User manual JBL CINEMA SOUND SET UP

CINEMA SOUND SYSTEM MANUAL January, 1998 JBL CINEMA SOUND SYSTEM MANUAL Table of Contents .2 I. INTRODUCTION.. .................................................................................................. .2 II. BASIC SYSTEM CONCEPTS.. .............................................................................. .2 A. Analog Film Formats.. ................................................................................ .4 B. Digital Film Formats ................................................................................... .5 C. A- and B-chains ......................................................................................... D. Evolving Dynamic Range Requirements in the Cinema.. ........................... .7 E. Integration of Loudspeakers into the Acoustical Environment ..................... 7 F. Power Response and Power-Flat Systems ................................................ .9 G. Coverage Requirements for Proper Stereo Reproduction .......................... 10 .12 III. ACOUSTICAL CONSIDERATIONS ...................................................................... A. Noise Criterion (NC) Requirements.. .......................................................... .12 B. Control of Reverberation and Discrete Reflections .................................... .13 C. The Role of the Acoustical Consultant.. ..................................................... .15 IV. SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERS.. ............... .15 A. Hardware Class vs. Room Size.. ................................................................ .15 17 B. Advantages of Biamplification ..................................................................... .17 C. Cinema Playback Level Calibration.. .......................................................... .18 D. New JBL Driver Developments .................................................................. E. Mechanical Details of JBL Screen Loudspeaker Systems ......................... .18 .26 F. Subwoofers ................................................................................................ .29 G. Surround Requirements.. ........................................................................... .30 H. Screen Losses.. ......................................................................................... I. Use of Multiple High Frequency Elements.. ................................................. .31 .31 V. MOUNTING REQUIREMENTS.. ............................................................................ 31 A. General Comments ..................................................................................... B. Platform and Baffle Construction.. .............................................................. .31 .32 C. Subwoofer Mounting.. ............................................................................... .33 Mounting ................................................................................... D . Surround .35 VI. ELECTRICAL INTERFACE .................................................................................. A. Wiring for Non-biamplified Installations.. ................................................... .35 B. Wiring Diagram for a Biamplified Installation.. ............................................ .35 .37 C. Wiring for Surround Channels.. .................................................................. D. Wire Gauges and Line Loss Calculations .................................................. .38 E. Dividing Network Characteristics.. .............................................................. .38 .39 F. System Setup and Checkout.. .................................................................... .41 References.. ............................................................................................................... page 1 I. INTRODUCTION The decade of the 1980' saw many improvements in the quality of cinema sound. Dolby s Laboratories had begun the cinema sound revolution during the middle 1970' with the introduction of s noise reduction and equalization of cinema loudspeaker systems. In 1981, JBL demonstrated the first flat power response loudspeaker systems at the Academy of Motion Picture Arts and Sciences. In 1983, Lucasfilm introduced the THX@ system, along with their program of cinema certification. As the 1980' progressed, Dolby stereo optical sound tracks gained in favor, increasing the number of stereo s houses significantly. The application of Dolby Spectral Recording (SR) to cinema release prints represented another step forward in sound quality. By the mid 199Os, three digital systems had been introduced into the cinema, Dolby SR-D. Digital Theater Sound (DTS), and Sony Dynamic Digital Sound (SDDS). These systems have similar digital performance characteristics, and they all provide analog stereo optical tracks for overall compatibility and operational redundancy, should the digital portion of the