Abstract:
A midrange loudspeaker for operation in conjunction with low-frequency and high-frequency loudspeaker modules in a theater sound system, having a reduced depth for deployment in limited space. The midrange module is configured with a plurality of drivers and a waveguide unit that provides uniform sound coverage throughout a theater auditorium with substantially seamless crossovers at 250 Hz and 1.5 kHz and with the vertical beam-width held substantially constant by an electrical filter network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/644,611, filed on Aug. 23, 2000, now abandoned, titled IMPROVED MIDRANGE LOUDSPEAKER MODULE FOR CINEMA SCREEN, which claims the benefit of U.S. Provisional Application Ser. No. 60/160,705, filed on Oct. 20 th , 1999, both of which are incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cinema sound systems and more particularly to mid-frequency range loudspeaker systems. 
     2. Related Art 
     When designing a cinema or theater loudspeaker system, it is desirable to provide uniform or consistent loudness and full mid-frequency range sound coverage to the seating locations in the cinema. Further, the perceived sound source needs to sufficiently coincide with the images projected on the screen, while operating with an efficiency that keeps the total audio amplifier power requirements within practical limits. 
     One design approach for cinema loudspeakers is the use of conventional horns or waveguides and drivers. One drawback with the use of conventional horns or waveguides is that frequency pattern control of conventional horns or waveguides require a relatively large mouth and overall size to provide the required directivity. For example horns of conventional designs are required to be about four to five feet in depth to achieve the required pattern control at frequencies in the order of 250 Hz. Conventional horns designs are therefore generally undesirable because they occupy a large area behind the cinema screen, decreasing the amount of usable cinema space. 
     Another design approach for providing cinema sound is with array loudspeakers. An array of loudspeakers may have multiple speakers with selective frequency response ranges similar to a home speaker unit with a high, mid, and low-range speaker. However, the unusual degree of beam width confinement and control required for successful implementation of an array of loudspeakers to function as a unified signal source presents additional design challenges. Furthermore, array loudspeakers are unable to compensate for phases between the different loudspeaker signals and are unable to control the vertical off-axis angle at which the summation between the signals is greatest. 
     Thus, a need exists for a loudspeaker system that is smaller than a conventional horn design yet provides the frequency pattern control of the horn design and the selective frequency responses of array loudspeakers to satisfy the size, coverage and power requirements of a cinema or theater. 
     SUMMARY 
     The loudspeaker system of the invention is a mid-range array loudspeaker for use in cinema or theater loudspeaker array systems. The mid-range array loudspeaker is designed as an acoustic waveguide loaded array of loudspeaker drive units that provides uniform loudness and full mid-frequency range sound coverage to the listening regions of the cinema or theater. 
     The mid-range array loudspeaker is comprised of multiple drivers positioned in a waveguide unit. By using multiple drivers, the size of the drivers may be smaller than those found in conventional mid-range array loudspeakers, thereby reducing power requirements, heat generation and the overall size of the loudspeaker. Further, the mid-frequency array loudspeaker of the invention not only has a shallow profile, not exceeding 18 inches in depth, but also provides substantially constant beam width down to a designated frequency, such as 250 Hz. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a front view of the mid-range array loudspeaker of the invention. 
         FIG. 2  is a cross-sectional side view of a four-element vertical stack mid-range array loudspeaker taken along line a—a of  FIG. 1 . 
         FIG. 3  is a block diagram of a filtering network for the driver of the mid-range array loudspeaker of  FIG. 1 . 
         FIG. 4  is an electrical diagram of a passive circuit implementation of the filtering network of  FIG. 3 . 
         FIG. 5  is an acoustical frequency response transfer function graph of the filtering network of  FIG. 4 . 
         FIG. 6  is a phase transfer function graph of  FIG. 5 . 
         FIG. 7  is a graph showing polar horizontal directivity of a mid-range array loudspeaker of  FIG. 1  taken at frequencies ranging from 200 Hz–400 Hz. 
         FIG. 8  is a graph showing polar horizontal directivity of a mid-range array loudspeaker of  FIG. 1  taken at frequencies ranging from 500 Hz–1 kHz. 
         FIG. 9  is a graph showing polar horizontal directivity of a mid-range array loudspeaker or  FIG. 1  taken at frequencies ranging from 1.25 kHz–1.6 kHz. 
         FIG. 10  is a graph showing polar vertical directivity of the mid-range array loudspeaker embodiment of  FIG. 1  taken at frequencies ranging from 200 Hz–400 Hz. 
         FIG. 11  is a graph showing polar vertical directivity of the mid-range array loudspeaker embodiment of  FIG. 1  taken at frequencies ranging from 500 Hz–1 kHz. 
