Abstract:
A full-frequency-range cinema loudspeaker system intended or deployment in limited space behind a perforated cinema screen is configured as three modules: (1) a high-frequency module having a compression driver working into a horn-shaped waveguide that is specially shaped to compensate for beam-spreading effects of the perforated screen, (2) a midrange module providing a specially shaped waveguide with a multiple throat portion that mounts four cone-type drivers in a vertical array, the four individual throat regions merging into a common mouth portion of the waveguide that flares out to the front of the module, and (3) a vented-port low-frequency module with two cone-type low-frequency loudspeaker units. The three modules are stacked with the high-frequency module on top and the low-frequency module at bottom; The modules are all made to have a uniform width and a depth of under eighteen inches. Each module is designed to provide uniform sound coverage throughout its designated audio frequency range and throughout a target auditorium area of the theater, including smooth and seamless crossovers between the ranges. In the midrange module, vertical beamwidth is held substantially constant by signal processing provided by an electrical filter network that results in a constant vertical polar response pattern independent of frequency To provide optimal coverage, the waveguides in the high-frequency and midrange modules are specially shaped and directed downwardly at a selected angle of 5 degrees. Additionally, mechanism is provided for tilting the midrange and high-frequency modules if necessary to accommodate a tilted screen.

Description:
PRIORITY 
     Benefit is claimed under 35 U.S.C. §119(e) of pending provisional application No. 60/163,137 filed Nov. 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of cinema sound systems and more particularly it relates to an improved full-frequency-range loudspeaker array in a modular system for providing defined full audience coverage in a theater where the system is deployed behind a conventional perforated cinema screen. 
     BACKGROUND OF THE INVENTION 
     The requirements for a cinema loudspeaker system can be stated simply: to provide uniform sound coverage as perceived at practically all seating locations in the theater with regard to both loudness and flatness of full frequency response, while causing the perceived sound source to coincide with the images projected on the screen, with sufficient overall efficiency to keep the total power requirements within practical limits. 
     Fulfilling these requirements is far from simple and requires special treatment for various portions of the total audio frequency spectrum and for various regions of the theater auditorium. 
     A principal design challenge in cinema sound, with the burden falling largely in the mid-frequency range, is the unusual degree of beamwidth confinement and selective control required in efforts to configure and deploy an array of loudspeakers that will satisfy the defined audience coverage requirements from the front to the back row and fully to the sides of the auditorium. Without special selective directivity, the audience coverage would be uneven, and much of the available acoustic energy would become lost, escaping to regions other than the seating area. 
     For defined coverage, cinema sound systems are required to provide controlled sound directivity, typically measured in a hemispherical free space to approximate the geometric conditions of anticipated final location in the front wall of a theater; the beamwidths at −6 dB coverage are typically required to be about 90 to 100 degrees horizontal by 40 to 50 degrees vertical. This beamwidth is a function of the loudspeaker system design at all frequencies down to about 250 Hz; below this, sound inherently becomes increasingly non-directional as the frequency decreases. 
     The system must meet the requirement of providing spatial accuracy, i.e. identifying the source of sound with differently located images on the screen. Typically this can be addressed satisfactorily by a three channel system having left, center and right vertical arrays or stacks behind the screen, however to preserve “stereo sound stage imaging” each of these stacks must be designed for defined coverage of the full theater area. Typically the three stacks are made physically identical, but they may be equalized individually for optimizing coverage and frequency response. 
     Since a solid screen generally require loudspeakers to be located above or below the screen, the majority of motion picture exhibitors utilize a perforated vinyl screen in order to preserve accuracy of sound sourcing by locating the loudspeaker system behind the screen. However, due to the small diameter holes, the low ratio of open area introduces frequency-dependent anomalies such as attenuation, reflections and beam spreading which degrade the audience coverage. 
     Frequency-dependent attenuation can be dealt with by equalization, however there are usually associated spatial anomalies that must be addressed as well. 
     In addition to auditorium acoustics, the region behind the screen and even the space between the screen and the speakers must be considered with regard to harmful reflections. 
