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D. Mechanical Details of JBL Screen Loudspeaker Systems:
Most JBL screen channel systems are intended for field assembly, and instructions for their assembly are
contained in the shipment. Here, we will show assembled and dimensioned line drawings in front and side views
so that the user can determine the actual space requirements behind the screen. The height from base to the
center of the HF or MF horn is also given as a guide to determining vertical positioning so that the horn will be
approximately at 2/3 the height of the screen.
Figure 13A and B shows front and side views, of the 3622N and 3678 systems. The model 4670D is
shown in Figure 14. Figure 15 shows front and side views of the 4622N passive system. The model 4675C is
shown in Figure 16. Figures 17, 18, and 19 show the large three-way models 5671, 5672, and 5674, respectively.
Figure 27 shows the 4632 and 3632 ScreenArray models.
E. Subwoofers:
Subwoofers are an integral part of cinema loudspeaker systems installed in mid and large size houses. In
specifying them, the designer must take into account the reduced sensitivity of the ear to low frequency sounds.
Figure 20 shows the Robinson-Dadson equal loudness contours. Note that, for a reference level of 85 dB at 1 kHz,
frequencies in the range of 30 to 40 Hz will have to be reproduced 15 to 20 dB louder in order to be perceived at
the same subjective level.
Because of the relative non-directionality of very low frequencies, the number of subwoofers specified
for a given cinema must bear some relationship to the total volume in the space. JBL’s experience in specifying
subwoofer units has led us to the following empirical recommendations:
1. For each 5,666 cubic meters (25,000 cu ft), specify one 2241 or 2242 transducer in a suitable enclosure.
2. All systems should be mounted in very close proximity to one another in order to maximize mutual coupling
among them.
3. All subwoofer systems should be mounted at the base of the screen, preferably hard up against the screen
wall and the floor. This boundary condition is known as a “quarter-space,” or “1-pi“ mounting, and this will
further increase the output of the subwoofers at very low frequencies. When specified in this manner, the entire
subwoofer array should be capable of generating low frequency sound pressure levels in the range of 110 to 115
dB in the 40 Hz range. (See mounting details in Section V-C.)
Figure 21 shows mechanical views of the JBL 4645C. Each subwoofer unit should be driven with its own
amplifier capable of producing up to 800 continuous watts of sine wave power into a rated impedance of 8 ohms.
Figure 22 shows views of the model 4641 subwoofer, which has a single 2241H driver rated at 600 watts
continuous. A pair of subwoofer modules can be driven by a single stereo amplifier that is capable of producing
continuous sine wave power of 600 watts into each of two 8-ohm loads. The model 4642A is shown in Figure 23
and contains a pair of 2241H drivers, each brought out to its own pair of terminals.
Figure 21: Front and side drawings of
4645C subwoofer
Figure 22: Front and side drawings of
4641 subwoofer
