Abstract:
A broadband sound generator and transmitter provides minimal attenuation of sound over the distance between the generators and a point at a selected distance. The transmission component includes a parabolic dish and a positionable framework for the sound generators. The sound generators are positioned in front of the dish and oriented to direct sound into the dish for reflection toward a target. Drive signal conditioning circuitry apportion components of the drive signal to the several sound generators and adjust the signal in terms of delay and phase to accommodate changes in position of the generators relative to the dish.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to directional loud speaker systems and more particularly to a acoustic source for delivering intense sound energy to a location spaced a substantial distance from the source. 
     2. Description of the Problem 
     A wide variety of acoustic transducers capable of absorbing substantial input energies to produce intense sound fields are available. Directional control of the sound produced and limiting the attenuation of sound field intensity may be effected using a number of types of enclosures and horns and careful positional arrangement of the transducers with respect to one another. The application of the sound system guides selection and blending of these techniques. Some systems, for example those intended for music, should minimize distortion. Many music amplification systems will limit themselves to use of an enclosure and a baffle around the transducers. A public address system tolerates some distortion, particularly at higher frequencies. This favors the use of a high degree of directional control to reduce the rate of drop off in sound pressure with increasing distance from the source. In a public address system it is common for the transducer to be horn loaded. 
     Of particular interest here is the possibility that a sound system can be adapted for use in the management of crowds or of individuals. It is well known that sound can be intensive enough to be disabling without threatening permanent injury. Were it possible to deliver a sound field of sufficient intensity to disable a person at a distance, or force his retreat, direct physical interaction between those charged with control of crowds, or limiting access to a facility, would be made easier. Such control would also appear far less dramatic and provocative to onlookers and those seeing recordings of the events on television. 
     Naturally it would of advantage to make such a system mobile. This factor dictates that the system be highly efficient and that sound generated by the system have a minimal drop off in intensity with distance. The directional control of the sound should also be high. The ability to optimize the sound field for the range to a target would also be of advantage. 
     SUMMARY OF THE INVENTION 
     The invention provides a broadband sound generator and transmitter. Sound generation is provided by a low frequency range transducer and a higher frequency range transducer array. The sound generators are located forward from a concave reflecting surface which has a forward radiant axis. The low frequency range transducer is located on the radiant axis and the higher frequency transducer array is located radially distributed about the forward radiant axis. The transducer and the transducer array are movable along the forward radiant axis to vary the focal point of sound radiated by the transducers into the concave reflecting source. A broadband input signal used to excite the transducers is applied to the transducers through signal conditioning circuitry connected between an input signal source and the transducers. The signal conditioning circuitry includes a cross-over module apportioning selected frequency components of the input signal between first and second channels, and phase and differential delay components adjusting for the changes in spacing between the transducers and the concave reflective surface. 
     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. 1  is a side elevation of a broadband sound projector as taught by the present invention suitable for transport on a vehicle. 
         FIG. 2  is a partial cutaway view of the sound generating and transmitting apparatus of the preferred embodiment of the invention. 
         FIG. 3  is a diagram depicting convergence of the sound field generated by the apparatus of the invention on a target. 
         FIG. 4  is a cross sectional view of the sound generating and transmitting apparatus of a second embodiment of the invention. 
         FIG. 5  is a plan view of a secondary acoustic lens. 
         FIGS. 6A-B  are block diagrams of signal conditioning circuitry for both the preferred and a second disclosed embodiment. 
         FIGS. 7A-C  are side, front and back views of an alternative embodiment of the invention. 
         FIG. 8  is a graphical depiction of the sound attenuation produced by the projector of the present invention versus conventional attenuation. 
         FIG. 9  is a side elevation illustrating an alternative support for a sound projector constructed in accordance with the invention. 
         FIG. 10  is a side view of yet another packaging arrangement for a minaturized version of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and in particular to  FIG. 1  there is illustrated a mobile sound projection system  10  mounted on a Vehicle V. Sound projection system  10  includes a telescoping mast  14  supported on a base  26 . Mounted at the upper end of mast  14  is a broadband sound projector  24 . Broadband sound projector  24  is attached to mast  14  by an altazimuth mounting  20  allowing substantial freedom of positioning of the projector. Manual controls for mast  14  may be located within the Vehicle V. Broadband sound projector  24  supports a range finder and targeting camera  12  aligned with the radiant axis of the projector.  FIG. 2  shows an alternative location for the range finder and targeting camera attached along the perimeter of parabolic reflector  32 . Operator controls may include a television screen allowing identification of a target and controls for aiming a collimated sound field SF from the broadband sound projector  24 . 
