Abstract:
An acoustic impulse source has been developed and used to simulate the highcoustic output of detonation driven acoustic sources. While detonation driven acoustic sources are capable of producing large acoustic signals (&gt;160 dB) at 10s and 100s of meters from the source, their laboratory use is limited due to the combustion byproducts. The present invention is capable of reproducing the downrange detonative source acoustic signals (170 dB) at short range, on the order of 1 meter, without noxious or large thermal byproducts. The present invention relies on the plasma formation resulting from electrical arc discharge in air to generate thermal impulses and the resultant acoustic signals. Utilizing multiple plasma channels and timing control of the formation of said channels, the present invention achieves increased efficiency and the ability to tailor the acoustic output signal to match the desired detonation source characteristic output.

Description:
FIELD OF THE INVENTION 
     Currently, different types of acoustic sources are being developed and tested as non-lethal weapons. Among these weapons are a class of detonation driven acoustic sources which are capable of repetitively generating extremely large acoustic impulses to deter people while avoiding lethality. The acoustic output from detonation driven sources are characterized by a sharp risetime, peaked amplitude and fast decay. These detonation sources generate heat, flames, and noxious gas byproducts as a result of the combustion process. In order to assess the weapons being developed, the acoustic signatures must be simulated in the laboratory to avoid the expense of testing at large outdoor ranges. The present invention was developed to provide the laboratory simulation necessary to assess the capability of impulsive acoustic weapons. By varying electrical parameters and the timing of the plasma jet formations, the acoustic output of the present invention can be tailored to reproduce the subject detonation source signature. The present invention also has applications for; gunfire simulators, dog training aids, fowl and rodent control, ceremonial noisemakers, and others. 
     DESCRIPTION OF RELATED ART 
     Many devices, which rely upon arc discharge, have been conceived to generate acoustic impulses, with a majority of the prior art concerning underwater uses. The arc discharge Electro-acoustic sources found in the literature can be divided into several categories. U.S. Pat. Nos. 3,715,710 issued to Julius et al., 3,879,698 issued to Pepper, 3,879,699 issued to Pepper, and 5,228,011 issued to Owen, relate to acoustic pulse sources which utilize a combination of electrical pulses to produce a net electrical current. The tailored current is used to drive an acoustic transducer to produce the desired acoustic signal. These patents use a spark gap to create the impulse electrical current, however the spark gaps are located external to the acoustic transducer. The energy dissipated in these spark gaps is lost and cannot contribute to acoustic energy conversion. 
     U.S. Pat. Nos. 4,651,311 and 4,706,228 both issued to Owen and Schroeder, relate to acoustic pulse sources in which the geometry of the output transducer is used to tailor the output acoustic signal. 
     U.S. Pat. Nos. 4,734,894 issued to Cannelli et al. and 4,764,906 issued to Clements et al, utilize triggered plasma jet arc discharge to produce acoustic pulses. The referenced patents utilize one plasma jet and rely upon adjustment of electrical parameters (current) to produce variable acoustic output. 
     U.S. Pat. No. 5,398,217 issued to Cannelli et al. utilizes multiple arc discharge output sections and relies upon adjustment of electrical circuit component values to produce the desired acoustic output. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high output acoustic source for simulating the acoustic output of high power detonation driven non-lethal weaponry. The present invention relies on electrical arc discharge to produce intense acoustic pulses. Electric arc discharge is compatible with laboratory testing in that the only hazards (no `hazmats` are used) besides high voltage is; minimal thermal output, intense light flashing (including UV) and ozone production, which are all easily abated in the lab with standard safety procedures. 
     The present invention utilizes one or more high voltage (HV) DC power supplies to charge multiple capacitor banks. Each capacitor bank is connected to one side of a spark gap, the other side grounded. Trigger electrodes are disposed in close proximity to the gap of each spark gap. Each trigger electrode is connected via high resistance cable to a coil, of the type used in automotive ignitions. Each coil is charged via a MOSFET switch connected to a 12-volt battery. The opening of the MOSFET switch, subsequent coil magnetic field collapse and trigger arc output is controlled via a fiber optic control cable. The high resistance output cable and fiber optics serve to isolate the HV system from the triggering/timing system. The high voltage produced by the collapsing coil (40,000-60,000 volts) &#34;over-volts&#34; the air gap causing air break down and current flow across the gap, and plasma forms. The high energy contained in the capacitor banks discharges into the plasma stream causing the plasma to continue, create a large thermal impulse and hence acoustic output. By choosing the proper values of capacitance, resistance and inductance of the HV system, the plasma channel time history can be controlled in amplitude and duration. This plasma signal however does not directly map into an identical acoustic output signal. 
