METHOD AND APPARATUS FOR IDENTIFYING THE SOURCE OF AN IMPULSIVE OR EXPLOSIVE EVENT

Abstract: This invention consists of a system with the capability of acquiring an acoustic signature with high precision, which can be used to identify artillery or other sources of explosive shock waves from their acoustic signatures. It includes two high-performance microphones (20, 22), one of which acts as a triggering microphone (22). This microphone (22) is harboured within a special chamber (12) containing a floating panel or wall (14), which is inserted in such a way that it is free to vibrate slightly. Any impulsive acoustic signal causes an internal impulse pressure to be built up, causing data acquisition to commence. The anti-alias filtered, digitised signal from the measurement microphone (20) is then processed in such a manner that certain parameters can be extracted. The most significant of these parameters are the "Modified Mach Number", the "Sound Intensity Level", the "Acoustic Shock Wave Energy Potential" and the "Decision Pattern" and two- or three-dimensional "Decision Function", which can be used to identify the origin of the explosive or impulsive shock wave.

Description: METHOD AND APPARATUS FOR IDENTIFYING THE SOURCE OF AN IMPULSIVE OR EXPLOSIVE EVENT The present invention relates to a method and system for identifying the source of explosive or impulsive shock waves, such as artillery, from their acoustic signatures. In the past, various attempts have been made to identify sources of explosions, such as artillery, from their acoustic signatures. These have involved the use of various transducers, hardware and software to acquire and analyse the explosive shock waves. However, poor performance was obtained for the following reasons: signals were not acquired with sufficient precision, partly because of inadequate hardware and partly because environmental conditions such as higher wind speeds can greatly reduce the signal-to-noise ratio of recordings, thereby causing extreme difficulty in the subsequent processing of these recordings; bulky and expensive equipment was used, which complicated and lengthened implementation in the field; continuous recording required time-consuming manual event pinpointing prior to pre-processing. The present invention seeks to provide improved identification of the source of an explosive or impulsive event. According to an aspect of the present invention, there is provided a method of identifying the source of an explosive or impulsive shock wave event including the steps of recording the event by converting acoustic sound pressure to an electrical signal using a sound pressure sensor, within a frequency range including at least part of the audio range, calculating at least sound energy level and the acoustic speed of the wavefront, and identifying therefrom the source of the explosive or impulsive event. The frequency range is preferably from zero Hertz up to several thousand Hertz, more preferably from 0 Hertz to 10,000 Hertz and most preferably from 0 Hertz to around 20,000 Hertz. The actual range of frequencies chosen for measurement depends upon factors such as the types of events sought to be identified and the accuracy of the identification required. Therefore, the preferred ranges given above could be changed, for example to start above zero Hertz, such as from a few tens of Hertz. According to another aspect of the present invention, there is provided a system for identifying the source of an explosive or impulsive shock wave event including a sound pressure sensor operable to record the event by converting acoustic sound pressure to an electrical signal, the sound pressure sensor being operable within a frequency range including at least part of the audio range, and processing means operable to calculate at least sound energy level and the acoustic speed of the wavefront and to identify therefrom the source of the explosive or impulsive shock wave event. The sound pressure sensor is preferably able to measure from a zero Hertz up to several thousand Hertz, more preferably from 0 Hertz to 10,000 Hertz and most preferably from 0 Hertz to around 20,000 Hertz. However, since the actual range of frequencies chosen for measurement depends upon factors such as the types of events sought to be identified and the accuracy of the identification required, the sound pressure sensor could measure a different range, for example starting above zero Hertz, such as from a few tens of Hertz. The inventors have discovered that the energy content at higher audio frequency ranges contains important features about the shock wave event, even for signals that have suffered attenuation over distance. According to another aspect of the present invention, there is provided a method of acquiring the signature of an explosive or impulsive shock wave event by delaying any data acquisition until triggering of a trigger device activated only by sound pressure from explosive or impulsive shock waves. According to another aspect of the present invention, there