     Beam spreading due to the screen perforations increases with frequency. FIG. 1A is a graph showing target directivity index for a cinema loudspeaker of known art, with no screen present, as being constant at 10 dB, which represents a gain at the strongest direction, typically on the major axis, relative to a omnidirectional point source of the same power. The corresponding beamwidth, shown in the curve of FIG. 1B, is seen to be constant at 100 degrees: cinema loudspeaker systems are typically designed for 90 to 100 degrees horizontal beamwidth. These target parameters have been used conventionally for design and evaluation of cinema loudspeakers with no screen present. However when deployed behind the cinema screen, the directivity index tends to decrease with increasing frequency as shown in FIG. 2A, which shows it reducing to 5 dB above 10 kHz; the corresponding beamwidth, shown in FIG. 2B, spreads to nearly double, increasing from 100 degrees to about 180 degrees which is practically omni-directional in the case of the movie theater since the sound source, i.e. the loudspeaker, is in effect mounted in one wall and thus working into a hemispherical field region. 
     FIGS. 3A-C are polar graphs showing horizontal directivity as provided by a conventional high-frequency module of known art measured at 2, 4 and 8 kHz respectively, with a cinema screen spaced away 2″, 8″, and completely removed. 
     FIGS. 3D-F are polar graphs showing the vertical directivity corresponding to FIGS. 3A-C. 
     This screen beam-spreading effect, increasing with frequency as seen here and in related FIG. 2B, wastes high-frequency audio power emanating in unwanted directions and generally degrades the high-frequency coverage of the system. It remains a problem in the attainment of required beamwidth at high-frequency for defined coverage of cinema sound systems: a problem that has not been adequately addressed in known art. Heretofore, transducer driver units, waveguides and/or stacks thereof for behind-the-screen cinema deployment have not been commercially available with capabilities to fully meet increasingly demanding requirements for defined coverage with full frequency high fidelity and space constraints: more specifically, with compensation for perforated screen spreading in the high-frequency range that increases with frequency and/or with sufficient directivity for defined coverage control in the lower mid-frequency range and/or in a sufficiently compact size for installations where there is only limited space available behind the screen, which can be as little as 18 inches. 
     DISCUSSION OF KNOWN ART 
     U.S. Pat. No. 4,569,076 to Holman for a MOTION PICTURE THEATER LOUDSPEAKER SYSTEM discloses such a system wherein the loudspeaker elements are made to be integral with an acoustical boundary wall constructed behind the screen in order to optimize the characteristics of vented bass woofers. 
     U.S. Pat. No. 4,580,655 to Keele Jr. discloses a DEFINED COVERAGE LOUDSPEAKER HORN wherein opposed sidewalls are constructed to direct portions of a sound beam toward a target over different preselected incident angles. 
     U.S. Pat. No. 5,233,664 to Yanagawa et al for a SPEAKER SYSTEM AND METHOD OF CONTROLLING DIRECTIVITY THEREOF discloses the use several different digital filters connected between a common input terminal and several speaker units arranged linearly, in a matrix or in honeycomb form. 
     U.S. Pat. No. 5,004,067 to Patronis for a CINEMA SOUND SYSTEM FOR UNPERFORATED SCREENS utilizes an exponential middle frequency horn, crossed-over at 150 and 600 Hz, physically combined with a constant directivity high-frequency horn, for mounting three such units above the screen while locating three direct radiator bass units at the floor position beneath the screen. 
     U.S. Pat. No. 5,020,630 to Gunness for a LOUDSPEAKER AND HORN THEREFOR discloses a high-frequency loudspeaker for projecting sound over a listening area having a driver and a horn in which the horn has a coupling portion communicating with an outwardly flaring portion, the horn forming an elongated slot at the interface, the slot being narrower at one end and flaring outwardly to the other end. The driver frequency range is 500-20,000 Hz, and directivity is shown at 2,000 Hz. This patent teaches a high central loudspeaker location, downwardly inclined at the front end of an auditorium; however it fails to address the particular requirements of theaters or deployment behind a cinema screen. 
     Speaker arrays, including electronically and/or acoustically filtered arrays have been used in known art for low-frequency pattern control, but have relatively low efficiency, limited bandwidth capability and/or excessive physical size. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide a loudspeaker system for deployment behind a perforated cinema screen, to accomplish defined uniform sound coverage with regard to loudness and full frequency range as perceived in virtually all listening regions of a theater auditorium. 