system fail, or momentarily go into a mute mode. DTS makes use of a synchronized CD-ROM for its digital program, while the other two include the digital information on the print itself. As new cinema complexes are being pianned and constructed, acoustical engineers are now more than ever before being engaged to deal with problems of architectural acoustics and sound isolation between adjacent exhibition spaces. More attention is being paid to the specification of sound equipment and its careful integration into the cinema environment. JBL has a strong commitment to the cinema sound market. We have become the acknowledged leader in the field, and our products are routinely specified for major studios and postproduction houses throughout the world. JBL continues its rapid pace in new product development aimed at increasing performance levels in the cinema. This manual has several goals. First, it will provide a background in basic systems concepts, and then move on to acoustical considerations in the cinema. The subject of electroacoustical specification will be discussed, as will the problems of mounting and aiming of the components. Electrical interface and system checkout will be covered in detail. JBL believes that the more dealers and installers know about the basics of sound in the cinema, the better will be the results of their work in all areas. II. BASIC SYSTEM CONCEPTS A. Analog Film Formats There are two film sizes for theatrical exhibition: 35 mm and 70 mm. The most common projection image aspect ratios (horizontal vs. vertical) for 35 mm can be either 1.851 (" ) or 2.35:1 flat" (" scope" Seventy mm prints are normally projected at a ratio of 2.2:1. The advantages of 70 mm ). have, in the past, been the availability of six magnetic tracks and large image area. The cost of a 70 mm print is quite high, and these prints have normally been made in limited quantities for exhibition in premier houses in large metropolitan locations. Today, the general practice with 70 mm is to use three channels behind the screen (left, center, and right) and a single surround channel feeding multiple page 2 loudspeakers. Options are to use the two remaining magnetic tracks for subwoofer signals and/or split (dual channel) surrounds. The 35 mm format was modified during the 1950' to handle four magnetic tracks: three screen s channels and a single surround channel. At the same time, the standard monophonic variable area optical track was maintained. Figures IA and B show the channel layout for both 70 mm and 35 mm magnetic standards. At present, the 35 mm magnetic standard is no longer in general use. A. 70 m m 0.35 m m A I ' MAGNETiC STRIPING Figure 1. 70mm six-track magnetic format (A); 35mm four-track magnetic format (B) Figure 2A. 35mm Dolby Stereo Optical format page 3 I LT 0 INPUTS ---ADAPTIVE MATRIX rOUTPUTS --QL RTO * + M A S T E R -_o C c LEVEL --_o R CONTROL -_o SURROUNDS I SUBWOOFER 0 I AUDIO DELAY i kHz LOW-PASS FILTER B~TYPE NR DECODER Figure 28. Block diagram of the Dolby Stereo Optical playback matrix Today, the Dolby Stereo Optical system is virtually a standard format on non-digital 35 mm film. In this process, the dual bilateral variable area optical sound tracks, which were formerly modulated with a monophonic signal, are now modulated in stereo, as shown in Figure 2A. Recording on the two sound tracks is accomplished through a matrix, which accepts inputs for the three screen channels and the single surround channel. The signals intended primarily for the left and right screen loudspeakers are fed to the left and right channels. Program material intended for the center screen loudspeaker, including most on-screen dialog, is fed to both stereo channels in phase. The in-phase relationship between the stereo channels triggers the playback matrix to steer that information primarily to the center screen loudspeaker, through a combination of gain control and altering of separation coefficients within the matrix. In a similar manner, information intended for the surround channels is fed to both stereo channels so that there is a 180" phase relationship between them. This phase relationship triggers the playback matrix to steer that information primarily to the surround loudspeaker array. Figure 2B shows details of the playback matrix used in Dolby Stereo Optical soundtracks. The surround channel is delayed relative to the other channels so that, by the precedence (or Haas) effect, the surround channel will not dominate the perceived sound field in the middle and back of the house. The reason for this is that the matrix output contains certain " leakage" signals that may be disturbing to a listener if such signals were to be heard from the surround loudspeakers. in practice, the surround channel is delayed with respect to the screen channels so that the most distant listener in the cinema will hear that channel delayed by a minimum of 20 milliseconds. Since the ear will " lock in" on