Abstract:
A single folded, expanding horn loudspeaker reproduces low frequency audible sound at high power output levels. A compact enclosure houses a plurality of identical transducers, characterized by small vibrational surface areas. The throats for each transducer into the horn are acoustically differentially spaced from the mouth of the horn. Transducer drive circuitry adjusts a drive signal for the transducers to compensate for the different impedance and propagation time to the horn mouth at each throat.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The invention relates to an electro-acoustical device and, more particularly, to a folded horn loudspeaker for reproducing low frequency audible sound at high output levels from electric-acoustic transducers having relatively small diaphragms.  
         [0003]     2. Description of the Problem:  
         [0004]     The reproduction of low frequency audible sound, with high fidelity and at high intensity levels, poses a number of challenges. To do so from a small, energy efficient package, portable enough to be moved and suitable for open air use is especially difficult. Generally, high output, high efficiency, low frequency loudspeakers have been built around a horn. A horn is in effect an acoustic transformer, allowing the designer to obtain the output performance of a driving unit having a large area diaphragm from a unit having a smaller area diaphragm while minimizing cone/diaphragm resonance issues that exist with direct radiator devices. Increasing the effective diaphragm area renders radiation impedance increasingly resistive with the result that increasing power is radiated at the desired low frequencies. However, increasing acoustic power output from most horn designs has required increasing diaphragm piston travel in order to move the required volume velocity of air. Piston travel becomes an important limiting factor relating to the amount of power that can be delivered to the horn.  
         [0005]     Another limitation on the total energy input that can be introduced to a horn has been the limited scalability of horns. Though examples of multiple driver horns are known, typically only a single driving unit for a given frequency range has been provided. One example of a multiple driver horn (U.S. Pat. No. 5,898,138) positions a pair of low frequency transducers having throats located equidistant from the horn&#39;s mouth. While effective such an arrangement is still not readily scalable.  
       SUMMARY OF THE INVENTION  
       [0006]     According to the invention there is provided a folded, expanding horn loudspeaker having a selectable plurality of acoustic drivers for a given frequency range. The loudspeaker unit provides a compact enclosure defining the folded, expanding horn and housing the acoustic drivers. A scalable number of identical acoustic drivers is provided, each having a relatively small cone or diaphragm, and each being located in a sealed back chambers (i.e. a closed box baffle). The acoustic drivers radiate into volumetrically identical high pressure chambers located in front of the drivers. The acoustic drivers are preferably positioned with respect to one another in a linear array and may vary in number from 2 to 12. Each high pressure front chamber is coupled to a summing throat for the horn by an extended port which operates as a air pressure or air volume velocity step up transformer. The outlets of the ports are acoustically spaced from one another and differentially spaced from the mouth of the horn. Transducer drive circuitry applies drive signals to the acoustic transducers derived from a common source. The signals to the respective acoustic transducers are delayed to reflect the distance of the throats for the respective acoustic transducers from the mouth of the horn. The source signal is also as filtered and phase adjusted as required for clear reproduction of the sound.  
         [0007]     Additional effects, features and advantages will be apparent in the written description that follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a perspective view of a loudspeaker enclosure;  
         [0010]      FIG. 2  is a perspective view of the loudspeaker horn.  
         [0011]      FIG. 3  is a cross sectional view of the loudspeaker enclosure of  FIG. 1  taken along section line  3 — 3 .  
         [0012]      FIG. 4  is a cross sectional view of the loudspeaker enclosure of  FIG. 1  taken along section line  4 — 4 .  
         [0013]      FIG. 5  is a cross section of a transducer housing taken along section line  5 — 5  in  FIG. 3 .  
         [0014]      FIG. 6  is a rear elevation of the enclosure of  FIG. 1  with the back panel of the enclosure removed.  
         [0015]      FIG. 7  is a block diagram schematic of drive circuitry for the loudspeaker.  
         [0016]      FIG. 8  is a block diagram schematic of the operation of the circuitry of  FIG. 7 .  
         [0017]      FIG. 9  is a side elevation of a high pressure chamber exited by paired drivers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring now to the figures and in particular to  FIG. 1  there is illustrated a loudspeaker system  10  for use as a high output, transportable unit. Loudspeaker system  10  comprises a right trapezoid enclosure or cabinet  11  which houses sound radiators and a folded waveguide or horn having a mouth  12  in front wall  14 . Mouth  12  directs sound radiated from loudspeaker system  10  forward from the unit. Enclosure  11  is constructed from front wall  14 , a back wall, a first side wall  16 , a second side wall (described below), a cover  18  and a base  20 . The bases and walls are conventionally made of plywood or some other material which does not readily absorb sound.  