is provided a system for acquiring the signature of an explosive or impulsive shock wave event including a trigger device activated only by sound pressure from an explosive or impulsive shock wave, data acquisition means, and control means operable to activate the acquisition means only upon triggering of the trigger device. The trigger prevents the recording of unwanted sound waves, that is any sound waves not associated with an explosive or impulsive shock wave event. Continuous measurement results in time-consuming manual event selection prior to pre-processing. This makes it practically impossible to provide real-time or substantially real-time identification of the source of an explosive or impulsive shock wave event. By contrast, adoption of the trigger device enables the exclusive recording of events of interest, resulting in a very substantial reduction in the amount of data recorded and therefore the possibility of providing real-time or substantially real-time identification of the source of an explosive or impulsive shock wave event. In the preferred embodiment, the trigger device comprises a chamber having a rigid or substantially rigid floating wall or panel within one wall, within which chamber there is provided a microphone. The floating panel or wall is caused to vibrate by the sound impulse produced by an explosive or impulsive shock wave event impinging upon it. The sound field in the chamber is a combination of the direct sound field from the source, transmitted through the entire surface of the chamber, the reverberant field and the emitted sound from vibration of the panel comprising a part of the surface of the enclosure. In the case of explosive waves, both the directly transmitted sound and the sound emitted from the panel to the microphone will have an impulsive nature. This will greatly enhance the sound field, and only in this case will the microphone apply a sufficiently high voltage to initiate data acquisition. Thus, the system will only commence data acquisition when triggered by an explosive wave. According to another aspect of the present invention, there is provided a trigger operable to initiate data acquisition of an explosive or impulsive shock wave event, including a chamber comprising a rigid or substantially rigid floating panel or wall and a sound pressure sensor within the chamber. The sound pressure sensor is preferably a microphone. The present invention can be used to identify sources of explosive or impulsive shock waves, such as artillery, from their acoustic signatures. The preferred embodiments incorporate both hardware and software to accomplish the tasks of digitally acquiring an acoustic signature with the required level of precision and then to analyse it to produce "decision patterns" and "decision functions" which can be used to identify the source of the explosive or impulsive shock wave. Such a source might be the gun muzzle "break" or the "burst" (fall of shot) from an exploding shell. For such sources, the preferred system enables such factors as the charge weight and type of gun to be determined. The preferred methods lend themselves to effecting identification in real-time. It is envisaged that embodiments of the invention can provide a practical solution to identifying the sources of explosions with a high degree of confidence and possibly using equipment which is more portable, more quickly deployed and more cost-effective than has previously been the case. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an embodiment of triggering chamber; Figure 2 is a schematic diagram of the principal components of the preferred embodiment of system for identifying the source of an explosive or impulsive shock wave; Figure 3 shows an example of a two-dimensional decision pattern producible by the system of Figure 2; Figure 4 shows an example of a three-dimensional decision pattern producible by the system of Figure 2; Figure 5 shows examples of three-dimensional decision patterns producible by the system of Figure 2 for two different types of artillery; and Figure 6 a two-dimensional pattern grouping decision function for different types of artillery. Referring to Figure 1, there is shown an embodiment of triggering device 10 which is formed of a chamber 12 of substantially rectangular cuboid shape. An open end of the chamber 12 is provided with a floating rigid or substantially rigid panel or wall 14, referred herein as an excitation or triggering panel. A supporting framework 16 supports the excitation panel 14 in a floating manner, and can take many forms, such as a channel which provides a loose fit of the panel 14 therein, a deformable support such as a viscoelastic material, or any other suitable form. The excitation (triggering) panel 14, typically of glass or other material with sufficient stiffness and density, is chosen to respond to explosive or impulsive shock waves. It is loosely supported within its framework in such a way that it is free to vibrate. Within the chamber 12, in the preferred embodiment, there