     Referring to  FIG. 2 , broadband sound projector  24  is illustrated in greater detail. Broadband sound projector  24  is based on a primary parabolic reflector dish  32  having a front concave reflecting surface  34  with a forward radiant axis A. Forward concave reflecting surface  34  preferably has a parabolic contour. Sound is reflected forward from concave reflecting surface  34  in a collimated sound field SF toward a far focus (shown in  FIG. 3 ) substantially forward from the reflecting surface. Forward from the concave reflecting surface  34 , lying along the forward radiant axis A of the concave reflecting dish  32 , is a loudspeaker enclosure  30  which in turn includes a secondary parabolic dish  46  and a lens cap  35  defining an acoustic cavity  36  (shown in  FIG. 4 ). 
     Sound field SF is fed by low frequency and high frequency acoustical transducers operatively positioned in a spaced relationship in front of the front concave reflecting surface  34  and centered on the forward radiant axis A. The acoustical transducers are mounted in the loudspeaker enclosure  30  and, more specifically, are mounted on a secondary parabolic dish  46  forming the end of enclosure  30  located closer to primary parabolic dish  32 . Low frequency sound is generated by a loudspeaker  40  which is centered on the forward acoustical axis A and oriented to direct sound from its forward side directly into concave reflecting surface  34 . The low frequency sound source is illustrated as a single driver; diaphragm unit, however other elements might be used. For example, the device could have multiple drivers. Higher frequency sound has as a source a plurality of horn loaded tweeters  39  which are disposed on the secondary parabolic dish  46  and oriented outwardly to direct sound toward the primary parabolic dish  32 . Again, other high frequency sound services could be used, e.g. high frequency diaphragm elements. Tweeters  39  are arrayed radially around forward radiant axis A in a circle and the projection axis of the sound they generate is canted outwardly from the forward radiant axis A of the concave reflecting surface  34 . Alternatively, the low frequency device could be a circular diaphragm disposed centered on the forward radiant axis with the HF sources located on or nearer to the forward radiant axis A. Whatever the arrangement, sound from both sets of transducers is reflected forward from concave reflecting surface  34  in a sound field SF collimated around null field NF. As described below, sound field SF slowly closes to a far focus F F  which may be displaced from sound projector  24  by hundreds of meters. 
     Enclosure  30  provides both support for the transducers and a framework  27  for moving the transducers in and out along forward projection axis A relative to concave reflecting surface  34 . By moving enclosure  30  the far focus F F  of the forward reflected sound waves can be changed from tens of meters to hundreds of meters by changing the apparent acoustic source F S  of the sound. Enclosure  30  is supported forward from concave reflecting dish  34  on framework  27  which is mounted to a rim  29  set on the perimeter of primary parabolic dish  32 . The framework includes a plurality of struts  42  extending from the rim  29  forward from concave reflecting dish  34 . Struts  42  converge on a perimeter ring  26  of smaller diameter than rim  27 . Enclosure  30  rides on tracks  38  supported by the perimeter ring  26 . Tracks  38  lie parallel to the forward radiant axis A. Linear motors (not shown) may be used to lock enclosure  30  in place on the tracks  38  and to move the enclosure to and fro along the forward radiant axis A as indicated by double arrow B. Movement of enclosure  30  changes the location of apparent source F S  of sound directed into the concave reflecting dish and also changes the point of convergence of sound field SF forward from the concave reflecting surface  34 . The object is to achieve beam collimation. 
     Conveniently mounted somewhere on the framework  27 , such as depending from rim  29 , is a range finder  12  which may include a television camera and laser range finder. Range finder  12  may also advantageously be mounted in lens cap  35  aligned with the forward radiant axis A of the concave reflecting surface  34 . Acoustic projector  24  is movable as a unit up and down and in a circle using a motorized altazimuth mounting  20  set on the upper end of mast  14 . 
       FIG. 3  illustrates convergence of a sound field SF projected from acoustic projector  24  on a target T located at a position displaced from acoustic projector  24 . The point of convergence or far focus F F  may be changed dynamically for a moving target. 