earlier arrival sounds, localization will be maintained in the direction of the screen for all patrons, while effects intended only for the surround channel will be clearly heard from the surround loudspeakers. This problem is further addressed by rolling off the response of the surround channel above about 7 kHz. B. Digital Film Formats The Dolby SR-D format, introduced in 1992, is shown in Figure 3A. It has exactly the same optical sound tracks as shown in Figure 2A with the addition of digital information located in the otherwise unused space between sprocket holes. This new digital format provides the usual three screen channels plus a split surround pair and a single low frequency (subwoofer) channel limited to 100 Hz. This is commonly referred to as a " 5.1' channel system and uses an elaborate perceptual encoding process known as AC-3. timecode data btween optical tracks and picture Figure 3A. Dolby SR-D Figure 38. DTS Figure 3C. SDDS Figure 3B shows the format used in DTS. Here we see only the stereo optical tracks and a sync channel for maintaining control of the associated CD ROM player. Figure 3C shows the format used with SDDS. In addition to the stereo optical tracks, there are two digital tracks, one at each edge of the film. Like Dolby SR-D, DTS and SDDS make use of perceptual encoding methods for reducing the amount of digital data required for system operation. DTS and SDDS support the 5.1 channel format used in most cinemas, but SDDS also supports as many as 5 screen channels for special applications. All digital formats discussed here have a fall-back (failsafe) mode in which the analog tracks will be used in case of failure of the digital portion of the systems. C. A- and B-chains For convenience in defining responsibilities for system specification and alignment, the playback chain is customarily broken down into the A-chain and the B-chain, as shown in Figure 4. The A-chain is comprised of the preamplifiers (optical or magnetic), light source (optical), magnetic heads, solar cells (optical), associated equalization (signal de-emphasis), and noise reduction and directional decoding required for flat electrical output at the end of that chain. For digital reproduction, a digital optical reader is used and the digital signal is fed to a digital-to-analog conversion system. The analog A-chain is shown in Figure 4A, and the digital A-chain is shown at B. The B-chain, including split surround channels, is shown at C. SIGNAL OUT FILM --$-I_ ___) `I LAMP ' I i h SOLAR CELL --) P R E A M P --) NOISE REDUCTION - Figure 4A. Block diagram of analog A-chain SIGNAL OUT Figure 48. Block diagram of digital A-chain SCREEN CHANNEL I1 of31 ,:,pg MASTER FADER SURROUND CHANNEL il Of 2) 1 3 OCTAVE _,, EO A SUB CHANNEL Figure 4C. Block diagram of B-chain with split surrounds The B-chain is comprised of one-third octave equalization, dividing networks (low- or high-level), power amplification, and loudspeakers. JBL Professional products are used extensively used in the Bchain of the system. D. Evolving Dynamic Range Requirements in the Cinema Figure 4D shows details of the headroom requirements of current and future cinema formats. According to Dolby Laboratories, the level of dialog in the cinema will remain as it currently is, while the added headroom will be used primarily for more realistic peak levels for sound effects and music. Depending on specific signal content, the peak level capability of Dolby SR analog tracks can be 3 dB greater in the mid-band than with Dolby A, rising to about 9 dB at the frequency extremes. The digital formats can provide 12 dB headroom relative to Dolby A, with overall characteristics that are flat over the frequency band. This peak capability translates into acoustical levels, on a per-channel basis, of 103 dB-SPL in the house. All of the loudspeaker systems discussed in this manual will meet these new specifications, consistent with the size of the cinema for which the systems will be specified. dE 110 100 SO 80 -1 375 63 125 253 500 1K 2K :r( 8K 16K tiz Peak power levels (Al A-type. (8) SF?. (CI SR.D Figure 40. Dynamic range requirements for Dolby-A, Dolby SR and Dolby SR-D formats E. Integration of Loudspeakers into the Acoustical Environment In order to present a clear picture of the interaction of loudspeakers and the acoustical environment, we will begin with the previous era in cinema loudspeaker design. Through the end of s, the 1970' the loudspeaker systems which were current in the cinema were the tried and true twoway designs composed of multicellular or radial high-frequency horns and hybrid horn/reflex lowfrequency systems. These systems had been developed by Bell Laboratories as far back as the 1930' and the versions used until just a few years ago were essentially the same as has been s, developed and refined by Lansing and Hilliard (1). These systems were well engineered in terms of efficiency, ruggedness, and low distortion, given