Abstract:
A performance venue with a main speaker system is provided having a target coverage area within the performance venue. A performance area is located within the performance venue outside the target coverage area of the main speaker system. A low frequency directed audio source is positioned with respect to the performance area for active cancellation of low frequency sound spilling over into the performance area from the main speaker system. Control of the low frequency directed audio source is effected by an audio sensor is located in the performance area for generating an output correlated with low frequency spill over into the performance area.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to active noise reduction and more particularly to suppression of low frequency spill over in a wide field, live performance type venue, through destructive interference of the spillover. 
     2. Description of the Problem 
     High fidelity sound systems for large auditorium and free field applications are readily made directional in the middle and upper frequencies. This helps direct sound energy toward the intended audience. However, most loudspeaker units perform increasingly as omnidirectional or monopole devices as the sound frequency reproduced decreases. This relatively difficult to direct low frequency, long wavelength sound energy can spill over into undesired areas, such as a stage set up in the field of view of the audience and on which an open microphone may be located. Where an open microphone is present spillover can lead to feedback, limiting the allowable gain from the microphone. On stage performers can find the spillover low frequency noise a distraction and highly misleading as to the character of the sound in the intended area of coverage. 
     Active sound cancellation is a developing field using destructive interference to produce a null sound field. The selective cancellation of low frequency sound would be made easier by availability of a directed beam low frequency device capable of relatively high levels of power output. 
     SUMMARY OF THE INVENTION 
     The invention provides an efficient, endfire array operating in conjunction with a conventional sound system to provide selective cancellation of spillover sound from the conventional sound system, particularly at frequencies where the conventional sound system functions as a monopole device. In a performance venue a main speaker system is provided having a target coverage area within the performance venue. A performance area is located within the performance venue outside the target coverage area of the main speaker system. A low frequency directed audio source is positioned with respect to the performance area for active cancellation of low frequency sound spilling over into the performance area from the main speaker system. An audio sensor is located in the performance area for generating an output correlated with low frequency spill over occurring with the performance area. A control system for the main speaker system and the low frequency directed audio source uses the output of the audio sensor as a control input. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1A  is a side schematic of a performance venue constructed with the sound system of the present invention. 
         FIG. 1B  is a top plan schematic of the performance venue of  FIG. 1A . 
         FIG. 2  is a perspective view of an endfire bass loud speaker array. 
         FIG. 3  is a cutaway view of the endfire bass loud speaker array of  FIG. 2 . 
         FIG. 4  is a cross sectional view of an endfire loudspeaker array usable in the invention. 
         FIG. 5  is a block diagram schematic of the control system for the loudspeaker system of the invention. 
         FIG. 6  is a polar graph of the sound field generated by the directed low frequency audio source used in the loudspeaker system of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and in particular to  FIGS. 1A-B , there is illustrated a performance venue  10  having a sound system  12 . Sound system  12  includes a main loudspeaker system  14  used for high fidelity sound reproduction and a directed low frequency audio source  16  used for selective, active cancellation of low frequency spillover from the main loud speaker system into a desired null zone  20 . 
     Performance venue  10  is intended as a location where audiences gather for live performances. An active zone  18  for coverage by main loudspeaker system  14  intersects an audience seating area (not shown) from which the audience can observe performers located on a stage or similar performing area corresponding to the null zone  20 . The oral and instrumental parts of performances within the null zone  20  are intended to be picked up using microphones, including what is termed here as an audio sensor  24 , amplified and directed into the audience over main speaker system  14 . In order to allow maximum gain to be applied to the sound pick up in the null zone  20  it is undesirable that sound from the main speaker system  14  be audible in the null zone  20 , since doing so could result in undesirable feedback and further because the inherent delay would be disruptive to many types of performances, particularly performances of contemporary music. 
     Ideally main speaker system  14  is highly directional, with a generated sound field corresponding to the active zone  18 . In actual application the coverage of an “active zone” is frequency dependent and tends to spread uncontrollably from conventional speaker arrays at the lowest frequencies (particularly below 250 HZ where most speaker systems become monopoles, that is, undirected). Thus, while for some performances a null zone  20  may be a natural result of the directionality of the main speaker system  14 , for other performances, particularly those with a strong bass component, the null zone may have to be generated, for example by physical variation of the performance venue  10 , or the gain of the microphones  24  limited. Of course, it is not always possible to control the physical aspects of the performance venue  10 , particularly if sight lines are to be kept open. Selective active sound cancellation is used in the present invention to produce a null zone  20  of selected size at the desired location. The present invention uses a directed low frequency sound source  16  to effect cancellation of spillover low frequency sound from the main speaker system  14  to produce the null zone  20 . A consequence of using active cancellation is an intermediary fringe zone  22  between the null zone  24  and the active zone  18 . The depiction of the fringe zone  22  is not intended to suggest that its existence is desirable, just that it is a byproduct of the process described here. 
     In order to produce a null zone  20  of a desired size and location it is highly beneficial to have a directed low frequency sound source  16 . Such is provided here by an endfire array of woofers as described below. 
       FIG. 2  illustrates a directed low frequency sound source  16  which generates a sound field which is strongest centered on a primary propagation axis A extending from one end of the device. Sound source  16  comprises a cannister  26  which houses the woofer array of the source. Sound is emitted from circumferential slots  28  located spaced from one another at locations along the primary propagation axis A. In a preferred embodiment the spacing between slots  28  increases from one to the next along the primary propagation axis A. 