     Referring to  FIGS. 4 and 5 , details of the mounting of loudspeaker  44  and horn loaded tweeters  39  on the secondary parabolic dish  46  may be seen. Bass or low frequency loudspeaker  44  is located centered in the secondary parabolic  46  and centered on forward radiant axis A of concave reflecting surface  34 . Although only one loudspeaker is shown a plurality of devices could be used. The plurality of horn loaded tweeters  39  are disposed radially from loudspeaker  44 , centered on forward radiant axis A, and canted outwardly from the radiant axis to direct sound toward the outer portion of the concave reflecting surface  34 , or into an adjustable outer circumferential section  55  of the concave reflecting surface, as provided by a second embodiment of the projector and as shown in  FIG. 4 . The HF source need not be horn loaded tweeters and could instead be planar devices, a HF diaphragm, compression driven devices, etc. 
     Shape control of outer circumferential section  55  provides improved efficiency, i.e. reduced attenuation of the higher frequency sound generated by the array of horn loaded tweeters  39  and projected forward by the primary parabolic dish  32 . Where primary parabolic dish  32  is divided into two sections  255 ,  55  the shape of the two sections can be better optimized relative to the predominant frequencies of the sound directed into the respective sections. Shape control of the outer circumferential section  55  is achieved by dividing the outer circumferential section into segments  155  which are independently positionable. (See  FIG. 7 ). Movement of the segments  155  can be made dynamic and is done under the control of shape control circuitry  54  and pneumatic pistons  52 . The input signal to the low frequency loudspeaker  44  and to the array of horn loaded tweeters  39  is processed by signal conditioning circuitry  58  as described below. Positioning control  56  of enclosure  30  is done responsive to target selection by a user. 
       FIGS. 6A-B  illustrate input signal conditioning circuitry  58 ,  158  in greater detail. Generation of a drive signal for the transducers  139  and  144  for the array of horn loaded tweeters  39  and the low frequency speaker  44  may be guided by one of several psychoacoustic objectives. Where the acoustic projector  24  of the invention is intended to alert individuals, a voice signal may be patched to conditioning circuitry. Where crowd control is desired one or more signal types are selected from a table of signals  60  stored in memory  64 . These signals may include large first and third order distortions to produce highly unpleasant or uncomfortable sound which, when combined with high volume levels, is directed to driving people off. The signal conditioning circuitry  58  is intended to allocate components of the signal between the two sets of differing types of loudspeakers, adjust the signal as to delay to optimize reflective efficiency based on distance of the speakers from the concave reflective surface  34 . 
     Generation of sound is initiated electronically upon microprocessor  62  receiving a trigger signal from operator inputs  102 . Simultaneously with receipt of indication from an operator that sound is to be projected, the range to a target identified by the operator is obtained by microprocessor  62  from range finder  68 . Range finder  68  may include a laser distance measuring element for this purpose. Or, a microphone may be built into the system for echo location. Aiming of the primary parabolic dish  32  is done under operator control by inputs from operator inputs  102  directed by microprocessor  62  as position control signals to positioning motors  92 . Where the primary parabolic dish  32  is divided into inner and outer sections shape control of the outer circumferential section  55  is provided by dish shape control  90 . This operation is informed by the frequency mix selected by microprocessor  62 , delay of the signal and the distance to target and may be made dynamic. 
     Microprocessor  62  generates a signal for application to an audio signal source  61  (which may be an output port of the microprocessor). Audio signal source  61  generates a signal which is in turn applied to an adjustable amplifier  70 . Microprocessor  62  controls the output amplitude to achieve an optimal typically non-lethal, sound pressure level at the target distance. The resulting signal is applied to an analog to digital converter  72  and the resulting digital signal is applied to a cross-over circuit  74  which passes selected frequency components to the signals to either of two channels. The channels, of course, correspond to the low and high frequency audio transducers. Each channel comprises four components, connected in series, and under the control of microprocessor  62 . The components are connected, in series and include dynamic delay lines  76 A-B, parametric equalization contour filters  78 A-B, dynamic phase filters  80 A-B and dynamic limiters  82 A-B, in each channel. Operation of these components is under the control of microprocessor  62 , which takes into the account the frequency and phase of the signals and the distance spacing the loudspeakers from the concave reflecting surface  34  to achieve near coherent summing of the signal mix to boost efficiency of the system. Before application of the signals to the respective sets of transducers, the signals are reconverted to analog signals by digital to analog converter  84 . The outputs of converter  84  are amplified by amplifiers  86  and  88  and the respective amplified drive signals are applied to transducer  144 , associated with low frequency loud speaker  44  and to audio transducers  139  associated with horn loaded tweeters  39 . 