         FIG. 12  is a graph showing polar vertical directivity of the mid-range array loudspeaker embodiment of  FIG. 1  taken at frequencies ranging from 1.25 kHz–1.6 kHz. 
         FIG. 13  is a graph with curves showing −6 dB horizontal and vertical beam width coverage versus frequency, based on data of  FIGS. 7–13 . 
         FIG. 14  is a flowchart of the steps for generating cinema sound with the mid-range loudspeaker module of  FIG. 1  and drivers of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is front view of an example implementation of a mid-range array loudspeaker  100  of the invention. The mid-range array loudspeaker  100  illustrated in  FIG. 1  is designed for use in cinema and theater loudspeaker array systems; however, the mid-range array loudspeaker may also be utilized for other applications. 
     As illustrated in  FIG. 1 , the mid-range array loudspeaker  100  is an acoustic waveguide unit  102  having foul transducer drivers  104 ,  106 ,  108  and  110 . The four transducer drivers  104 ,  106 ,  108  and  110 , commonly referred to as drivers, are typically round units ranging from approximately 6.5 to 12 inches in diameter and are mounted on the rear of the acoustic waveguide unit  102 . The cone of each driver  104 ,  106 ,  108  and  110  provides a separate waveguide for each driver  104 ,  106 ,  108  and  110 . The cones of each driver are then integrated into common waveguides  112 ,  114  and  116 . The waveguides  112 ,  114  and  116  together form part of the overall waveguide unit  102 , which functions to uniformly radiate the energy of the acoustical sources with the plurality of drivers  104 ,  106 ,  108  and  110  and generate a frequency response from approximately 250 Hz to approximately 1.5 kHz. 
       FIG. 2  is a cross-sectional side view of the mid-range array loudspeaker taken along line a—a of  FIG. 1 .  FIG. 2  illustrates the integration of the cones and drivers into the waveguide unit  102 . For the upper and lower drivers  104  and  110 , respectively, the exterior walls  203  of waveguide unit  102  form the exterior surfaces of the waveguide for drivers  104  and  110 , respectively. Vanes  112  and  114  are defined by a rounded nose shape and form the opposing walls of the waveguide unit  102  for the drivers  104  and  110 , respectively. Drivers  106  and  108  are separated from one another by a central vane  116  that is also defined by a rounded nose shape. As seen in  FIG. 2 , the central vane  116  is slightly larger than the vanes  112  and  114 . Vanes  112  and  114  form the waveguide walls opposing the central vane  116  for the waveguide drivers  106  and  108 , respectively, and the walls opposing the exterior walls  203  for waveguide drivers  104  and  110 , respectively. 
     Each driver is mounted to the backside of the waveguide unit  102  of the array loudspeaker  100 . As illustrated by  FIG. 2 , the mounting surfaces for the drivers  104 ,  106 ,  108  and  110  are not perpendicular to the face of the waveguide unit  102 , but are tilted from vertical to optimize the defined coverage. The mounting surfaces are titled such that each driver has a design axis angle that is aimed downward from the nominal on-axis angle by approximately 5 degrees. In other embodiments, the nominal on axis angle may be greater or less than five degrees. In yet other embodiments, the nominal on axis angle may be zero degrees. 
     When using 6.5 inch drivers, the center-to-center spacing dimension “d 1 ” for the upper and lower driver pairs  104 ,  106  and  108 ,  110  is approximately 7.75 inches, and the spacing dimensions “d 2 ” for drivers  106  and  108  is approximately 11.25 inches. Dimension “d 3 ”, the setback of vane  116  from the front plane, is approximately 3 inches, and dimension “d 4 ”, which is the setback of vanes  112  and  114  from the front plane, is approximately 6.5 inches. 
       FIG. 3  illustrates a block diagram of a filtering network for the drivers of the mid-range array loudspeaker  100  shown in  FIG. 1 . The low pass filters  302  and  306  receive an electrical signal from the input “IN”. After the electrical signal has been transferred through a common electrical node to the plurality of low pass filters, the electrical signals are filtered by the low pass filters into filtered electrical signals. Thus, the one electrical signal is routed among the multiple paths created by the low pass filters  302  and  306 . The electrical signal through the lowest frequency path is commonly referred to as the lower mid-range signal and is passed directly to an associated output. The output is in communication with the lowest mid-range driver. 
     The other path passes the filtered electrical signal from the low pass filter  302  through an all-pass filter  304  to an upper mid-range output. Tile all-pass filter  304  functions as a frequency dependent phase delay device that introduces a frequency dependent phase delay between the low pass filter  302  and the upper mid-range output that compensates for the different phases between different loudspeakers (drivers, horns, and waveguides). The upper mid-range outputs are each similarly connected to an associated upper mid-range driver. 