     It is a further object to provide a full-frequency range loudspeaker array system having a shallow profile less than 18 inches in depth. 
     It is a further object to provide a modular loudspeaker system that satisfies the defined coverage requirements for a theater installation utilizing a horizontal array of several physically identical modular vertical stacks, typically three, each stack formed from separate waveguide acoustically-loaded modules, each of which is dedicated to a different portion of the frequency spectrum, and which can be manufactured, tested, marketed and/or deployed independently. 
     SUMMARY OF THE INVENTION 
     The abovementioned objects have been accomplished in the present invention in a system of modular stacks that can be deployed in multiples, typically a row of three, behind a cinema screen. Each stack constitutes a three-way vertical line array having a high-frequency module stacked on top of a multi-driver midrange module which in turn is stacked on top of a dual-driver low-frequency module. The crossover frequencies are 250 Hz and 1.2 kHz. 
     The high-frequency module utilizes a compression driver coupled to a horn waveguide with a special orientation, vertical asymmetry and three-dimensional waveguide shaping to provide controlled directivity that increases with frequency, to compensate for cinema screen spreading and to optimize defined coverage uniformity. 
     The midrange frequency module is an integrated multi-band waveguide assembly configured to provide a vertical array of four contiguous specially-shaped waveguide regions each driven by a cone type transducer driver. The required defined coverage is accomplished through a combination of special shaping of the waveguide directing surfaces with vertical asymmetry to provide controlled directivity vertically and horizontally, and frequency-selective filtering in a passive network that accomplishes the required overall coverage by splitting the drive power into two paths with different special transfer functions allocated to the lower two transducers as a low-frequency portion and the to the upper two transducers as a high-frequency portion of the midrange assembly. The four drivers are separated by partitions shaped with strategic spacing dimensions, each driver working into an individual waveguide throat portion, and each directed at an inclined angle downwardly from horizontal, to optimize defined coverage uniformity. The throat portions combine smoothly into a common flared mouth portion which extends to the substantially rectangular shape of the front outline of the midrange module. 
     The low-frequency module is a vented bass enclosure deploying a vertical stack of two 15″ cone type transducers with response extending down to 30 Hz at −6 dB. 
     The resulting cinema loudspeaker system provides high efficiency, high sound level capability and well-controlled coverage, compensated for screen spreading at high-frequency, and maintained substantially constant for beamwidth over the high and midrange frequency range (16 kHz to 250 Hz) in both vertical and horizontal directions. Beamwidth as well as amplitude response are matched at the 250 Hz and 1.2 kHz crossover frequencies for seamless acoustic integration of the three modules. The combination of a waveguide designed for use with a filtered line array with a well-designed filtered line provides significantly better performance than current designs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, features and advantages of the present invention will be more fully understood from the following description taken with the accompanying drawings in which: 
     FIG. 1A is graph showing the target flat frequency response of the horizontal directivity index of a conventional high-frequency cinema loudspeaker unit, with no cinema screen present. 
     FIG. 1B is a graph with a curve showing horizontal beamwidth at −6 dB coverage corresponding to the directivity curve shown in FIG.  1 A. 
     FIG. 2A is a graph showing typical frequency response of horizontal directivity index for a conventional high-frequency cinema loudspeaker as in FIGS. 1A and 1B when it is deployed behind a perforated cinema screen. 
     FIG. 2B is a graph showing the frequency response of horizontal beamwidth at −6 dB coverage corresponding to the directivity curve shown in FIG.  2 A. 
     FIGS. 3A-C are polar graphs showing horizontal directivity as measured on a conventional cinema loudspeaker of FIGS. 1 and 2 measured at 2, 4 and 8 kHz, with the cinema screen spaced 2″, 8″ and removed. 
     FIGS. 3D-F are polar graphs showing vertical directivity corresponding to FIGS. 3A-C. 
     FIG. 4A is a functional diagram showing a cross-sectional side view through a central plane of the full frequency range loudspeaker array embodiment of the present invention. 
     FIG. 4B is a functional diagram showing a front view of the loudspeaker array of FIG.  4 A. 