     It is not necessary that all loud speakers in an array be driven synchronously. Speaker drive channels can be divided so that groups of speakers, or individual speakers, are independently controlled. Circuitry to effect such operation can take a number of different forms. Similarly, digital signal processors can be programmed in a number of different ways to implement a given equivalent circuit.  FIG. 6B  is a possible implementation of a circuit to differentiate the signals applied to groups of speakers, but is by no means exhaustive of the possible forms such a circuit could take. The circuit of  FIG. 6B  is substantially identical to the circuit of  FIG. 6A , except for the final stages of the high frequency channel. A multiplexor/buffer element  183  is connected to take the output of dynamic limiter  82 B. A control signal from microprocessor  62  may be applied to multiplexor/buffer to direct signals received from dynamic limiter  82 B among one of four channels. In effect, signal source  61  supplies four signals for four groups of H.F. loud speakers in a time division multiplexed format. Mux/buffer  183  operates to space divide the signals among 4 arrays of buffers, the output of which may be sequentially applied to D/A converters  184 A-D for application to amplifiers  244 ,  344 ,  444  and  544 , respectively. Amplifiers  244 ,  344 ,  444  and  544  supply loudspeaker arrays  139 A-D with differentiated signals. Those skilled in the art will now realize that each speaker in the arrays would be individually driven by a separate amplification channel. 
       FIGS. 7A-C  illustrate a two section primary parabolic reflector  732  in accordance with a second embodiment of the invention. An inner parabolic section  255  is centered on the focal axis A and provides a reflecting surface for low frequency sound radiation. Outer parabolic section  55  provides the primary reflecting surface for higher frequency acoustic radiation and is adjustable. A plurality of panels  155  extend radially from inner parabolic section  255 , to which the outer section panels  155  are connected by hinges  65 . Outer section panels  155  swing on hinges  65  between more open positions and more closed positions by use of positioning pneumatic pistons  52 , with at least one being connected between each outer section panel  155  and the inner parabolic section  255 . Individual outer section panels  155  are separated by ribs  66  which extend outwardly from inner parabolic section  255 . 
       FIG. 8  illustrates the reduction in attenuation of sound intensity at distance where attenuation is reduced from 6 DB per doubling of distance to 3 DB per doubling of distance. With an initial intensity of 150 DB sound intensity is still 126 DB at 256 meters instead of 102 DB as would occur with a point source in free space. At the higher intensity levels possible with the invention it is energy efficient to deliver uncomfortable sound to a precise location without use of deadly force and without the need for contact between crowd control personnel and people which are to kept at a distance. 
     Sound projection system  10  may be dismounted from a vehicle and set up as a stand alone unit powered by a local generator or battery (not shown). As illustrated in  FIG. 9 , sound projection system  10  has been mounted by mast  14  on a tripod  900 . A control panel  18  is located nearby for use in aiming the system. 
       FIG. 10  illustrates a hand held unit sound projector unit  810 . The primary parabolic dish  832  is attached at its base to a housing  815  which encloses the signal generating and conditioning circuitry. Transducer arrays  809  are disposed forward from the primary parabolic dish  832 . Visible on the lower portion of housing  815  are a mode selection screen  803 , a mode selection keypad  804 , a battery charge indicator  802 , and a microphone  808  for use when the system is used for public address functions. A handle  820  extends below housing  815  providing a grip for a user allowing easy use of an on/off trigger  807 . A replaceable battery pack  801  attaches to the bottom of handle  820 . 
     The present invention provides a sound system adapted for use in the management of crowds or of individuals. Intensive, highly directed sound may be directed toward an isolated human target and disable or drive away the target without threatening permanent injury. Such a sound field makes it possible to disable a person at a distance, or force his retreat, without direct physical interaction between those charged with control of crowds, or limiting access to a facility, would be made easier. Such control should appear far less dramatic and provocative to onlookers and those seeing recordings of the events on television. 
     While the invention is shown in only a few 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.