JBL Cinema 300 - Page 13

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loudspeakers is not justied. We will therefore use inverse square calculations in determining loudspeaker levels in
the cinema. Because of their frequency range and low directivity, subwoofer calculations can be made taking into
account total room volume.
Guidelines for digital lms state that full modulation of a digital sound track should be capable of
producing, per screen channel, peak levels of 105 dB SPL at a distance approximately two-thirds the depth of the
house. The following table gives average dimensions for houses of various seating capacities:
We will now consider the peak output capability of the 3000 and 4000 Series screen loudspeakers in
these four environments:
When all three screen channels are running at full modulation, the loudspeaker output level will be
approximately 5 dB greater than for a single channel. As we can see from Table 1B, the models 3622N and 3678
can deliver the required total output in rooms up to 50,000 cubic feet (200 to 250 patrons) when driven with rated
power, but should not be specied for larger spaces.
Both 4670D and 4675C models can handle the largest spaces listed here with no problem at all. In fact,
a single screen channel is capable of delivering 105 dB at the required distance in the 250-seat house. JBL
recommends driving these models with ampliers equal to their rated power, regardless of the application.
We will now consider the maximum output capability of the 4675C-4(8)LF system in biamplied mode,
taking into account the 6 dB crest factor of the test signal. This is the absolute peak output capability of the
system:
JBL recommends that the 4675C-4(8)LF system be used in biamplied mode in cinemas seating 500 or
more patrons.
Table 1A. Seating Capacity and Room Dimensions:
Seating: Room Volume: Two-thirds Distance: (front-to-back)
75-125 695 m
3
(25,000 cu ft) 12.0 meters (40 feet)
125-250 1390 m
3
(50,000 cu ft) 14.2 meters (47 feet)
250-500 2780 m
3
(100,000 cu ft) 16.4 meters (54 feet)
500-1000 5560 m
3
(200,000 cu ft) 18.0 meters (60 feet)
Table 1B. Output Capabilities of 3000 and 4000 Series Loudspeakers:
Model: Sensitivity: (1W@1m) Rated Power: Maximum continuous level (dB) at:
1 m: 12 m: 14.2 m: 16.4 m: 18 m:
3622N 101 dB 400 W 127 105.4 104 103 102
3632 104 dB 500 W 131 109.4 108 107 106
3678 98 dB 300 W 123 101.4 100 99 98
4622 101 dB 600 W 129 107.4 106 105 104
4632 106 dB 800 W 135 113.4 112 111 110
4670D 100 dB 600 W 128 106.4 105 104 103
4675C 100 dB 600 W 128 106.4 105 104 103
Table 1C. Output capability of the 4675C-4(8)LF:
Sensitivity (1W@1 m Rated Power Maximum level (dB) at:
1 m: 12 m: 14.2 m: 16.4 m: 18 m:
100 dB 1200 W (LF) 130.8 109 107.7 106.5 105.7