     FIG. 4C is functional diagram showing a cross-sectional view of the high-frequency module taken at horizontal axis  4 C- 4 C′ of FIG.  4 B. 
     FIG. 4D is functional diagram showing a cross-sectional view of the mid-range module taken at horizontal axis  4 D- 4 D′ of FIG.  4 B. 
     FIG. 5A is a graph showing the horizontal beamwidth at −6 dB coverage targeted for the high-frequency module of the cinema loudspeaker system of the present invention, as would be measured in a free space environment with no cinema screen present illustrating the compensation for screen spreading. 
     FIG. 5B is a graph showing the substantially constant horizontal beamwidth at −6 dB coverage as targeted for the high-frequency module of the cinema loudspeaker embodiment of the present invention as in FIG. 5A, as would be measured with the loudspeaker deployed behind a perforated cinema screen. 
     FIG. 6 presents the mathematical basis of the waveguide wall shape in the midrange and high-frequency modules of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 7A is a functional block diagram of the filtering network for the transducer driver elements of the midrange module of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 7B is a schematic diagram of a passive circuit implementation of the filtering network of FIG.  7 A. 
     FIG. 8A is a graph with curves showing electro-acoustic magnitude/frequency response transfer functions of the lower and upper frequency drivers and their combined response as provided by the filtering network of FIG. 7B in combination with the mid-range loudspeaker module of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 8B is a graph with curves showing the corresponding phase transfer function of the functions shown in FIG.  8 A. 
     FIG. 9A is a graph with a curve showing the overall electro-acoustic magnitude/frequency transfer function of the combined mid-range and high-frequency modules of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 9B is a graph with a curve showing the overall electro-acoustic magnitude/frequency transfer function of the combined low-frequency, midrange and high-frequency modules of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 10A is a graph with a curve showing directivity index measured on the combined low-frequency, midrange and high-frequency modules of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 10B is a graph with curves showing horizontal and vertical beamwidth as measured on the combined low-frequency, midrange and high-frequency modules of the cinema loudspeaker array embodiment of the present invention, corresponding to the directivity index shown in FIG.  10 A. 
     FIGS. 10C-E are graphs with families of curves showing normalized horizontal, vertical up and down responses measured on the combined midrange and high-frequency modules of the cinema loudspeaker array embodiment of the present invention. 
     FIGS. 11A-B are polar graphs showing the midrange horizontal directivity of the cinema loudspeaker array embodiment of the present invention. 
     FIGS. 11C-E continue from FIGS. 11A-B, showing the high-frequency horizontal directivity. 
     FIGS. 11F-G correspond to FIGS. 11A-B, showing the midrange vertical directivity. 
     FIGS. 11H-J correspond to FIGS. 11C-E, showing the high-frequency vertical directivity. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A-3F have been discussed above in connection with the background of the invention. 
     FIG. 4A is a functional diagram showing a cross-sectional side view through a central plane of the full frequency range linear loudspeaker array  10  in an embodiment illustrative of the present invention. Three modules of uniform width and maximum depth dimensions are stacked to form the vertical array; each module is rear-enclosed to contain the back wave and prevent reflections between the screen and the rear cinema wall. The front plane  10 A fits closely near the cinema screen typically separated by a spacing in the range of 2 to 8 inches. 
     In the high-frequency module  12 , at the top, a high-frequency driver  12 A is coupled to a waveguide with asymmetric upper and lower walls  12 B and  12 C as shown, extending to a transitional plane  12 D where the flare increases to an opening at the front plane  12 E, extending as a flange in region  12 F. The waveguide is dimensioned to be effective down to 600 Hz, i.e. one octave below the 1.2 kHz crossover frequency, and, as a departure from known art, is specially shaped to increase in directivity with increasing frequency to counteract screen spreading. The cross-sectional area increases from the driven end at driver  12 A to a vertical transitional plane  12 D located at approximately 90% of the total waveguide length, where the flare shape transitions in a smooth tangential manner to a greater curvature extending tangentially to the exit opening at the vertical front plane  12 E, where a flat surface  12 F extends vertically to the top of the enclosure of module  12 . The walls of the waveguide are shaped in a special and novel manner so as to cause an increase in directivity, i.e. a decrease in beamwidth, with increasing frequency, in order to compensate for beam spreading caused by the perforated cinema screen. 