     The network of low pass filters and all-pass filters may be increased in number with in multiple upper mid-range outputs. However, the lowest mid-range output passes only through an associated low pass filter  306 . Further, amplifiers (not shown) may be placed in the electrical signal path prior to the electrical signals being, sent to the different drivers. The filtering network may be implemented with either analog or digital circuitry, and may be inserted either before or after the power amplifiers that provide the electrical signals to the midrange drivers. 
     Although  FIG. 3  represents block  304  as all-pass filter, in an alternate embodiment, the frequency dependent phase delay device that introduces a frequency dependant phase delay may be a delay line. In yet another embodiment, all-pass filters and delay lines may be used to introduce the frequency dependent phase delay. Thus, the electrical input signal that results in the upper mid-range signal would pass through a low pass filter  302  and a delay line or delay line and all-pass filter. The delay line could be digital or analog and optionally implemented at a low signal level followed by power amplification. Further, the frequency dependent phase delay may be introduced by a combination of all-pass filters and delay lines within the same array loudspeaker. An alternate implementation may also be accomplished totally or in part by a delay caused by the physical location of the appropriate transducer driver element with regard to setback from the front plane of enclosure and the other elements. 
     Turning now to  FIG. 4 ,  FIG. 4  is an electrical diagram  400  of a passive circuit implementation of the filtering network shown in  FIG. 3 . An input “IN” is connected to an inductor L 1    402  that is connected to another inductor L 2    404  and a capacitor C 1    406 . The two inductors L 1    402 , L 2    404  and capacitor C 1    406  are configured to function as a low pass filter (represent by block  302  in  FIG. 3 ). The output terminal of inductor L 2    404  is connected to a capacitor C 2    408 , which is connected to one end of an inductor L 3    412  and an output terminal “OUT”. The opposing end of the inductor l 3    412  is connected to a ground and to one end of a capacitor C 3    414 . The other end of capacitor C 4    414  is connected to another output “OUT” and inductor L 4    410 , which is connected to a ground. The configuration of capacitor C 2    408  and C 3    414  along with inductor L 3    412  and L 4    410  is commonly know as an all-pass filter (represented by block  304  in  FIG. 3 ). Another inductor L 5    416  is connected at one end to the input “IN” and inductor L 1    402 . The other terminal of inductor l 5    416  is connected to inductor L 6    418  and capacitor C 4    420 . The two inductors L 5    416 , L 6    418  and capacitor C 4    420  form a second low pass filter (represented by  306  in  FIG. 3 ) 
       FIG. 5  is a frequency response graph  500  showing the resulting acoustic response of the filtering network with an all-pass filter of  FIG. 4  when used with the mid-range array loudspeaker  100  of the invention. The graph has three curves “C”  502 , “U”  504  and “L”  506  that illustrate the acoustical frequency in dB SPL (Sound Pressure Level). Curve “U”  504  is the transfer curve for the frequency response over from the upper mid-range drivers  104  and  106  while curve “L”  506  is the transfer curve for the frequency response over from the lower mid-range drivers  108  and  110 . Curve “U”  504  emphasizes the full mid-range with high frequency outputs, while curve “L”  506  shows the narrower bandwidth due to attenuation at the high frequency end. The combined curve “C”, shown as a dashed line, indicates the overall acoustical summation of the frequency response curve “U”  504  and curve “L”  506  for the entire mid-range module extending from about 150 Hz to 1.3 kHz. 
       FIG. 6  is a phase transfer function graph  600  of  FIG. 5 . This graph further illustrates the effect that the all-pass filter  304  has on the upper mid-range frequency band. The upper line  602  is an approximate upper mid-range frequency driver acoustic phase response without the all-pass filter  304 . Line  604  is the lower midrange frequency driver acoustic phase response. The third line  606  is the upper midrange frequency driver acoustic phase response with an all-pass filter. Together, the three lines  602 ,  604 , and  606  demonstrate that the all-pass filter is compensating for phase but not magnitude, i.e. the phase is independent of magnitude. The upper mid-range frequency acoustic phase with the all-pass filter approaches the ideal case where the phase response of the upper mid-range  606  and lower mid-range frequency  604  driver acoustic phase response are significantly closer. Thus, the maximum summation at the target vertical angle and phase compensation may be achieved. 
       FIG. 7  is a graph showing polar horizontal directivity of a mid-range array loudspeaker  100  of  FIG. 1  taken at frequencies ranging from 200 Hz–400 Hz. The graph has four plots taken at one-third-octave frequency ranges (200 Hz, 250 Hz, 315 Hz, and 400Hz) with no screen deployed. Each radial step is 6 dB magnitude as indicated, so the −6 dB beam width in degrees of each curve is indicated by the crossings of the −6 dB circle by each curve. 
