User manual JBL CINEMA SOUND SET UP

[. . . ] CINEMA SOUND SYSTEM MANUAL January, 1998 JBL CINEMA SOUND SYSTEM MANUAL Table of Contents . 2 I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] In older cinemas with balconies, such reflections were often generated by the balcony front (or fascia) itself. Substantial acoustical damping had to be placed on that surface in order to diminish the problem. In most cinemas constructed today, echo problems can generally be dealt with by ensuring that the back wall is very absorptive and that substantial damping is installed behind the screen on the baffle adjacent to the loudspeakers. C. The Role of the Acoustical Consultant An acoustical consultant should be chosen on the basis of previous jobs well done. There is much that is learned simply by having encountered- and solved - many problems. Stating it another way, an experienced consultant has probably seen most of the common mistakes and knows how to spot them before they become problems. While much of what a consultant does may seem obvious, and even simple, it is the breadth of experience that qualifies a good consultant to take on a difficult task and succeed at it. In addition to the points discussed so far in this section, the consultant will look for potential difficulties in the following areas: 1. When acoustical isolation has been addressed in wall construction, flanking paths through, or around, the wall may become significant. For example, sound often leaks through electrical or air conditioning conduit, even though the wall itself may act as a good barrier to sou, nd transmission. Such paths can crop up in many places and need to be identified early in the construction phase of the project. Many building contractors routinely take shortcuts, and somebody needs to watch them carefully. The acoustical isolation of double wall construction can be nullified by the presence of material left between them bridging the air barrier between the two sections. These are some of the most difficult problems to fix, since they are literally ` built in. 'Plumbing noises, elevator motors, and air handling machinery located on the roof are just a few of the offenders here. Once the installation has been made, the problem is very expensive to correct, and a good consultant will have an eye out for such things at the design stage of the project. Related problems, such as projector noise and other noises associated with concession activities need to be identified early in the project and corrected before construction begins. As standards for film exhibition continue to improve, such points as we have raised here will become more important. In a 1992 monograph (5), loan Allen of Dolby Laboratories stressed the need for noise ratings in the cinema lower than NC-25, with NC-30 representing the worst acceptable case. Room Size In all but the smallest cinemas, dual low-frequency systems, such as the JBL 4670D and the 4675C, should be specified. Normally, there will be three of the systems behind the screen in left, center, and right positions. The 4670D has the Flat-Front Bi-Radial236OA horn, while the 467% has the large 2360A Bi-Radial horn. The differences in performance are basically high-frequency vertical pattern control in the range from 500 to 1000 Hz. [. . . ] Figure 34A and B shows typical HF and LF response curves for electronic dividing networks used for cinema applications. The curves shown at A have 12dB/octave slopes with HF power response equalization for 2360 series uniform coverage horns. Curves shown at B are for 16 dB/octave slopes. -30 2 3 4 5 676910 tm 15 2 3 4 5 676910 15 2 r. aa 3 4 5 676910 15 lO. Dx 2 2 3 1111111 4 5 676910 1CQ 1. 5 2 3 4 5 671910 I5 I. 066 2 FRQMlcym I1111111 3 4 5 676910 lO. CCQ 15 2 FFZCUENCVW A. with power reqonse correction for 236OA Series horns 8. 500HzlEd~~, withpcmnreeponcleanreQionfor238044 S3ries hams Figure 34. [. . . ]