     The vertical asymmetry of the waveguide shape causes the central axis to incline downwardly at an angle A, which is made to be 5 degrees in a preferred embodiment, so as to co-operate with the waveguide shape in accomplishing the defined coverage in typical theaters. 
     The high-frequency module  12  operates in a frequency range from the crossover frequency of 1.2 kHz up to 20 kHz at −6 dB with a rated power-handling capability of 50 watts AES; the recommended amplifier capability is 200 watts. 
     The high-frequency module  12  is made 762 mm×450 mm max×381 mm high (30″×17.75″ max×15″). 
     The midrange module  14  is a four element vertical array driven by four identical cone-type transducer drivers  14 A-D, typically round units within a size range of 6.5 to 12 inches in diameter with a power-handling capability of 100 watts each. 
     Drivers  14 A-D are each mounted on a mounting surface that is inclined downwardly at an angle B at the rear of the multiple waveguide assembly which provides a separate waveguide for each driver. As seen in the vertical cross-section, drivers  14 A-B and  14 C-D are separated by a center-to-center distance d 1  and share a partition  14 E with a cone-shaped cross-section as shown, whose opposite sides forms a waveguide surface for each, the other waveguide surface extending to the front plane  12 E. Drivers  14 B-C are separated by a greater center-to-center distance d 2  and share a larger partition  14 F which extends to a point set back from the front plane  10 F by dimension d 3 , while partitions  14 E are set back by a greater distance d 4 . 
     The four drivers are open-basket round units and are enclosed at the rear by a common cover  14 J that confines the rear acoustic radiation. 
     Drivers  14 A-B work together as an upper midrange portion driven from a branch of a filter network, and drivers  14 C-D work together as lower midrange portion separately driven from a different branch of the filter network. While each of the these portions could be implemented by a single driver unit, the preferred embodiment deploys two in each portion for greater power handling capability and pattern control down to 250 Hz: the low midrange crossover frequency. 
     The required directivity of the four-speaker array in the midrange module  14  for defined coverage is accomplished by the shaping of the four waveguides and by the dimensioning of d 1 - 4  and the downward mounting angle of the drivers. In a preferred embodiment of the midrange module  14 , d 1  is made 7.75″, d 2  is made 11.25″, d 3  is made 3″, d 4  is made 6.5″, and the downward driver mounting angle B is made 5 degrees. the same as in the high-frequency module  12 . 
     The midrange module  14  operates between the crossover frequencies 250 Hz and 1.2 kHz with a rated power-handling capability of 400 watts AES; the recommended amplifier power capability is 600 watts. 
     For the midrange module  14 , the outside dimensions are 762 mm×450 mm max×1143 mm high (30″×17.75″ max×45″). 
     The high-frequency module  12  and the midrange module  14  are attached rigidly to each other at the front, and the midrange module  14  is attached to the top of the low-frequency module  16  with a pair of pivots  14 K near the front, and a pair of adjustable support members  14 L attached to cover  14 J at the rear. Support members  14 L are provided with a series of attachment holes so that, as an option to the normal condition with the front of the three modules in a common vertical plane, the attachment to cover  14 J can be altered to support the high/midrange assembly at a selection of + or − inclined angles relative to the low-frequency module. 
     As a further option, one or more of the drivers  14 A-D could be mounted in a manner to make the mounting angle B different than 5 degrees and/or to make angle B adjustable individually for on-site coverage optimization. 
     The low-frequency module  16  contains two  15 ″ cone type transducers  16 A in a vented port configuration with a rated power-handling capability of 800 watts AES; the recommended amplifier capability is 1200 watts. The low-frequency module  16  operates from the crossover frequency of 250 Hz down to 40 Hz at −3 dB, and 30 Hz at −6 dB. 
     The enclosure of the low-frequency module  16  is made 762 mm wide×450 mm deep×883 mm high (30″×17.75″×34.75″). 
     FIG. 4B is a functional diagram showing a front view of the loudspeaker array  10  of FIG.  4 A. 