is provided a sound pressure sensor, in this example a microphone (not shown), described in further detail below. The actual structure, shape and size of the triggering device 10 can be chosen by simple experiment by the skilled person in dependence upon the particular application. A cubic shape is the simplest to manufacture. As a guide, a chamber 12 whose internal distance between its walls of 10 to 100 times the diameter of the microphone provides good triggering. Further details of the triggering chamber are given below. Figure 2 shows the preferred embodiment of identification system. The system includes two transducers, typically microphones, 20, 22, the first of which, transducer 20, is the measurement transducer, while the second transducer 22 is located inside the trigger chamber 12 to act as the trigger sensor. The transducers are preferably high precision, advantageously condenser microphones and most preferably electret-condenser high-precision microphones. The analogue signal received by the two transducers 20, 22 is passed to a digital signal processing (DSP) chip 26 and an analogue-to-digital converter 28 via anti-aliasing filters 24. The initial part of the digitised signal held in memory 32 is added to the main body of the digitised signal 30 to form the digitised time-history data file 34. This is passed to processing units 36 to 46, described in more detail below. In use, acoustic sounds are converted, in this embodiment, into electrical voltages using the electret-condenser high-precision microphones 20, 22. Two microphones are used, one acting as a trigger microphone 22 (positioned inside the triggering chamber 12), the other acting as the measurement microphone 20. There is no acquisition of data from the measurement microphone 20, apart from a short period of "pre-trigger" data constantly replaced, until the triggering microphone 22 has received a triggering signal. Thus, there is no need for large memory space for storing continuous streams of data, most of which would be redundant. Any output from the triggering microphone 22 is passed to the DSP unit 26, which commences acquisition if the signal it receives is of a sufficient voltage level. Following this, the output from the measurement microphone 20 is acquired and digitised by the analogue-to-digital (A/D) converter 28. The initial (pre-triggered) part of the signal (i.e. that which is acquired before the trigger microphone triggers data acquisition) is then added from memory 32 to the beginning of the main digitised time history. A Modified Mach Number 36, as well as other key parameters, is then calculated, using selected portions of the digitised time history signal, as is described in detail below. Following this operation, time windowing is applied and overlaid FFTs 40 are performed. Sound intensity over a predefined frequency bandwidth is then calculated. Finally, a three-dimensional "decision pattern" 44 and "decision function" 46 are produced, showing how certain key parameters compare with those from various types of artillery. From these, the source of the acoustic shock wave signal of interest can be identified. The preferred embodiment of signal processing is now described in detail. The preferred methodology can be considered as including three distinct processes: data acquisition, data analysis and results processing. Data acquisition Acoustic sounds from artillery are first converted into electrical voltages using sensitive high-precision transducers, typically electret-condenser microphones 20, 22. Such microphones have a flat frequency response over the audio frequency range. They also have the required sensitivity. The outputs from the measurement microphone 20 and triggering microphone 22 are initially routed through anti-aliasing filters 24. A special feature of this embodiment is the "pre-triggered acquisition", allowing the inclusion of the initial part of the signal from the measurement microphone 20 (i.e. that which is acquired before the trigger microphone 22 triggers data acquisition). The excitation (triggering) panel or window 14 is loosely supported within its framework in such as way that it is free to vibrate. At any point within the chamber 12, the sound field is a combination of the direct field of the source, the reverberant field and emitted sound from panel 14 vibration at the surface of the chamber 12. A portion of the direct field sound from impulsive waves plus the emitted sound from the excitation panel will enter the chamber 12 and be reflected within it. This creates a reverberant field. Thus, the total sound energy measured at a point within the chamber 12 is the sum of the sound energy due to the direct field and that due to the reverberant field. The advantage of this excitation (triggering) panel or window 14 set-up is to prevent the system being triggered either by strong winds or other non-impulsive acoustic waves, such as high level noise from passing lorries. Consequently, only "good" signals are acquired, and this is of great benefit because