       FIG. 8  is a graph showing polar horizontal directivity of a mid-range array loudspeaker  100  of  FIG. 1  taken at frequencies ranging from 500 Hz–1 kHz. The graph has four plots taken at one-third-octave frequency ranges (500 Hz, 630 Hz, 800 Hz, and 1 kHz) and the 500 Hz being one-third-octave from the 400 Hz of  FIG. 7 . Each radial step is 6 dB magnitude as indicated, so the −6 dB beam width in degrees of each curve is indicated by the crossings of the −6 dB circle by each curve. 
       FIG. 9  is a graph showing polar horizontal directivity of a mid-range array loudspeaker  100  of  FIG. 1  taken at frequencies ranging, from 1.25 kHz–1.6 kHz. The graph has plots taken at one-third-octave frequency ranges of 1.25 kHz and 1.6 kHz. The 1.25 kHz plot is one-third-octave from 1 kHz of  FIG. 8 . Each radial step is 6 dB magnitude as indicated so the −6 dB beam width in degrees of each curve is indicated the crossings of the −6 dB circle by each curve. 
     As illustrated by  FIGS. 7–9 , the coverage in the horizontal direction is relatively constant. The coverage in the present embodiment is maintained from 200 Hz up to 1.6 kHz in the horizontal direction. Further, the results of the graphs  700 ,  800  and  900  demonstrate that the coverage of the loudspeaker array has the desirable 90-degree coverage in the horizontal direction. 
       FIG. 10  is a graph showing polar vertical directivity of the mid-range array loudspeaker  100  taken at frequencies ranging from 200 Hz–400 Hz. As in  FIGS. 7–9 , the plots are taken at one-third-octave frequency ranges at 200 Hz, 250 Hz, 315 Hz, and 400 Hz. The five-degree downward aiming of the mid-range loudspeaker drivers  104 ,  106 ,  108  and  110  of  FIG. 1  is evident. 
       FIG. 11  is a graph showing polar vertical directivity of the mid-range array loudspeaker  100  of  FIG. 1  taken at frequencies ranging from 500 Hz–1 kHz. The plots are taken at one-third-octave frequency ranges at 500 Hz, 630 Hz, 800 Hz, and 1 kHz. With the 500 Hz plot being a one-third-octave higher that the 400 Hz plot of  FIG. 10 . The five-degree downward aiming of the mid-range loudspeaker drivers  104 ,  106 ,  108  and  110  of  FIG. 1  is still evident. 
       FIG. 12  is a graph showing, polar vertical directivity of the mid-range array loudspeaker  100  of  FIG. 1  taken at frequencies ranging from 1.25 kHz–1.6 kHz. The plots are taken at one-third-octave frequency ranges at 1.25 kHz and 1.6 kHz. With the 1.25 Hz plot being a one-third-octave higher that the 1 kHz plot of  FIG. 11 . The five-degree downward aiming of the mid-range loudspeaker drivers  104 ,  106 ,  108  and  110  of  FIG. 1  is still evident. 
       FIG. 13  is a graph  1300  with curves showing −6 dB horizontal and vertical beam width coverage versus frequency, based on the data of  FIGS. 7–13 . The graphs demonstrates the beam-width characteristics of the described mid-range array loudspeaker and demonstrates how the plurality of drivers and mid-range waveguide unit shape the vertical polar acoustical response to maintain substantially constant vertical beam-width within a predetermined frequency range of the mid-range array loudspeaker. Further, the substantially constant vertical beam-width is shown to be within a 50 degrees arc in  FIGS. 7–13 . 
       FIG. 14  is a flowchart  1400  of the steps for generating cinema sound with the mid-range array loudspeaker of  FIG. 1 . The steps start  1402  with the electrical signal being routed to a plurality of low pass filters that are in separate frequency bands and include a lowest mid-range frequency  1404 . The routing is accomplished by a common electrical node that has the electrical signal entering the electrical node and multiple paths out of the electrical node to the low pass filters. Each of the low pass filters is in a separate frequency band. In an alternate embodiment, more than one low pass filter may be combined within a frequency band. 
     The electrical signals exiting the electrical node are then filtered with the low pass filter in each of the plurality of frequency bands  1406 . If the frequency band is not the lowest mid-range frequency  1408 , then the plurality of signal frequency band is modified by an all-pass filter  1410 . After the frequencies are modified by the all-pass filters, they are provided to a driver that generates an audio frequency in an associated waveguide  412 . If the frequency band is the lowest mid-range frequency  1408 , then the filtered electrical signal of the lowest mid-range frequency is provided to a diver that generates an audio frequency in an associated waveguide  1412 . The audio frequencies are then adjusted by the waveguides  1414 . The process is shown, as stopping in step  1416 , but in practice the process may be continuous as long as an electrical signal is present. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.