     In the high-frequency module  12 , the front elevational view shows the cross sectional shape of the waveguide. At the driven end the shape is a circle of 1″ or 1.5″ diameter for engaging a conventional compression driver  12 A. The waveguide shape evolves smoothly to the transitional plane  12 D, where the cross-sectional shape is “keystone”-like with the sidewalls  12 G and  12 H bowed inwardly and inclined so as to become narrower at the top by a varying upwardly-converging angle B as shown: this shape is key to the attainment of the desired overall uniform high-frequency coverage pattern, including compensation for screen spreading effect as described above. 
     In the midrange module  14  it is seen that sidewalls  14 G extend from the two vanes  14 E and the central vane  14 F to a front opening  14 M flanked by flat flange surfaces  14 H at the front plane  10 A, in a manner to form for each driver a waveguide that extends in a symmetrical flare to the front corners of the enclosure of module  16 , and flares vertically to either the enclosure top/bottom front corner or to the front extremity of a corresponding vane  14 E. 
     In the low-frequency module  16 , the locations fo the two low-frequency transducers  16 A and their circular bass reflex vents  16 B are shown. 
     FIG. 4C is an enlarged cross-sectional view of the high-frequency module  12  taken at horizontal axis  3 C- 3 C′ of FIG. 4B showing the two waveguide sidewalls  12 G to be symmetrical and to diverge in a smooth curvature from driver  12 A to a front opening  12 F flanked by flange surfaces  12 E at the front plane  10 A. 
     FIG. 4D is an enlarged cross-sectional view of a waveguide in the mid-range module  14  taken at horizontal axis  3 D- 3 D′ of FIG. 4B, showing the two sidewalls  14 G to be symmetrical and to diverge in a smooth curvature from the cone type transducer driver  14 A to an opening  14 J flanked by flange surfaces  14 H at the front plane  10 A. 
     FIG. 5A is a graph showing target horizontal −6 dB beamwidth coverage as a function of frequency for the high-frequency module  12  of the cinema loudspeaker system  10  of the present invention, including compensation for screen spreading, as would be measured in a free space environment with no cinema screen present. The objective is to narrow the horizontal beamwidth, from its midrange value of 90 degrees, to 40 degrees at 16 kHz. This increase in directivity at high-frequency, a novel departure from conventional loudspeaker performance, is accomplished in the present invention mainly by configuring the shape of the waveguide in high-frequency module  12  in a manner to narrow the beamwidth (i.e. increase he directivity) with increasing frequency as shown in FIG. 5A, so that when the loudspeaker, compensated in this manner, is deployed behind a perforated screen, the resultant beamwidth will be remain substantially constant at the desired nominal value, 100 degrees, over the full frequency range. 
     FIG. 5B is a graph showing target horizontal beamwidth at −6 dB coverage as a function of frequency for the compensated high-frequency module  12  of the cinema loudspeaker embodiment  10  of the present invention as in FIG. 5A, but as would be measured with the loudspeaker deployed behind a perforated cinema screen. The desired response is substantially constant horizontal beamwidth at −6 dB coverage, over the frequency range up to 16 kHz, as shown: in this example, a horizontal beamwidth of 100 degrees. 
     FIG. 6 presents the mathematical basis of the waveguide wall shape in the midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 7A is a functional block diagram of the filtering network for the transducer driver elements  14 A-D of the midrange module  14  of the cinema loudspeaker array embodiment  10  of the present invention. 
     FIG. 7B is a schematic diagram of a passive circuit implementation of the filtering network of FIG.  7 A. Low pass filters  20  and  24  are implemented by L 1 , L 2  and C 1  and L 5 , L 6  and C 4  respectively, each in a T configuration. All-pass filter  22  is shown implemented by two series voltage divider branches: C 2 , L 3 ,and L 4 , C 3  returned to common ground as shown. All-pass filter  22  could alternatively be implemented by a delay line (digital or analog) optionally implemented at low signal level followed by power amplification: this implementation could also be accomplished totally or in part by the physically location of the appropriate transducer driver element with regard to setback from the front plane of the enclosure and the other elements. 
