System and method for acoustically identifying gunshots fired indoors

Abstract

A system and method for acoustically detecting the firing of gunshots indoors employs multiple microphones (15, 20) which are utilized individually and in combination to detect sounds inside a building or other structure and, upon sensing a loud impulsive sound which is indicative of a gunshot, processing signals from both microphones (15, 20) to determine if the sound is that of a gunshot. The system and method relies on the acoustic signature of the noise as collected, with the acoustic signature being analyzed to arrive at values which are then compared to adjustable levels that signify a gunshot.

Claims

1. A method of acoustically detecting a gunshot comprising the steps of: a) identifying when an incoming acoustic signal sensed with a first microphone, having a low sensitivity, has a peak amplitude level greater than a trigger threshold established for a potential gunshot; b) if a potential gunshot is identified in step a), analyzing output signals sensed by the first microphone corresponding to the potential gunshot to establish operational and nominal threshold values for the first microphone and a second microphone which is more sensitive than the first microphone to determine a maximum amplitude, respectively, for the first microphone and the second microphone, and analyzing the output signals sensed by the first microphone in multiple, distinct frequency ranges to determine a ratio for summation values to enable gunshot false positive identification; c) only if a potential gunshot is identified in step a), comparing a value calculated based on signals from the second microphone corresponding to the incoming acoustic signal, with a threshold value corresponding to a time after the output signals sensed by the first microphone window with respect to a specific time window, wherein the threshold value corresponds to the operational and nominal threshold value for the second microphone to determine sound dissipation of the incoming acoustic signal; and d) determining that an occurrence of a gunshot has been detected based on results from both steps b) and c).

2. The method of claim 1, further comprising: determining a time for the potential gunshot which is prior to a time when the incoming acoustic signal is sensed with the first microphone.

3. The method of claim 2, further comprising: basing the time for the potential gunshot based on amplitudes of signals from the first microphone at multiple, different times.

4. The method of claim 1, further comprising: performing enhanced autocorrelation on signals from the first microphone.

5. The method of claim 4, further comprising: calculating a maximum of the enhanced autocorrelation within a defined frequency range.

6. The method of claim 5, wherein the defined frequency range is between 15 kHz and 25 kHz.

7. The method of claim 1, wherein analyzing signals sensed by the first microphone in multiple, distinct frequency ranges includes calculating a sum of amplitudes in a first frequency range.

8. The method of claim 7, wherein the first frequency range is from 10 kHz to 25 kHz.

9. The method of claim 7, wherein analyzing signals sensed by the first microphone in multiple, distinct frequency ranges further includes calculating a sum of amplitudes in a second frequency range which is lower than the first frequency range.

10. The method of claim 9, wherein the second frequency range is from 2 kHz to 5.5 kHz.

11. The method of claim 10, wherein analyzing signals sensed by the first microphone in multiple, distinct frequency ranges further includes calculating a ratio of the sum of amplitudes in the first and second frequency ranges.

12. The method of claim 1, wherein comparing a value calculated based on signals from a second microphone includes determining a root-mean-square value of signals from the second microphone over a predetermined time period and comparing the root-mean-square value with the threshold value.

13. The method of claim 1, wherein the method is limited to determining the occurrence of a gunshot within a building or other structure.

14. The method of claim 1, further comprising: alerting emergency personnel when the occurrence of a gunshot has been detected.

15. The method of claim 1, wherein the first microphone has a sensitivity of below −40 dBFS.

16. The method of claim 1, wherein the second microphone has a sensitivity that is at least 70% greater than the sensitivity of the first microphone.

17. The method of claim 1, wherein only outputs from the first microphone are initially, continuously analyzed for a peak amplitude level greater than the trigger threshold.