the acoustical performance demands of the day. Their designers had also successfully coped with the problems of frequency division and arrival time page 7 differences between high and low frequency sections. The chief weakness of these systems was their lack of uniform coverage. System design stressed output conversion efficiency, because of the small power amplifiers available at the time. Figure 5A shows the on- and off-axis curves of a typical horn/reflex system, while polar response of a typical multicellular horn is shown at B. Note that the off-axis response of the lowfrequency system falls off considerably at higher frequencies. The typical reverberant room response of a system composed of these elements is shown at C. Note here the double hump, which indicates that the total power output of the system is far from uniform. At the same time, however, the on-axis response of the system may be fairly flat, when measured under non-reflective conditions. A. Off-axis response of ported horn system C. Reverberant (power) response of a cinema system composed of elements similar to those shown in Figures 5A and 58 I +1c +5 - - - c -5 II c5 I I 4 I 3 I '5 I 3'5 I 63 ( -Ii: '25 .Hi 8. Polar characteristics of a 2 x 5 multicellular horn Mulhcellular horn (2 x 5) 1000 Hz vertical (so/id); horizontal (dashed) Multicellular horn (2 x 5) 2000 Hz vertical (soliu); horizontal (dashed) Multicellular horn (2 x 5) 10 kHz vertical (solid); horizontal (dashed) Figure 5. Theatre equalization of old-style cinema system If any attempt is made to equalize the response of this system in the cinema, then the response along the major axis of the system will be anything but flat. This is precisely the problem which Dolby Laboratories encountered when they introduced equalization into cinemas during the 1970' s. Page 8 F. Power Response and Power-Flat Systems The discrepancy between on-axis and reverberant room response in the older systems was solved with the introduction of a new family of systems based on uniform coverage high-frequency horns and simple ported low-frequency enclosures. Figure 6A shows the horizontal off-axis response of the JBL 4648A low-frequency system. Note that the response is uniform below 500 Hz over a wide angle. At 6B we show the vertical off-axis response of the 4648A system. Note that the response begins to narrow just below 200 Hz. The net result of this pattern narrowing in the horizontal and vertical planes is that they make a good match for the pattern control of the JBL 2360A horn at the normal crossover frequency of 500 Hz. Figure 6C shows the off-axis response curves for the 2360A Bi-Radial horn, coupled to a JBL 2446J high-frequency driver which has been equalized for flat power response. Note that the off-axis curves are essentially parallel, indicating that the horn produces a solid radiation angle which is uniform with respect to frequency. The need for equalization of the compression driver comes as a result of the natural high frequency roll-off which occurs in high frequency drivers above about 3.5 kHz. This frequency is known as the " mass break point" and is a function of diaphragm mass and various electrical and magnetic constants in the design of the driver. When the 4648A or 4638 low-frequency system and the 2360/2446 combination are integrated into a full range system for cinema use, the -6 dB beamwidth above 500 Hz is smoothly maintained at 90" in the horizontal plane and 40" in the vertical plane out to 12.5 kHz. At lower frequencies, the system' coverage broadens, eventually becoming omnidirectional in the range below 100 Hz. s A I3 - 1 Figure 6. (A) Horizontal response; (B) Vertical response; (C) Off-axis response of a JBL 236OA horn equalized for flar power response When the system described above is equalized in a typical cinema environment, both direct sound and reverberant sound can be maintained quite smoothly, as shown in Figure 7A. The system' s reverberant response is proportional to its power output, or to its power response, and the matching of the system' on-axis and power response indicate that the reflected sound field in the cinema will s have the same spectral characteristics as the direct sound from the loudspeaker. When this condition exists, sound reproduction, especially dialog, will sound extremely natural. (The frequency response contour shown in Figure 78 is the so-called " X-curve" recommended for cinema equalization, as specified in IS0 Document 2969.) - ON *xis RESPONSE --POWER RESWNSE UNEOUALIZED EQUALIZED Figure 7. Cinema equalization of power flat systems JBL pioneered the concept of flat power response in the cinema (2,3). It has become the guiding principle in much of JBL' product design, and it has been adopted by the industry at large. s G. Coverage Requirements for Proper Stereo Reproduction In the cinema, it is expected that all patrons will be able to appreciate convincing stereo reproduction. By contrast, standard