     FIG. 8A is a graph showing magnitude versus frequency response curves of the electrical-to-acoustic transfer functions of the lower and upper frequency drivers provided by the filtering network of FIG. 7B in combination with the mid-range loudspeaker module  12  of the cinema loudspeaker array embodiment  10  of the present invention. Curve U for the upper midrange drivers  14 A and  14 B shows a −6 dB cutoff frequency of about 1.4 kHz, while curve L for the lower midrange drivers  14 C and  14 D shows a cutoff frequency of about 700 Hz. The combined curve C, shown as a dashed line, indicates a  6 dB bandpass from about 160 Hz to 1.3 kHz, and the dashed curve showing the overall response as the combination of curves U and L, showing the −6 dB bandwidth of the midrange portion extending from 150 Hz to 1.2 kHz. 
     FIG. 8B is a graph showing the corresponding phase transfer function of the function shown in FIG.  8 A. Curve U′ shows the upper driver acoustic phase response without the all pass filter  22 ; curve U″ shows the upper driver acoustic phase response with the all-pass filter  22 . Curve L shows the lower frequency driver acoustic phase response. 
     FIG. 9A is a graph showing the overall electro-acoustic magnitude/frequency response in half-space, i.e. 2 pi steradians solid included angle, for the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment  10  of the present invention. 
     FIG. 9B is a graph showing the overall electro-acoustic magnitude/frequency response in half-space (2 pi) for the total cinema loudspeaker array embodiment  10  of the present invention, including the low-frequency module  16 , midrange module  14  and high-frequency module  12  deployed together. 
     In the graphs of FIGS. 10A through 10E and  11 A through  11 J, the curves shown are taken in a free-field environment with no cinema screen present. 
     In the graph of FIG. 10A, the curve shows directivity index measured on the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment of the present invention. 
     In the graph of FIG. 10B, curve H shows horizontal beamwidth and curve V shows vertical beamwidth at −6 dB coverage measured on the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment of the present invention. 
     FIG. 10C is a graph with a family of curves showing normalized horizontal response measured on the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment  10  of the present invention. 
     FIG. 10D shows the normalized vertical down off-axis response measured on the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment  10  of the present invention. 
     FIG. 10E shows the normalized up off-axis response measured on the combined midrange module  14  and high-frequency module  12  of the cinema loudspeaker array embodiment  10  of the present invention. 
     FIGS. 11A-B are polar graphs showing the midrange horizontal directivity of a loudspeaker array embodiment  10  of the present invention as in FIGS. 1A-3, measured at eight ⅓ octave frequency ranges from 200 through 1 kHz, with no screen deployed. Each radial step is 6 dB magnitude as indicated, so the −6 dB beamwidth in degrees of each curve is indicated by the two crossings of the −6 dB circle by each curve. 
     FIGS. 11C-E continue from FIGS. 11A-B showing the high-frequency horizontal directivity at twelve ⅓ octave frequency ranges from 1.25 kHz though 16 kHz, with no screen deployed. 
     FIGS. 11F-G show the midrange vertical directivity, and FIGS. 11H-J show the high-frequency vertical directivity, corresponding to FIGS. 11A-B and  11 C-E respectively. 
     The effect of the 5 degree downward aiming of the drivers is evident in FIGS. 11F-J, and the high-frequency compensation for screen spreading is evident in FIG.  11 J. 
     The tilt angle A in the high-frequency module  12 , the tilt angle B in the midrange module  14 , and the value of the upwardly converging angle C (FIG.  4 B), the asymmetry of the upper and lower waveguide walls  24 B and  24 C (FIG. 4A) and the symmetry of sidewalls  10 G and  10 H as shown in the illustrative embodiment are subject to “fine-tuning” variations for particular circumstances and objectives, that can be practiced within the scope of the invention. 
     A key aspect the invention, i.e. compensating a loudspeaker for screen spreading at high-frequency by configuring the high-frequency waveguide in a manner to narrow the beamwidth with increasing frequency, may be implemented with alternative shaping of the waveguide that may yield equivalent results, i.e. directivity that increases with frequency at the high end. 
     The invention could be practiced with a different quantity of acoustic driver units in any or all of the three modules, and these could driven by electrical signals distributed selectively in groups or individually. 
     This invention may be embodied and practiced in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments therefore are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations, substitutions, and changes that come within the meaning and range of equivalency of the claims therefore are intended to be embraced therein.