JBL Motion Picture Loudspeaker Systems Instruction Manual

Instruction ManualA. Introduction:Although the company's roots extend backto the early days of motion picture sound, ithas only been in the last few years that JBLhas made a significant impact on the commercialtheater market. Some of the reasons for this arethe company's preeminence in high-quality soundreinforcement in general and the development ofthe notion of flat power response in particular.In just the last year, JBL theater systems havebeen incorporated into some of the most presti-geous dubbing theaters and screening rooms inthe film industry. We now feel that the time isright to prepare a comprehensive applicationsmanual for our theater dealers to aid them insystem design and specification and to give theman edge over their competitors.B. Systems for Smaller Houses:1. A summary of house types:By far, most of the theaters being builttoday are long rectangular rooms. They areusually built in groups called multiplextheaters for reasons of economy. Most ofthese theaters are mono, with a singleloudspeaker behind the screen, and theseating capacity usually ranges from 200to 500. In general, the larger houses, thoseseating 1000 or more, were built some yearsago, and these are usually equipped withthree or five loudspeakers behind the screenand will have a multi-speaker surroundchannel located on the rear and side walls.A very few houses, usually found only inlarger cities, will have special effectslow-frequency channels, such as Sensurround,for special screenings of important films.Motion PictureLoudspeaker Systems2. General Acoustical Characteristics:Historically, motion picture theaters,whatever their size, have been acoustically"dead" rooms; that is, they have consider-able absorption on the boundaries, and rever-beration times, even in the larger houses,rarely exceed 1.25 seconds. More likely, wewill find the reverberation times in smallerhouses to be in the 0.5 second range. Thereason for this of course has to do with arti-culation and clarity of dialog.The most common method of treating theinterior of theaters is with velour draping.This is an interior treatment only; structurally,the architect relies on solid block walls tominimize sound leakage from adjacent theaters.One of the most common problems encoun-tered in theaters is a slap or reflection offthe back wall, which often goes untreated.Should the back wall be curved, then theeffect may be even worse, due to focussing ofthe reflection back to the front.3. Sound Level Requirements:Average peak levels of 85 dB-SPL are commonin the theater. A minimum headroom figureabove this would be 10 dB, but JBL recommends15 dB for a greater measure of safety. Thefollowing factors are taken into considerationin determining the kind of system to specifyand the amount of amplifier power which willbe required:a. Room volumeb. Room boundary areac. Reverberation timed. System sensitivity and powerratinge. System directivity index