       FIG. 3  illustrates the internal arrangement of an exemplary directed low frequency sound source  16 . A stack of four woofers  30 A-D, all centered on and aimed along the primary axis A of sound propagation are provided. Woofers  30 A-D are preferably progressively spaced in the primary direction of sound propagation. If the spacing between woofers  30 A and  30 B is “x”, than the spacing between woofers  30 B and  30 C is “2x” and between  30 C and  30 D is “3x”. Each woofer  30 A-D is located to radiate between a sealed backchamber ( 42 ,  43 ,  44 ,  45 ) and a front chamber ( 32 ,  34 ,  36 ,  38 ). Appropriate internal baffles ( 31 ,  33 ,  35 ,  37 ,  39 ,  40  and  41 ) are provided to form walls of the back and front chambers so that the backchambers are equal in size to one another and the front chambers are equal in size to one another. Slots  28  from the front chambers should no more than half the size in area as the diaphragm of the respective woofers  30 A-D. This achieves compression throat loading improving device efficiency. The longer the array can be made, preferably by inclusion of ever greater numbers of woofers, effects ever longer frequencies of sound which can be actively cancelled. The total length of the device (i.e. the distance separating the first and last woofers in an array) defines the lowest frequency which can be cancelled. 
     It is not essential that the spacing between adjacent woofers be increased with each successive woofer moving in the direction of intended primary radiation.  FIG. 4  illustrates a directed low frequency sound source  16  in which the spacing between adjacent woofers  130 A-E is equidistant. Slots  128  are equidistantly spaced, but otherwise the device is essentially similar to the device of  FIG. 3 , incorporating sealed back chambers  139 ,  140 ,  141 ,  142  and  143 , and front chambers  132 ,  134 ,  136 ,  138  and  137 . Performance for array  126  for a unit of identical length using increasing spacing between loudspeakers is somewhat improved, but at the expense of including a larger number of loudspeakers. 
     The back chambers of either device are sealed but tuned via volume and free air resonance to the driver. The front chambers are also tuned via volume, but ported by the slots. It is expected that the end fire arrays, mounted in cylinders, would be built with 4 to 8 inch diameter cone transducers, though any conventional acoustic transducer could be used. An effective bandwidth from 40 HZ to 1 KHZ is anticipated. Substantial feedback rejection is anticipated. 
       FIG. 6  illustrates an ideal two lobed sound field produced by an endfire array of woofers with a deeper, wider array being propagated in forward direction, and a narrow, shallow lobe in the reverse direction. Where the array is flown in a hall it is expected that the reverse lobe will be directed into the ceiling. 
       FIG. 5  is a block diagram of a control system  50  for a sound system  10 . Control system  50  includes inputs from a performer(s)  17  and feedback from both the endfire array  16  and the main speaker system  14 , all shown as directed through a microphone (mike/sound sensor)  24 . The output of microphone  24  is coupled back to a main channel digital signal processor (DSP)  52  and a cancellation channel digital signal processor (DSP)  54 . DSP  52  provides audio engineering control of mix and gain for optimal propagation of the sound in a venue based on venue parameters and possibly variables such as audience size. 
     The main channel of the sound amplification and reproduction from DSP  52  to the main speakers  14  is completely conventional and is depicted at a high level. The sound is divided into three bands (high, middle and low) by applying the signal to appropriate band pass filtering stages  56 ,  58  and  60 . The output of the high frequency band pass stage  56  is applied to delay and phase adjustment stages  62  and  68  (and possibly an amplification stage, not shown) before application to appropriate drivers in the main speaker system  14 . The output of the high frequency band pass stage  56  is applied to delay and phase adjustment stages  62  and  68  (and possibly an amplification stage, not shown) before application to appropriate drivers in the main speaker system  14 . The output of the medium frequency band pass stage  58  is applied to delay and phase adjustment stages  64  and  70  (and possibly an amplification stage, not shown) before application to appropriate drivers in the main speaker system  14 . The output of the low frequency band pass stage  60  is applied to delay and phase adjustment stages  66  and  72  (and possibly an amplification stage, not shown) before application to appropriate drivers in the main speaker system  14 . A low pass filter  74  is illustrated as interposed between the main speakers  14  and microphone  24 . This is not intended to imply that an electronic circuit element is located here, but to reflect the physical effects of location of the microphone/audio sensor  24  outside the main coverage zone of the main speakers  14  but in a low frequency spill over area relative to the main speakers  14 . 
     The drive circuitry for the end fire array/LF directional sound source  16  is essentially the same as that for the main speakers  14  except that a dynamic delay stage  78  is applied to the drive signal. The delay is varied on the basis of a difference signal generated by a differential amplifier  96 , which compares the low frequency components of the processed drive signal from DSP  54  and LF bandpass filter  76  and the output of a low frequency band pass filter  94  taken directly from the output of microphone  24 . In other words the dynamic delay stage  78  introduces a delay based on what is occurring on stage and what is input to the main channel. A distinct drive signal is produced for each driver of the end fire array  16 . These signals have a fixed delay and phase adjustment (delay stages  80 ,  82 ,  84  and  85 , phase adjustment stages  86 ,  88 ,  90  and  92 ) relative to one another based on the physical parameters of the end fire array  16 . End stage amplification (stages  93 ,  95 ,  97 ,  99 ) is provided as required by the venue (e.g. distance from the stage). 
     The invention provides active noise cancellation for selected zones of a performance venue, typically the stage areas, where the potential for feedback is strong and the possibility of disruption of the performance is strong. 
     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.