaker alignment first, as it is the most important. An
electrical reference
signal, generally wide band pink noise generated within the cinema processor at a
specific level, is used to align each of the screen speakers to give the desired response
(the X-curve or other reference response) and
acoustical reference
level for each
individual speaker (again, 85 dBc, slow response, measured with a wide-band SPL
meter). T his procedure ties the cinema processor’s
electrical reference
to an
acoustical
reference
in the cinema, similar to what was done on the dubbing stage.
