T he meas ur ement is made acous ti cally, not electrically. T ypically, each band of the
analyser in the pass band of the s ubwoofer will then measure about 80 dB SPL. If a
screen speaker and the L FE subwoofer loudspeaker are each measured wi th a wide
band SPL meter, the s ubwoofer will typically measure appr ox i matel y 5.5 dB higher.
8.4
W hy we do it this way.
T he r eas on for lowering the electr i cally recorded l evel of the s ubwoofer channel and
making i t up by turning up the pl ayback gain dates back to 70mm film. T he s ubwoofer
recording l evel was lowered to pr event saturation of the magneti c t r ack and the
playback gain was increased to compens ate. The loss of signal to nois e per for mance
was inconspicuous because the s ubwoofer signal was sent through a l ow pass filter
and did not reproduce hi ss. This level difference convention has been retained in the
digital world, where i t serves to give addi ti onal effective headr oom for the pl ayback of
low fr equency sound effects through the s ubwoofer loudspeaker.
8.5
W hat it means.
T he cons equence of having 10dB more acoustical gain in the s ubwoofer monitoring
channel is that the dubbing engineer will tend to turn down the electrical recording
level of the s ubwoofer by about 10dB compared to what she woul d have done wi thout
the i ncr eas ed moni tor gain. However, unlike the s cr een speaker channels, the engi neer
does not use a cons ol e meter to check the el ectr i cal recording l evel of the s ubwoofer
(except perhaps to guar d agai ns t overload condi ti ons ).
8.6
Setting t he digit al screen speakers in the cinema.
In the cinema, the s cr een speakers are adj us ted i n the almos t same manner as they
were on the dubbi ng s tage. This calibration, which encompasses the ci nema
processor’s main fader, equalizers, power amplifiers, crossovers, loudspeakers, screen,
and r oom respons e, is referred to as the B- chain alignment. Consider the di gitally
driven loudspeaker alignment first, as it is the mos t important. An electrical reference
signal, generally wide band pi nk noise gener ated wi thi n the ci nema pr ocessor at a
specific l evel , is used to align each of the s cr een speakers to gi ve the des i r ed r es pons e
(the X- cur ve or other reference r es pons e) and acoustical reference level for each
individual speaker (again, 85 dBc, slow r es pons e, measured wi th a wi de- band S PL
meter). This procedure ties the ci nema pr ocessor’s electrical reference to an acoustical
reference in the cinema, similar to what was done on the dubbi ng s tage.
