JBL CINEMA SOUND SET UP Manual

C. The Role of the Acoustical ConsultantAn acoustical consultant should be chosen on the basis of previous jobs well done. There is d,much that is learned simply by having encountered- and solved - many problems. Stating it anotherway, an experienced consultant has probably seen most of the common mistakes and knows how tospot 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 potentialdifficulties in the following areas:11. Flankina leakage Daths. When acoustical isolation has been addressed in wall construction,flanking paths through, or around, the wall may become significant. For example, sound often leaksthrough electrical or air conditioning conduit, even though the wall itself may act as a good barrier tosou,nd transmission. Such paths can crop up in many places and need to be identified early in theconstruction phase of the project.2. lntearitv in construction. Many building contractors routinely take shortcuts, and somebody needs towatch them carefully. The acoustical isolation of double wall construction can be nullified by thepresence of material left between them bridging the air barrier between the two sections.3. lmoact and structure-borne noise. These are some of the most difficult problems to fix, since theyare literally ‘built in.’Plumbing noises, elevator motors, and air handling machinery located on the roofare just a few of the offenders here. Once the installation has been made, the problem is veryexpensive to correct, and a good consultant will have an eye out for such things at the design stage ofthe project. Related problems, such as projector noise and other noises associated with concessionactivities 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 willbecome more important. In a 1992 monograph (5), loan Allen of Dolby Laboratories stressed the needfor noise ratings in the cinema lower than NC-25, with NC-30 representing the worst acceptable case.IV. SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERSA. Hardware Class vs. Room SizeIn all but the smallest cinemas, dual low-frequency systems, such as the JBL 4670D and the4675C, 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% hasthe large 2360A Bi-Radial horn. The differences in performance are basically high-frequency verticalpattern control in the range from 500 to 1000 Hz. Whenever possible, the 4675C systems should bespecified, but there are situations where space behind the screen is limited, and the smaller horn mustbe used.