two-channel stereo in the home environment often imposes strict limitations on where the listener must sit in order to perceive correct stereo imaging. The factor that makes the big difference in the cinema is the presence of the center channel. Not only does the center loudspeaker lock dialog into the center of the screen, it further reduces the amount of common mode information the left and right channels must carry, thus making it possible for listeners far from the axis of symmetry to hear the three channels with no ambiguity or tendency for the signal to " collapse" toward the nearer loudspeaker. In the Dolby stereo matrix, the same convincing effect is largely maintained through gain coefficient manipulation during playback. Ideally, each patron in the house should be within the nominal horizontal and vertical coverage angles of all the high-frequency horns. This requirement can usually be met by using horns with a nominal 90" horizontal dispersion and by toeing in the left and right screen loudspeakers. In very wide houses, the spreading of high frequencies above approximately 5 kHz, as they pass through the screen at high off-axis angles, actually helps in providing the desired coverage. Another desirable condition is maintaining levels as uniformly as possible throughout the house. We have found that aiming the screen systems, both high- and low-frequency, toward the back wall helps in this regard, by offsetting normal inverse square losses with the on-axis " gain" of the screen systems. Measurements made at the Goldwyn Theater of the Academy of Motion Picture Arts and Sciences in Beverly Hills, California, show that, over most of the frequency range, front-to-back levels in the house are maintained within a range of 5 dB. By contrast, aiming the high-frequency elements toward the audience would produce front-to-back level variations of up to 10 to 12 dB. An important requirement here is that the back wall of the cinema be as absorptive as possible. If the rear wall is not highly absorptive, then tilt the high frequency loudspeakers down, with the horn' axis s pointing at the seating area two-thirds of the way back in the house. This performance is seen in Figure 8. At A, we show in plan view the direct field coverage given by a JBL 2360 horn aimed at the absorptive back wall of a large theater with sloped floor. Coverage at 2 kHz is within a range of +/- 3dB, front to back. If the horn is aimed downward to a point two-thirds the distance from front to back, the coverage is as shown at B, and coverage at the rear of the house is compromised. The coverage given by one of the outside horns, aimed at the rear wall, is shown at C. It is customary to toe in the left and right systems toward the center, whether or not the screen itself is curved, and the aim is to provide adequate coverage for all patrons, with response maintained within a total range of 6 dB. n Figure 8. (A) Direct field coverage at 2kHz, aimed at rear wall; (B) Same, horn aimed 2/3 distance front to back; (C) Coverage of single outside horn. page 11 L The surround ensemble of loudspeakers, if properly specified, can easily produce a sound field that is uniform throughout the back two-thirds of the house, and level variations can often be held within a range of 2 to 3 dB. Details of surround system specification will be covered in a later section. When all of the above points are properly addressed, the sound in a cinema can approach that which we take for granted in a post-production screening facility - which is, after all, how the picture director intended it to sound. It is only when such details as these have been carefully worked out that the effects intended by the sound mixer can be appreciated by the viewing audience. III. ACOUSTlCAL CONSIDERATIONS A. Noise Criterion (NC) Requirements .-. The usual sources of noise in a cinema, outside of the patrons themselves, are air handling and transmission of noise from the outside. In the case of multiplex installations, there can be leakage from adjacent cinemas as well. Not much can be done about a noisy audience, but it is true that at the post-production.stage, mixing engineers take into account certain masking noise levels which may be encountered in the field and even do the final mix under simulated noisy conditions (4). 63 125 2% 94 IK 2ll .