L-Acoustics 112XT, Руководство оператора

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L-ACOUSTICS XT Manual V1.1 2/11/2003
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All 3-way XT presets have been optimized to provide a 6 dB low frequency contour when
subwoofers are used with 112XT or 115XT enclosures in a 1:1 XT:subwoofer ratio. In order to
compensate for sensitivity differences between subwoofer models, relative sub, low and high output
channel gains have been adjusted for individual presets so that the same 6 dB low frequency contour is
obtained, regardless of the subwoofer model in use.
4.8.1 Combining XT With Subwoofers
Two cases can be considered: ground stacked systems where XT and subwoofer enclosures are
closely coupled physically or separate flown XT loudspeakers with ground stacked subwoofers. For
both cases, processing will depend on the intended purpose of the subwoofers, i.e., in some
applications, subwoofers are used as an effect and are not driven with the same signal as the main XT
system (separate auxiliary send from the console) while in other cases, the subwoofers are used as a
low frequency extension of the array and are driven with the same signal in 3-way mode.
In general:
3W Presets have a 100 Hz 24 dB/octave Linkwitz-Riley (LR24) low pass filter for the subwoofers with
a complimentary 100 Hz LR24 high pass filter for the 112XT or 115XT low section. Complimentary
100 Hz crossover filtering optimizes the power bandwidth for the subwoofer and the 112XT or
115XT low section. 3W presets are recommended for all stacked XT/subwoofer applications where
the enclosures are closely coupled physically.
3WX presets have an 80 Hz LR24 low pass filter for the subwoofers and an overlapping LR24 high
pass filter for the XT low section (45 Hz HPF for 112XT, 40 Hz HPF for 115XT). Due to the overlap
in the operating bandwidths of the sub and low sections, subwoofer polarity may have to be inverted
due to the phase shift of overlapping crossover filters (depending on the selected preset and how the
subwoofers are driven, i.e., via AUX send or with the same signal as sent to the XT system)
4.8.2 Ground Stacked Systems
For ground stacked systems, XT and subwoofer enclosures are physically close to each other and time
alignment is simplified throughout the audience area since there are no arrival time (path length)
differences between the flown XT system versus ground stacked subwoofers. Typically, 3W presets
are used for ground stacked configurations in order to optimize power bandwidth for the subwoofer
and XT low section and the overall system is driven in either 3-way mode or with separate AUX drive
(although 3WX presets can also be used).
4.8.3 Separate Flown XT Array with Ground Stacked Subwoofers
For a separate flown XT array with ground stacked subwoofers, time alignment of subwoofers is
required due to the geometric path difference between the two systems. This is illustrated in Figure
17, where the distance from the measurement microphone position to the subwoofers is d
SUB
while
the distance to the flown XT system: d
FLOWN
= d
SUB
+ PATH DIFFERENCE. Delaying the subwoofers
by the geometric path difference will time align the subwoofers at the reference position.
Note: Selection of the reference position for time alignment is always a compromise since the geometric
path difference will vary with position.