         [0019]     Referring to  FIG. 2 , enclosure  11  is presented in phantom at a reverse angle from the view of  FIG. 1 . Folded horn  22  is optimized for low frequency use and is constructed from flat sides and incorporates a flair, as is conventional. Folded horn  22  is disposed along side walls  16 ,  17  and the back wall  15  of enclosure  11  which form portions of the horns walls. Folded horn  22  expands cross-sectionally along its entire length from a base end  161 , adjacent which the horn has its minimum cross-sectional area, to end  163  where the mouth of the horn is located. Folded horn  22  expands initially both vertically and horizontally, but eventually only in the horizontal dimension. A summing throat  61  is disposed along side  17  which expands in both the vertical and horizontal directions to a fold  151 , where it matches with a first backside section  121 . Backside section  121  continues to expand in two mutually perpendicular directions up to a second backside section  123 . Section  123  is characterized by horn  22  having a constant vertical dimension, however expansion continues in the horizontal dimension at a rate consistent with the horn&#39;s flair constant. Vertical expansion is stopped in second backside section  123  not for functional reasons, but for external packaging reasons. Section  123  meets the final horn section  125 , which continues to expand in the horizontal dimension, along fold  153 . Loudspeakers are nestled in the pocket  200  formed by and partially enclosed by the exterior faces of folder horn  22 .  
         [0020]      FIG. 3  is a cross sectional view of enclosure  11  taken along section line  3 - 3  in  FIG. 1 . Four walls form the perimeter, exterior sides of enclosure  11  including front wall  14 , first side wall  16 , back wall  15  and a second side wall  17 . The perimeter formed by these walls is broken only by mouth  12  which provides a radiating outlet from the waveguide, i.e. folded, expanding horn  22 . Folded horn  22  comprises four major sections and two bends or folds and, as described above, a rectangular cross sectional shape. A horn flare is provided by increasing the area of the section with distance through the horn  22 . Initially, the cross-sectional dimensions of folded horn  22  increase in both the vertical and horizontal dimensions, but eventually only in the horizontal. Folded horn  22  includes a summing throat  61  into which four ports or extended throats  58 ,  60 ,  62  and  64  are directed. Folded horn  22  expands both vertically and horizontally for the entire length of summing throat  61 . Folded horn  22  is divided into two sections  121  and  123  along back wall  15  of enclosure  11 . Section  121  continues the two dimensional cross sectional expansion of fold horn  22  from summing throat  61 . Section  123  expands only horizontally. Running from section  123  to mouth  12  is the final horn section  125 , which also expands only in the horizontal direction.  
         [0021]     Four acoustic drivers or transducers  26 ,  28 ,  30  and  32  are positioned in enclosure  11  (the latitudinal positions of which are illustrated in phantom) and oriented to direct sound downwardly into four high pressure (or preload) chambers  34 ,  36 ,  38  and  40  located directly above base  20 . The upper surface of base  20  forms the bottom surfaces of high pressure chambers  34 ,  36 ,  38  and  40  which are aligned with one another. Acoustically absorbent pads  42 ,  44 ,  46  and  48  are positioned on the upper surface of bottom board  20  within each of chambers  34 ,  36 ,  38  and  40  to deaden resonance. Pads  42 ,  44 ,  46 ,  48  correspond to and are vertically aligned with acoustic drivers  26 ,  28 ,  30 ,  32 , respectively. High pressure chambers  34 ,  36 ,  38  and  40  have acoustic outlet ports formed by extended throats  58 ,  60 ,  62  and  64 , respectively. Extended throats  58 ,  60 ,  62  and  64  direct energy into summing throat  61 . The outlets from extended throats  58 ,  60 ,  62  and  64  act as diaphragms aligned along one side of the summing throat  61  of folded throat  22 . These outlets are at different distances from mouth  12  and, as a consequence, see different output impedances and have different propagation times for the sound energy they emit to mouth  12 . The phase and frequency response of horn  22  will differ with respect to extended throats  58 ,  60 ,  62  and  64 , sometimes in ways difficult to predict in advance for particular horn parameters and thus empirical evaluation may be required to determine the best dynamic phase adjustments, frequency band widths and roll offs to be used with the drive signal for each of the acoustic drivers  26 ,  28 ,  30  and  32 . High pressure chambers  34 ,  36 ,  38 ,  40  each have the same volume as one another and the throats  58 ,  60 ,  62  and  64  have the same cross sectional areas as one another.  