(* xa FREOUENCV (Hz) 1K 4K FREOUEW (Hz) Figure 9. Noise Criterion (NC)wrves, octave band data Figure 10. Sound Transmission Curves Acoustical engineers make use of what are called Noise Criterion (NC) curves in attempting to. set a noise performance goal for cinemas. The octave band values of these curves are shown in Figure 9. In implementing this data, the acoustical designer settles on a given criterion and then determines the cost and other factors involved in realizing it. Low-noise air handling requires large ductwork and is expensive. Even more likely to be a problem is through-the-wall isolation from adjacent cinemas. The general recommendation made by Lucasfilm Limited (5) is that interference from adjacent cinemas should be audible no more than 1% of the time. Considering that NC-30 may represent a typical air conditioning noise level for a cinema, the desired degree of isolation between adjacent spaces does not represent a hardship in terms of wall construction. The need for improving NC standards in cinemas is a natural consequence of better recording technology and is the only way that the benefits of Dolby SR and digital formats can be fully appreciated. As an example of what may be required, let us assume that the normal maximum levels in a multiplex cinema are 95 dB-SPL, with levels exceeding this value only about 1% of the time. It is clear that the isolation from an adjacent cinema must be on the order of 65 dB if the NC-30 criterion is to be met, and this will call for a wall structure that will satisfy a Sound Transmission Class (STC) of 65 dB. There are a number of double wall, or single concrete block wall, constructions that will satisfy this requirement, and economic considerations usually take over at this point. Acoustical engineers and consultants are usually on firm scientific ground in these matters. Typical standard STC curves are shown in Figure 10 . The isolation task is certainly easier with new construction, since buffer areas can be designed between adjacent exhibition spaces. The most difficult problems occur when older spaces are to be subdivided to make multiplex cinemas, inasmuch as the chances of coupling through walls or through common air handling are compounded. It is obvious that the architect must work closely with an acoustical engineer if the job of isolating adjacent spaces is to be done correctly. All of this yields to straightforward analysis, but the job is often a tedious one. B. Control of Reverberation and Discrete Reflections After the problems of sound isolation have been addressed, the acoustical engineer then turns to those problems that are generated entirely within the cinema itself; i.e., reverberation and echoes. The acoustical ` signature' of a cinema should.be neutral. Reverberation per se is not generally apparent in most houses, and any perceived sensa of reverberation or ambience during film exhibition normally comes as a result of surround channel program. This is not to say that the cinema environment should be absolutely reflection-free. Strong initial reflections from the sides of the house may be beneficial in a concert hall, where they are needed to produce a sense of natural acoustical space; however, in the cinema, pronounced initial reflections from any direction should be eliminated. Traditionally, reverberation time in auditoriums increases at low frequencies and decreases at high frequencies. This is a natural consequence of the fact that many surfaces that are absorptive at middle and high frequencies are not very effective sound absorbers at low frequencies. At higher frequencies, there is additional absorption due to the air itself, and this excess attenuation of high frequencies tends to lower the reverberation time. Figure 11 shows the normal range of reverberation time, as a function of the value at 500 Hz, while Figure 12 shows the acceptable range of reverberation at 500 Hz as a function of room volume. .-. tm no 500 l.om FREWEKVIHz) zaa 5.m 1O.OLl-J 0.1 Glikl 3 &icf* 3 2760 m' mMw ni Figurn 11. Variation of reverBem tion time with frequency Figure 12. Suggested Em of revetiration The requirements of specifying the right finishing materials, along with any special needs for added low-frequency absorption, fall squarely in the hands of the acoustical designer. In smaller houses, there is often little choice but to make the space acoustically ` dead;' however, some degree of reflectivity, even though it may not be perceived as such, will be beneficial. 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. 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 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. 2. lntearitv in construction. 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. 3. lmoact and structure-borne noise. 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. d, , 1 IV. SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERS A. Hardware Class vs. 