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8 Subwoofers
- taken from tech note T N99051701 "setting the subwoofers", by C raig C onnelly
8.1
I nt r oduct ion
T he topic of setting subwoofer levels is the subject of numerous articles and even
more numerous opinions, but of only a single proposed standard (SMPT E RP 200, and
I T U 10-11R/T emp/11-E). T here are many reasons for this. First, the use of the modern
digital audio subwoofer is unique in comparison to the other channels and it is
recorded with a different monitoring reference. T he purpose of the subwoofer has
changed with the evolution of cinema sound, from compensating for the poor low
frequency response and bass power capability of older screen speakers, to adding
power to low frequency effects even in theatres having full range screen speakers and
capable amplifiers. Secondly, different post production facilities and even different
engineers have followed their own alignment conventions. Finally, equipment and even
meter standards are different when comparing the USA, Europe, and elsewhere. T his
wordy article is only an overview and is not meant to comprise a specific calibration
process or set of instructions.
8.2
S et t ing t he acoust ical r ef er ence level on t he dub st age.
A signal, generally wide band pink noise, is sent through the dubbing console to each
of the monitoring system' s screen speakers at the electrical reference level (also
known as Dolby Level). Using a real time third octave analyser, the monitor system is
adjusted for the preferred acoustical response at each loudspeaker, given the
properties of the loudspeaker, the room, the screen, and the "X-curve" of SMPT E
202M. T he electrical gain of each channel in the monitoring system is then adjusted to
give an acoustical reference level in the room from each screen speaker.
8.3
S et t ing t he subwoof er acoust ical gain using a scr een speaker as a
r ef er ence.
T he channel used for subwoofer or LFE (low frequency effects) in the dubbing theatre
is then adjusted as above, except that the SPL is measured differently. T o make this
adjustment properly, a multi-channel real time analyser must be used. Analyser bands
in the flat-response region of a screen speaker (generally, the centre speaker) are
taken as a reference. T hese bands will not individually measure 85dB SPL, but will be
somewhat less, depending on the bandwidth of the analyser' s bands, typically about
70 dB SPL for a 1/3-octave analyser as required by SMPT E 202M. T his flat-response
region, between the low frequency roll off caused by the loudspeaker cabinet and the
high frequency roll off due to the screen and X-curve, is referred to as the in-band
(acoustical) response of the monitor speaker. T he subwoofer electrical gain is adjusted
such that the analyser channels in its in-band region are 10dB greater than those in
the screen speaker' s in-band region. T his is referred to as "10dB of in -band gain"
(subwoofer level relative to each screen speaker level).
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8 Subwoofers
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