data sorting does not have to be effected at a later stage. Such time-saving is particularly important when considering real-time implementation. If the energy densities of the direct and reverberant fields are added at any point, an estimate of the total acoustic energy density is obtained. Thus, in principle, the sound pressure level at that point due to the combined effect of the direct and reverberant sound field is: Lp=Lw+ 10Logl0(D + 4/R) (1) where Lw is the sound pressure level generated by transmission from a source, D is a characteristic factor related to the position and direction of microphone arrangement within the chamber and R is the room/chamber constant. With the microphone gain set accordingly, data acquisition via any microphone 22 inside the chamber 12 can thus only be triggered by explosive or impulsive shock wave events. The data recorded during this fixed short time duration can be then added to the main body of recorded data (time history) after recording has ceased. Data acquisition requires the employment of dedicated Digital Signal Processing (DSP) device 26. An example would be the Texas TMS series. A sample and hold circuit along with an analogue-to-digital (A/D) converter 28 is provided to enable data acquisition and digitisation. As soon as the DSP device 26 receives a positive voltage of a certain level from the trigger microphone 22, data acquisition using the measurement microphone 20 commences for a pre-determined time period. Data analysis The complete time history 34 is then subjected to various computerised mathematical data analysis operations in order to extract certain key parameters from it, the most important being a new parameter termed the "Modified Mach Number", or "M-Mach number". A unique feature of the preferred embodiment is the derivation and subsequent application of the "M-Mach number" . Mach number represents the non-dimensional speed of shock wave front propagation, being a function of the sound over-pressure level of the shock wave and of atmospheric pressure as well as temperature. The definition of wave number is k=Cp/Cv (specific heat of constant pressure and constant volume, which are functions of absolute temperature). Mach number = function(£,Patm,SPLove.) = fiιnction(Temperature,Patm,SPLove.) (2) The Mach number of a shock wave is also a function of temperature, atmospheric pressure and measured over-pressure level at the receiving point. It represents the unique acoustic characteristics of the resulting shock front and is affected by many factors, such as the distance between receiving point and the centre of source of sound, temperature, atmosphere pressure and other meteorological conditions. In principle, Mach number is a dimensionless ratio N/a, the speed of the wave front (V) over the speed of sound (a). A mathematical equation of Mach number in terms of peak sound pressure of the blast wave front can be deduced from analysing the pressure change when an air stream flows through the stepped wave front. The larger the Mach number, the greater the likelihood that pressure variation in the flow will cause significant density changes. At low Mach number, the pressure change will be normally insufficient to cause the density to change appreciably, and the flow will behave as if it is incompressible. Then the speed of the wave front becomes the speed of sound. From analysis of the pressure jump from the shock wave front, the Mach number can be deduced as: Mach = (3) where p over is the over-pressure. The Modified Mach number (M-Mach) can be interpreted as being proportional to the force necessary to stop the fluid over the force necessary to compress it by a certain fraction. It may be defined as: (4) where Mach ref = 10x10 Mach ref is based on the smallest number the DSP system 26 can measure (that is, the system resolution). From the above analysis, it can be seen that the level of M-Mach indicates the proportion of force that compresses the air ahead of the blast wave front and overcomes the force for stopping the blast wave propagation. This is a function of the total energy of the blast wave and the inertia of the wave mass. Although different types of shell explosion, field-gun shots or 'breaks' can have a similar level of M-Mach number, it has been found that blast waves with lower total energies will degenerate differently than those with higher ones. Since the impulsive signal from artillery is very short, time windowing is utilised to eliminate noise from the rest of the record, thus improving the signal/noise ratio of the result. In order to obtain the energy distribution over a frequency range, overlaid Fast Fourier Transform (FFT) analysis of the short transient signal is then applied 40. This time-dependent frequency analysis splits the signal into overlapping segments. The result is A(t, f) in both the time and frequency domain: A(t,f) = FFT{P t (t)} P{t) = Time history (5) Results processing This process generates 44, 46 two-dimensional "decision patterns" and both two- and three-dimensional "decision functions". A time average of the overlaid FFT 40 is calculated over a defined frequency bandwidth, from which the Sound Intensity Level (SIL) is deduced, as follows: P Total = SIL Total W ref = \Q .