     Because thermal impulses form much slower than the electric arc, the electrical current provided to the plasma channel does not directly translate into an identical acoustic signal. This is further complicated in that air is a nonlinear medium for acoustic impulses. To provide further tailoring of the acoustic output directly, multiple plasma channels are formed at staggered time intervals such that in air the acoustic impulses overlap and add to produce the desired waveshape. That is, the present invention utilizes addition of multiple acoustic output pulses in space. Furthermore, multiple plasma channels increase the net resistance in the gaps (output section) thereby leading to more efficient formation of thermal impulses and hence acoustic output. 
     The electrodes (2 or more per spark gap) forming the spark gap and the trigger electrodes are fixedly mounted in a tube. One end of the tube is open providing and output face. The opposite end of the tube is closed by a moveable back plate and by adjusting the back plate farther or closer to the open end, the tube length behind the spark gaps can be changed. This allows the timing of the thermal pulse reflection from the back plate to be adjusted and hence provides for further tailoring of the acoustic output. 
     The output tube is further provided with an air line to allow pressurized air or other gas (such as nitrogen) to be supplied to the interior of the tube during operation. The supplied gas helps abate the production of ozone, provides cooling for the tube, and acts to purge the tube of ionized air and byproducts. Purging of the tube provides for a more consistent output, particularly during repetitive operation. 
     Accordingly it is an object of the present invention to provide a high output repeatable acoustic signal which is a scaled image of the output from high power detonation driven acoustic weapons. 
     It is a further object of this invention to provide a variable acoustic waveform through time staggered formations of multiple independent plasma channels. 
     It is yet another object of the present invention to provide a remotely operated and electromagnetically isolated multiple triggering and timing system to initiate multiple time staggered plasma formations. 
     It is still another object of this invention to provide an acoustic source with increased efficiency by forming multiple plasma channels in the output section, thereby achieving a net increase in gap resistance, whereby all arc discharges contribute directly to acoustic signal formation. 
     These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed descriptions when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an equipment diagram depicting the major components of the present invention. 
     FIG. 2 is a side view schematic depicting the detail of the output tube of the present invention. 
     FIG. 3 is a front view schematic depicting the detail of the output tube of the present invention. 
     FIG. 4 is a detailed schematic of the triggering/timing system. 
     FIG. 5 is a waveform plot comparison depicting the acoustic output generated from one spark gap and the acoustic output resulting from two gaps. 
     FIG. 6 is a waveform plot comparison depicting the acoustic output from two spark gaps timed to overlay and the acoustic output resulting from three spark gaps with a delayed firing of the third gap. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 depicts the acoustic source tube and supporting controls and supplies. For illustration only, three stages (three plasma channels) are shown. The heart of the present invention lies in the source tube 8. High voltage power supplies 2, supply DC voltage to a charging network, one per stage, consisting of resistors 5 and capacitors 6. Upon electrical arc initiation, the discharge circuit consists of capacitors 6, inductors 7 and any resistance provided by cables, connections and the inductors. Fresh gas 3 is supplied to the tube 8 to provide cooling and to purge the tube of ionized gas and debris. Trigger generators 4 receive trigger signals from the trigger/timing control 1 and generate high voltage impulses to `over-volt` the spark gaps in tube 8. A signal line connects trigger control 1 and the high voltage supplies 2 and communicates the &#34;full charge&#34; signal from the supplies to the trigger control. The trigger control inhibits triggers until the high voltage supplies indicate a full charge condition. 
     Detailed in FIG. 2 is the output tube 8. The tube housing 16 is preferably constructed of ceramic material because of its&#39; thermal stability and electrical isolation. On the open end of the tube is a mounting flange 17 that provides mounting means for a horn or other acoustic antenna. The closed end of the tube contains means for accepting a threaded rod 15, which enables the back plate 13 to be moved within the tube. The back plate is sealed to the inner diameter of the tube via O-ring 14. A fitting 12 is attached to the tube housing allowing gas transport from a gas line to the interior of the tube housing. For illustration, three spark gaps are shown in FIG. 2, each consisting of a positive electrode 9, a negative electrode 10 and a trigger electrode 11. A front view of tube 8 is shown in FIG. 3 and depicts the layout of a spark gap. A connector 18 is provided on the trigger electrode for connection of a high impedance cable. 
     FIG. 4 is a detail of the trigger control and timing system. The trigger controller 1 sends a trigger fire signal to each of the spark gap channels. Each trigger fire signal may be individually delayed from the next and is sent via fiber optic signal cable, which isolates the trigger system from the high voltage system and associated electromagnetic noise. Fiber optic receivers 19 receive the fire signal and energize the gate of a MOSFET for predetermined period of time and then release the gate. When on, the MOSFET shorts to ground causing current flow in the primary coil of transformers 18 from a 12 volt battery supply. As the MOSFET turns off, the circuit opens and the magnetic field in the primary of the coil collapses, transferring energy to the coils secondary winding. The high voltage output from the transformers is conducted to each trigger electrode 11, via a high impedance cable 20. The high impedance cables serve to isolate the trigger system from the high-energy discharge in the tube during spark gap break down. 