         [0022]     Each extended throat  58 ,  60 ,  62  and  64  has a cross sectional area which is at least 20% of the area of diaphragm for the corresponding acoustic drivers  26 ,  28 ,  30  and  32  and 100% of that area. of the corresponding diaphragms. Preferably the diaphragms of drives  26 ,  28 ,  30  and  32  are each about 3½ times the area of the cross section of the extended throats. As the diaphragms move back and forth in alternating fashion to form compression waves in the air mass, the air in high pressure chambers  34 ,  36 ,  38  and  40  varies in pressure. The extended throats are relatively constricted in area when constructed the preferred ratio and function as pneumatic amplifiers increasing the volume velocity of the air. Accordingly the movement of driver diaphragms  326 ,  328 ,  330 ,  332  can be made much smaller than is the case on the prior art because changes in air pressure in high pressure chambers  58 ,  60 ,  62  and  64  are relatively stiff. At the same time, the high pressure compression chambers  58 ,  60 ,  62  and  64  absorb much more power per unit of movement of the diaphragm allowing much larger driver motors  226 ,  228 ,  230  and  232  to be employed than in prior art devices. These motors may be two to three times as powerful as is conventional, for example, a 6-inch woofer may be driven by a watt drive coil. For maximum power input diaphragm  326 ,  328 ,  330  and  332  may be pushed at velocities up to the point of destructive turbulence in the extended throats  58 ,  60 ,  62  and  64 .  
         [0023]     The high pressure chambers, back chambers, extended throat and summing throat  61  are formed in part by vertical interior walls supported from base  20 . Vertical interior wall  33  provides a portion of one side of folded horn  22  adjacent mouth  12  opposite the side provided by first exterior side wall  16 . Vertical interior wall  33  and side wall  16  diverge from one another toward mouth  12  to provide an expanding cross-sectional area for horn  22 . Vertical side wall  33  also provides an interior wall for each of extended throat  58 ,  60 ,  62 ,  64  and for front chambers  34 ,  36 ,  38  and  40 . The horizontal perimeter of preload or high pressure chamber  34  is completed by vertical walls  25 ,  41  and  70 . The horizontal perimeter of high pressure chamber  36  is completed by vertical walls  27 ,  41  and  72 . The horizontal perimeter of high pressure chamber  38  is completed by vertical walls  29 ,  41  and  74 . The horizontal perimeter of high pressure chamber  40  is completed by vertical walls  31 ,  41  and  76 . Wall  41  is broken in three places by outlets from throat extension guides  61 ,  62  and  64 . The horizontal perimeters of high pressure chambers  34 ,  36 ,  38 ,  40  are broken by throats  50 ,  52 ,  54 ,  56 . The horizontal perimeter of throat extension section  58  is completed by vertical walls  14  and  70 . The horizontal perimeter of throat extension section  60  is completed by vertical walls  25  and  72 . The horizontal perimeter of throat extension section  62  is completed by vertical walls  27  and  74 . The horizontal perimeter of throat extension section  64  is completed by vertical walls  29  and  76 . Vertical walls  33 ,  50 ,  72 ,  74  and  76  all continue upwardly to provide perimeter elements of substantially sealed back chambers (or closed-box baffles) for acoustic drivers  26 ,  28 ,  30  and  32 .  
         [0024]     Referring to  FIG. 4 , which is a cross sectional view taken along section line  4 — 4  in  FIGS. 1 and 2  and to  FIG. 5 , which is view taken into enclosure  11  along section line  5 — 5  in  FIG. 4 , the positioning of acoustic drivers  26 ,  28 ,  30 ,  32  over high pressure chambers  34 ,  36 ,  38  and  40  is illustrated. Acoustic drivers  26 ,  28 ,  30 ,  32  are housed in sealed back chambers  80 ,  82 ,  84  and  86 , respectively. The term “sealed” as used here has its conventional meaning in the acoustical arts to mean that the back chambers have no acoustic outlet port. The only acoustic opening from sealed back chambers  80 ,  82 ,  84  and  86  are those directly in front of the diaphragms of acoustic drivers  26 ,  28 ,  30  and  32 . Back chambers  80 ,  82 ,  84  and  86  do slowly exchange air with their ambient environment, as is conventional.  
         [0025]     In  FIG. 5  the position of extended  60  in front of wall  72  illustrates the interface of a representative high pressure chamber  36  to its extended throat  60  and further into summing throat  61 . Because the upper cover section  91  is not horizontal, but slants upwardly from the base of summing throat  61  toward the back wall  15 , the outlet from extended throats into summing throat  61  differs for each extended throat. Extended throat  60  includes some freeboard on wall  41  above the outlet and below upper cover section  91 . As illustrated in  FIG. 6  and described with reference to the figure below, the amount of freeboard for each port will differ. Acoustic driver  28  rests on a support plane  93 . Sealed back chamber  82 , like the remaining back chambers, is closed on one side by a planer wall  95 .  