--1 l 2 z w.m In the equations below, SILi is the calculated sound intensity level for a frequency range from f to (fi + Δf), that is over a predefined bandwidth: Af = f™ x f min , i = 1, 2, 3... n • SPLover: 20 times the logarithm of the ratio between measured sound pressure and reference sound pressure. The measured sound pressure Pover is called the sound over pressure, which is the absolute sound pressure minus the atmospheric pressure. • Pover: the absolute sound pressure minus the atmospheric pressure. • Patm: atmospheric pressure. The application of sound intensity level over a given frequency bandwidth of an impulsive wave is another important feature of this embodiment. For following parameters or features can be computed and used in the formation of "decision patterns" and "decision functions": • Modified Mach number: represents the acoustic speed of the wave front of a shock wave; • Sound Intensity Level within a narrow frequency bandwidth: the total sound energy (as a time average) within that frequency bandwidth; • Decision Pattern: the level of sound energy distribution over a frequency bandwidth. The energy distribution pattern reflects the characteristics of acoustic signals from different types of guns/explosions; • Frequency Bandwidth Number: taken as i, where f + Δf - fi (see integral in the above equation); • Acoustic Shock Wave Energy Potential: SILi (see the above equation); • Decision Function: this is deduced from the Decision Pattern. Its role is to aid in the identification of type of guns/explosions. Automatic artillery classification can be effected if based upon this function. The "decision pattern" and "decision function" plots, described in further detail below, can be compared with those from known sources of explosions, such as various types of artillery. Thus, the source of the acoustic signal under scrutiny can be identified. Referring now to Figure 3, a two-dimensional "decision pattern" is shown for three different types of artillery. The decision patterns consist of a series of markers. The Y co-ordinate of any marker represents its sound energy level within that frequency band. Joining all the markers with a straight line forms a decision pattern for that type of artillery event. The x co-ordinate of a marker will depend upon its Modified Mach number. The magnitude of the Modified Mach number indicates either the artillery charge weight or the distance from which the event has emanated. Figure 4 shows two and three-dimensional "decision functions", in which the 2-D decision function is based upon the third Frequency Bandwidth Number of the first of the two 3-D decision functions shown. The two axes are Acoustic Shock Wave Energy Potential and Modified Mach Number. 2-D plots of this kind assist in the discrimination process. Here, different event types are clearly represented as bands or groups. 3-D Decision Function Plots give a good overall view. It can also be seen that different artillery types can be grouped. The three axes for these plots are Acoustic Shock Wave Energy Potential, Modified Mach Number and Frequency Bandwidth Number. This methodology can be used to discriminate between gun/artillery fire ("breaks") and fall of shot ("bursts") from individual types of artillery, as well as other sources of explosions. Figures 5 and 6 show examples of two- and three-dimensional decision pattern producible by two different types of artillery. The described system can be used to identify other acoustic signatures, such as those from small-arms, air/sea/ground vehicles; indeed of any device or event which produces an impulsive sound. It is also possible to enhance the system to provide location of the origin of an impulsive sound and thus the artillery, vehicle and so on from which it originated. This can be achieved using any of the known principles for geographical location currently known. Indeed, since the preferred embodiments allow for real-time identification of the source of an impulsive sound, its location can also be determined in real time, which can be a substantial advantage in some situations. In summary, the described embodiments provide a system, consisting of hardware and software, with the capability of acquiring an acoustic signature with high precision and ultimately producing "Decision Patterns" and "Decision Functions", which can be used to identify artillery or other sources of shock waves from their acoustic signatures. They include two transducers (typically high-performance microphones, such as condenser microphones), one of which acts as a triggering microphone. This microphone is harboured within a special enclosure containing a panel or wall, which is inserted in such a way that it is free to vibrate slightly. A combination of the direct field of the source, the reverberant field and the emitted sound from panel