8 Subwoofer s
SDDS Print Master Guidelines, Page 24 of 26
of its low pass filter, or even eliminate the s ynthes i z ed s i gnal altogether, depending on
the capabilities of the s cr een loudspeakers and the audi tor i um’s acoustical response.
8.10
Subwoof er polarity.
As a s i de note, it is often difficult to deter mi ne the correct polarity of the s ubwoofer ,
digital or analogue. Even if pink noise i s sent to the s ubwoofer and centr e channels
simultaneously, in many cases there will be no appar ent difference i n combined wi de
band S PL measurement when switching s ubwoofer polarity. In the end the bes t
polarity may come down to an aesthetic j udgement on the par t of the ci nema
technician, based on listening to actual films.
8.11
R ule-of-thumb met hods.
Measuring the l evel of a s ubwoofer with a wi de-band S PL meter is of questionable
value except as a r ul e- of - thumb method to r e-check a t heat r e that has already been
properly calibrated. The r eas ons for this include the fact that the meas ur ed acous ti cal
output of a s ubwoofer (or any loudspeaker system) depends on the bandwi dth of the
signal ultimately being meas ur ed. That will be affected by the bandwi dth of the i nput
test signal, any equalizer or low pass filter settings, the audi tor i um’s response, the
loudspeaker cabinet response, and the accur acy of the C wei gh t i n g r es pons e at low
frequencies of the S PL meter used for the meas ur ement. A s ubwoofer signal with a
wider bandwidth may measure the s ame as another with a narrower bandwidth but
higher level (as indicated by analyser bands within the pass band of the s ubwoofer ),
using the s ame s peaker cabinet. Considering the DF P-D3000, the bandwi dth of the
pink noise s ent to the s ubwoofer is affected by a l ow pass filter that can be adj us ted
from 80Hz to 330Hz—a di fference of three octaves or eight times in acoustical energy.
No s tandar ds exist for such wide band S PL measurements of subwoofer s .
Although the meas ur ed S PL of wide band pi nk noise thr ough the s ubwoofer will
change if the s ubwoofer ’s low pass filter is adjusted, this will not change the actual
playback level of the di gi tal LFE channel from film. So long as the filter frequency is
not set too low, the s i gnals recorded on the dubbi ng s tage will determine what is
heard i n the ci nema when the pr oper in-band gai n difference i s established
acoustically, as previously described. This is why an engineer who us es a wi de- band
SPL meter to s et the di gi tal subwoofer level is engaging in self-deception. In general, a
low pass filter setting for the di gi tal subwoofer (LFE) of 125 Hz, 160 Hz, or even
200 Hz should s er ve i n most installations, and all settings should s ound the s ame when
playing digi tal multichannel material. A l ow pass filter setting of 125 Hz will give a wi de
band s ubwoofer SPL pink noise meas ur ement of appr ox i mat el y 91 dBc. T his rule-of-
thumb r es ul t is only appr oxi mate and s houl d not be us ed for the pr i mar y alignment of
a t heat r e.
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