User manual JBL CINEMA SOUND SET UP

[. . . ] CINEMA SOUND SYSTEM MANUAL January, 1998 JBL CINEMA SOUND SYSTEM MANUAL Table of Contents . 2 I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] Discrete reflections are likely to be a problem only if they clearly are displaced from the direct sound in both time and spatial orientation. Side wall reflections are usually perceived by the listener well within a time interval which does not allow them to be heard-as such. However, a reflection off the back wall can rebound from the screen itself, creating a ` round trip' echo that may be delayed by as much as 100 milliseconds. The effect here is to render dialog difficult to understand. 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. [. . . ] Tvpical HF and LF response curves for active frequency dividing networks F. System Setup and Checkout Tfie vast majority of system performance problems can be avoided through proper design procedures and proper assembly. If all has gone well, the system will work, and the field crew can proceed with final calibration and equalization of the system. When a loudspeaker has been assembled, either in the shop or in the field, it should be tested with as oscillator-amplifier combination to ensure that there are no buzzes or rattles. [. . . ]