     Referring to FIGS. 1-4 in total, a &#34;shot&#34; or series of shots is requested through the trigger/timing control 1, and the power supplies 2 are allowed to charge to their fill charge setting (front panel selectable). The capacitors 6 are charged to the power supply voltage level setting through resistors 5 until full charge is achieved. At full charge, spark gaps electrodes 9 `sit` at the power supply voltage with reference to electrodes 10 (ground). The distance between electrodes 9 and electrodes 10 must be chosen such that the voltage potential between the two will not &#34;self break&#34;. Upon receiving a full charge status, the trigger/timing control issues a trigger signal to each of the spark gaps at the appropriate delay time entered for each channel. Each fiber optic receiver 19 receives its trigger signal and energizes the gate of a MOSFET for a predetermined time. The energized MOSFET shorts the primary of transformers 18 to ground allowing current flow through the primary from a 12-volt battery and thereby saturating the transformer&#39;s magnetic core. As the receiver releases the gate of the MOSFET, and the circuit opens, the magnetic field in the core collapses causing a amplified (transformed) voltage on the transformer&#39;s secondary. 
     The transformers secondary voltage is supplied to the trigger electrodes 11 situated between the spark gap electrodes. The trigger voltage (40-60 kV) is sufficient to &#34;over volt&#34; the spark gap and an arc occurs between the spark gap electrodes. The arc current forms the plasma and thermal heating of the air causes a pressure expansion. This pressure pulse propagates out of the open end of tube and constitutes the acoustic signal. The arc current continues for a period of time determined by the time constant of the charge capacitance 6, the inductance 7 and the resistance in the plasma channel, and has a damped sinusoidal shape. The acoustic pulse is however, for each plasma channel, a well-defined impulse. After a shot has been fired, another shot may be fired immediately thereafter, limited only by the high voltage DC power supplies&#39; rate of recharge, time constant of the discharge circuit, and the recombination time of ionized air in the tube. 
     The waveforms in FIG. 5 are the acoustic output measured at 1 meter from the tube output. The microphones used to measure the acoustic signals are inverting, the impulses are actually positive pressure. The data is has not been scaled or corrected for calibration but a direct comparison of the two waveforms is valid. The lower trace (trace 2) is the acoustic output generated from one spark gap using 10 kV power supply charge, 12 uF capacitance and no inductors. The upper trace (trace 1) is the acoustic output resulting from two spark gaps (timed to overlay and add acoustic amplitude), 10 kV supply charge, no inductors and 6 uF capacitance per spark gap. Given the same supply charge and total capacitance, the two gap acoustic signal was approximately 12% higher in amplitude. This efficiency increase is due to the resistance in the second gap doubling the net plasma resistance. The increased resistance causes more energy dissipation (heating) in the plasma zone and yields higher acoustic output. 
     The waveforms in FIG. 6 are a comparison of acoustic output for a three spark gap excitation. The upper trace (1) shows the acoustic output (raw microphone data) due to three spark gaps, 12 kV supply charge, no inductors, and 4 uF capacitance per each stage. The three gaps in trace 1 were timed to overlay acoustically and create an amplified peak on the impulse. The lower trace (2) shows the acoustic signal resulting from two gaps timed to overlay and the third gap delayed in time. Trace 2 shows the initial peak resulting from two gaps, a second peaked structure approximately 100 usec later. A small third structure is shown and is the result of the third gap reflection from the back plate. Although trace 2 has lower amplitude than trace 1, as expected, the pulse width (zero crossing) on trace 2 is approximately 50 usec greater than that of trace 1. Although the resultant waveform shown in trace 2 is jagged, it is obvious that with more spark gaps and smaller timing delays between each gap, a smooth lengthening of pulse width can be achieved as nulls in the waveform are filled in. By timing some gaps to be coincident at a point in space, the resultant acoustic amplitude may be adjusted, and by timing some of the gaps to be noncoincident at the same point in space, the resultant acoustic signal pulse width may be adjusted. By adjusting the capacitor values, the inductor values, the DC power supply voltage level and the relative timing of all the spark gap discharges; the resultant acoustic signal can be tailored to simulate the desired detonative source acoustic signal. 
     If the spark gaps are located too close together, the discharge from a preceding gap can ionize the air in the tube and cause succeeding gaps to self-break prematurely. In cases where long time delays are needed the required spacing between the spark gaps may led to an unacceptably long output tube. Additionally, during repetitive use, heating of the tube and electrodes may prove problematic. In these cases, multiple tubes may be used with a single spark gap arrangement. Where a large number of spark gaps are desired for simulation fidelity, multiple tubes with multiple spark gaps per tube are beneficial. 
     In light of the present disclosure, one skilled in the art can envision numerous uses, benefits, modifications, and embodiments of the present invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein and not depart from the true spirit and scope of the present invention and the appended claims.