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. Whenever possible, the 4675C systems should be specified, but there are situations where space behind the screen is limited, and the smaller horn must be used. Both systems are capable of the same acoustical output, inasmuch as they are both limited by the power handling capabilities of their identical low-frequency sections. Table 1 indicates the sustained maximum sound pressure level in the reverberant field which these systems can produce, based on room volume. Levels for a single unit, as well as for the three units, are given. Median reverberation times as given in Figure 11 are assumed in these calculations, as are system directivity index and estimated room surface area. 27omp (10,ooo al. L) 540m3 (20,ooo cu. ft.) 13SOm3 (!5o,ooo cu. ft.) 2700m3 (100,ooo cu. R) 5400m3 (200,ooo al. ft.) ++7zT 113 118 150 800W 108 113 300 800W 108 111 500 800W 104 109 loo0 800W Table 1A: Maximum Reverbwant Levels1 for JBL 467OD and 467% Systems in Cinsmas of various sizes (non-b&m plifiedmode) Taking the information presented in Table lA, we can now determine the actual power requirements to produce target levels in the house of 105 dB per channel: g$!f (10,ooo cu. ft.) 540m3 (20,aoo al. It.) 13SOm3 (!yJ.ooo al. ft.) 7510100 1oow JBLMPX3oo(onechamel) = 150 25ow JBL MPX800 (one chamd) 300 400W JBL MPX800 (one chard) Tabls 1 B: Power Requirements for Targst Revebmnt Levels' of 105 dB in Smaller Houses (nowbiam~lifisd mode) lReverberant levels, as calculated in Tables 1 A, 6, and C, represents the maximum level that would exist at a point about two-thirds from the front of the house to the back. For spaces of 2700 m3 or greater, JBL recommends that the model 4675C be specified in biampiified mode. d 2700m3 (100,ooo cu. ft.) S400m3 (200,ooo cu. ft.) 103dB 113dB 500 tt%z HF:25OW LF:4oow HF:2!5OW 106 111 loo0 ._ Table 1C. Maximum Reverberant Levels' for JBL 4675C Systems in Large Cinemas (biimpliied mode). B. Advantages of Biamplification The importance of biampiification in large cinemas cannot be overestimated. Even though the systems detailed in Table 1 B use the same amplifier model as the systems detailed in Table 1 A, the reallocation of the power through biampiification has important beneficial effects. Specifically, intermodulation distortion is reduced at-high operating levels, and available power can be more directly matched to the specific HF or LF load. C. Cinema Playback Level Calibration The actual level requirements in the film makel' dubbing cinema are established by relating s them directly with modulation level on the recorded medium. For magnetic media, this is established zero level,' or 185 as 85 dB-SPL in the house when the modulation on the tape is so-called ` nanowebers/meter. This last quantity has to do with Mording technology, and we need not concern ourselves with it further, except to note that modulation peaks often exceed zero level by 8 to 10 dB. Thus, the peak output per loudspeaker may be only 95 dB. Good engineering practice allows additional headroom of 6 to 8 dB above this, so it is clear that the values we have listed in Tables 1A and B are not excessive in the cases of the larger houses. in the smaller houses, we can certainly make do with smaller amplifiers than indicated in the table; but even then, the cost of the added power is very slight, and the benefit substantial. The powers recommended in Tables 1A and B are in accordance with the suggestions made by Lucasfilm Limited (6) in the specification of THX systems. 2JBL amplifier model MPA400 with appropriate front-end frequency division and power msponse equalization, is recommended for these applications. The LF loudspeaker section presents a 4-ohm load, to which the amplifier can deliver 400 watts; the HF section presents an S-ohm load, for whii the ampliier can deliver 275 watts. : lm page 17 D. New JBL Driver Developments Our studies have indicated that, in passive systems, maximum power input to the screen loudspeakers is essentially network limited. As a result of this, many cinema applications ordinarily will not require the high power Vented Gap Coolingm (VGC) performance designed into the JBL 2226 driver. A more recent model, the 2035, was subsequently designed with a 76 mm (3 in.) voice coil, retaining the same sensitivity of the 2226. Resulting economies can thus be passed on to the user. In biamplified systems for larger houses we strongly recommend that the 2226 transducers be used, because of their higher peak power and transient capabilities. Figure 13 shows the on-axis response of the dual low frequency 4638 system, which incorporates two of the 2035 transducers. ml20 xl im 200 5m 1.000 2.03l 5.m) ro.om 2o.clm REQUEW WJ On-axis response of dual 360 mm (15 in.) 4636TH LF system. E. Mechanical Details of JBL Screen Loudspeaker Systems The main JBL loudspeakers recommended for behind-screen use are discussed in this section. Since all of these systems are intended for field assembly, we will show them in exploded views, along with a parts list and a wiring diagram for use with a high-level dividing network. Figure 14 shows dimensional aspects on field assembled 4670D and 467X systems, clearly indicating their overall space requirements. The models4670D-HF, 4671 B, 46736,4675CHF, and 4638TH are shown, respectively, in Figures 15 through 19. Passive network hook-up details are shown in Figure 20. Wiring instructions for biamplification will be discussed in a later section. .-. Y - - .I- I-- lrn' - - 17s/r' - FQure 14. Complete system assembly diagram for 46700 and 46756.