vibration at the surface of the enclosure corresponding to a certain voltage threshold will cause triggering and data acquisition to commence. The anti-alias filtered, digitised signal from the (other) measurement microphone is then processed in such a manner that certain parameters can be extracted. The most significant of these parameters are the "Modified Mach Number", the "Sound Intensity Level", the "Acoustic Shock Wave Energy Potential" and the "Frequency Bandwidth Number". These may be plotted to produce a two-dimensional "Decision Pattern" and two- or three-dimensional "Decision Function", which can be used to identify the origin of the shock wave. The disclosures in British patent application no. 0022488.1, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims: CLAIMS 1. A method of identifying the source of an explosive or impulsive shock wave event including the steps of recording the event by converting acoustic sound pressure to an electrical signal using a sound pressure sensor, within a frequency range including at least part of the audio range, calculating at least sound energy level and the acoustic speed of the wavefront, and identifying therefrom the source of the explosive or impulsive event. 2. A method according to claim 1, wherein the frequency range measured is from a few Hertz to several thousand Hertz. 3. A method according to claim 1 or 2, wherein the frequency measured is from around zero Hertz up to several thousand Hertz, from around 0 Hertz to around 10,000 Hertz or from around 0 Hertz to around 20,000 Hertz. 4. A method according to any preceding claim, including the step of providing a measurement trigger which causes initiation of data acquisition of the sound pressure of an explosive or impulsive shock wave event, measurement of the sound pressure of the event being carried out immediately following triggering. 5. A method according to any preceding claim, including the step of determining a modified Mach number associated with measured sound overpressure. 6. A method according to any preceding claim, including the step of determining sound intensity level from measured sound pressure. 7. A method according to claim 5 and 6, including the step of determining a decision pattern on the basis of the determined modified Mach number and the determined sound intensity level. 8. A method according to claim 7, including the step of determining a decision pattern on the basis of the determined modified Mach number, the determined sound intensity level and the acoustic shock wave energy potential. 9. A system for identifying the source of an explosive or impulsive shock wave event including a sound pressure sensor operable to record the event by converting acoustic sound pressure to an electrical signal, the sound pressure sensor being operable within a frequency range including at least part of the audio range, and processing means operable to calculate at least sound energy level and the acoustic speed of the wavefront and to identify therefrom the source of the explosive or impulsive event. 10. A system according to claim 9, wherein the sound pressure sensor is able to measure from zero Hertz up to several thousand Hertz, more preferably from 0 Hertz to 10,000 Hertz and most preferably from 0 Hertz to around 20,000 Hertz. 11. A system according to claim 9 or 10, including a measurement trigger device arranged to be activated only by sound pressure from an explosive or impulsive shock wave. 12. A system according to claim 9, 10 or 11, including processing means operable to determine one or more of a modified Mach number associated with the measured acoustic data, a sound intensity level of the measured acoustic data and acoustic shock wave energy potential. 13. A system according to claim 12, wherein the processing means is operable to determine a decision pattern on the basis of two or more of the determined modified Mach number, the determined sound intensity level and acoustic shock wave energy potential. 14. A method of acquiring the signature of an explosive or impulsive shock wave event by delaying any data acquisition until triggering of a trigger device activated only by sound pressure from explosive or impulsive shock waves. 15. A system for acquiring the signature of an explosive or impulsive shock wave event including a trigger device activated only by sound pressure from an explosive or impulsive shock wave, data acquisition means, and control means operable to activate the acquisition means only upon triggering of the trigger device. 16. A trigger operable to cause initiation of data acquisition of an explosive or impulsive shock wave event, including a chamber comprising a rigid or substantially rigid floating panel or wall and a sound pressure sensor within the chamber.

Inventors: DREW RODERICK CHARLES, LI MENG, DREW, RODERICK, CHARLES, LI, MENG
Applicants: DREW RODERICK CHARLES, LI MENG, SERCO LTD, DREW, RODERICK, CHARLES, LI, MENG, SERCO LIMITED
Application year: 2001
Main classification: G01V1
IPC codes: F41H11, G01S5, G01V1