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loudspeakers is not justified. 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 films 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 specified 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 amplifiers 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 biamplified 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 biamplified 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