         [0026]     Referring now to  FIG. 6 , which is an end view of enclosure  11  with back wall  15  removed, the interior of folded horn  22  is illustrated in greater detail, particularly the summing throat  61 . Summing throat  61  is formed by portions of side wall  17 , cover  91 , base  18  and wall  41 . Summing throat  61  collects sound output from the four throat extension sections  58 ,  60 ,  62 ,  64 , the radiating outlets of which are visible along a side of summing section  61  defined by vertical wall  41 . The surfaces forming summing throat  61  diverge from one another moving toward the back wall  15  from the base of the horn along front wall  14 . The divergence of the upper and lower surfaces of folded horn  22  is provided in the upward slant of board  97 . While the output port from extended throat  58  has a vertical extent substantially equal to the local height of summing throat  61 , the outlets of downstream extended throats  60 ,  62  and  64 , which are all of the same height, will have increasing amounts of freeboard.  
         [0027]     Any given horn has differing horizontal and polar frequency responses. And while a horn may operate well at certain frequencies its performance can degrade markedly at other frequencies. These changes in performance are highly dependent on the length of the horn. While each of transducers  26 ,  28 ,  30 ,  32  is coupled to the folded horn by an identical high pressure chamber and extended throat, the extended throats in  56 ,  60 ,  62  and  64  are coupled to summing junction  61  at points which are differently spaced from the mouth  12 . In other words, horn  22  will have different performance characteristics for each transducer including, different optimal frequency operating range. Accordingly, each driver circuit differentially treats the signal applied to each transducer.  
         [0028]     Producing sound of maximum intensity from loudspeaker system  10  requires that acoustic pressure waves from the outlets of extended throats be synchronized at the points where they merge. Due to the different distances sound travels to reach mouth  12  from the outlets from extended throats  58 ,  60 ,  62  and  64 , the drive signal applied to transducers  26 ,  28 ,  30 ,  32  is time differentiated so that the sound waves constructively reinforce one another in summing section  61  rather than cancel or interfere with one another. While the same signal is the genesis of the signal used to drive each of the four transducers  26 ,  28 ,  30 ,  32 , this source signal must be processed differently before application to the respective transducers&#39; voice coils to assure good phase matching at the mouth  12  and a good match of output from the extended throats  58 ,  60 ,  62  and  64  to the frequency response characteristic of folded horn  22  for a given outlet port from one of extended throats  58 ,  60 ,  62  and  64 . The signal for the transducer associated with the throat radiating end removed by the greatest distance from mouth  12  is delayed least, while the signal driving the transducer associated with the throat radiating end closest to mouth  12  is delayed by the greatest period. Differences in impedance matching of the extended throat for each driver to summing section  61  require some band pass filtering and shading of the source signal for optimal system performance. The source signal may require dynamic phrase adjustment (i.e. adjustment of the signal phase as a function of frequency) of the source signal due to the frequency response characteristics of the horn which vary with frequency at each extended throat outlet port.  
         [0029]     Referring to  FIGS. 7 and 8 , a common source  711  of audio frequency signals is applied to four inputs of a digital signal processor (DSP)  709  which differentially processes the signals to accommodate the relative positions of acoustical drivers  26 ,  28 ,  30 ,  32 . DSP  709  provides the four differentiated outputs on each of four channels  713 ,  715 ,  717 ,  719  to four amplifiers  701 ,  703 ,  705  and  707  associated with acoustical drivers  26 ,  28 ,  30 ,  32 . In general, the input signal is processed in the same general way for all four channels, with only the parameters applied by the processing steps changing. For each channel, the signal is fed through a band pass filter  801  which passes frequency ranges best handled by a particular horn/driver configuration. Typically, the broadest band of frequencies is applied to the acoustic driver couple to the summing junction  61  at the furthest point from mouth  12 . The roll off of the signal range applied to a driver may also be adjusted. Next, the filtered signal is applied to a time delay  803  which synchronizes the signals based on the differing distances of the speakers from the horn mouth. Lastly, the filtered, delayed signal for a channel is applied to a dynamic phase adjustment module  805 , which adjusts the phase of the signal as a function of frequency. The specific parameters used will change along with changes in horn dimensions and the number of transducers used.  
         [0030]      FIG. 9  illustrates an alternative embodiment of the invention whereas two acoustic drivers  930 A and  930 B are coupled to a common high pressure chamber  900 . Sound is coupled from compression chamber  900  to a horn  922  by an extended throat  902  which has a cross sectional area which is a fraction of the area of the diaphragms of acoustic drivers  930 A-B. Extended throat  902  operates as a kind of pneumatic amplifier greatly accelerating air velocity. Still other arrangements of transducers and horn types will now occur to those skilled in the art.  
         [0031]     The invention provides high acoustic output power for low frequency sound from a minimally sized, portable cabinet, suitable for use at outdoor, temporary venues. The package is well suited for bass line reproduction required for rock music reproduction.  
         [0032]     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.