18. A system for acoustically detecting a gunshot within a building or other structure comprising: a sensor including a first microphone having a low sensitivity and a second microphone which is more sensitive than the first microphone; and a controller configured to determine an occurrence of a gunshot within the building or other structure based on signals received from each of the first and second microphones, wherein only if a potential gunshot is identified with the first microphone are signals analyzed from the second microphone; wherein the controller analyzes output signals sensed by the first microphone corresponding to the potential gunshot to establish operational and nominal threshold values for the first microphone and a second microphone which is more sensitive than the first microphone to determine a maximum amplitude, respectively, for the first microphone and the second microphone, and analyzing the output signals sensed by the first microphone in multiple, distinct frequency ranges, and the controller determines a ratio for summation values to enable gunshot false positive identification, and only if a potential gunshot is identified, the controller then compares a value calculated based on signals from the second microphone corresponding to the incoming acoustic signal, with a threshold value corresponding to a time after the output signals sensed by the first microphone window with respect to a specific time window, and wherein the threshold value corresponds to the operational and nominal threshold value for the second microphone to determine sound dissipation of the incoming acoustic signal.

19. The system of claim 18, wherein the sensor further includes a network port configured to connect the sensor to a remote computer.

20. The system of claim 18, wherein the first and second microphones are MEMS microphones.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically indicates structure associated with the sensor of the invention;

(2) FIG. 2 is a flowchart of a calculation algorithm employed in accordance with the invention; and

(3) FIG. 3 is a flowchart of a comparing algorithm employed in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) With initial reference to FIG. 1, a gunshot detection sensor designed for mounting within a building or structure to be monitored for gunshots in accordance with the invention is generally indicated at 5. In the embodiment shown, sensor 5 includes a single computer board 10 linked to a first microphone 15 and a second microphone 20. As depicted, first and second microphones 15 and 20 are preferably arranged orthogonal to each other and connected to a CPU 25 (particularly a multi-core processor for fast signal processing) which is electrically powered, such as through a 5V battery 30, a micro USB port or the like. Also provided as part of sensor 5 is a network connector, such as an Ethernet, USB or the like connection port indicated at 35. At this point, it should be noted that sensor 5 can actually take on various forms while functioning and operating in the manner which will be detailed below. Certainly, it should be recognized that sensor 5 could be electrically powered in various ways, including being electrically hardwired, and need not be network hardwired but rather can incorporate a wireless interface. In general, it is important that CPU 25 is capable of sampling acoustic signals received from both microphones 15 and 20, specifically at a minimum of 192 KHz.

(5) In the most preferred form of the invention, each microphone 15, 20 constitutes a MEMs microphone which is omnidirectional. In accordance with the invention, one microphone 15 has a low sensitivity while the other microphone 20 is more sensitive. In accordance with the invention, a low sensitivity is defined as below −40 dBFS while, by “more sensitive” it is meant that microphone 20 has a sensitivity which is at least 70% greater than the sensitivity of the “low sensitivity” microphone 15. In an exemplary embodiment, microphone 15 has a low sensitivity of −46 dBFS, but with a high clipping level, specifically greater than 130 dB. On the other hand, microphone 20 has a sensitivity of −26 dBFS. Although various known microphones could be employed in connection with the invention, in one specific embodiment, currently available MEMs microphone models INMP621ACEZ-R7 and MP34DBO1TR which are digital, 16 bit microphones manufactured by InvenSense, Inc. are utilized for the first and second microphones 15 and 20 respectively.

(6) In general, the system and method operates by initially identifying an incoming acoustic signal which could potentially be from a gunshot. For this purpose, only outputs from microphone 15 are initially, continuously analyzed for a peak amplitude level large enough to be preliminarily identified as a gunshot. Basically, since microphone 15 has a low sensitivity, microphone 15 only provides an output for very loud sounds and is essentially deaf to normal, everyday sounds emanating from within the building or structure and therefore will likely not reach a necessary threshold on any noise other than the loudest sounds. By way of example, a typical trigger value would be −5 dBFS (corresponding to a digital value of approximately 18000 based on the 16 bit unit). After a possible gunshot is identified in this manner, the system then processes acoustic signals to determine if the sound was actually from a gunshot in the manner detailed below.

(7) Reference will now be made to FIG. 2 in describing a preferred methodology employed in accordance with the invention. Here, it can be seen that steps 50 and 60 represent the initial possible gunshot identification routine outlined above which utilizes outputs from first microphone 15 and compares peak signal amplitudes with a pre-established trigger value, e.g., 18000. Assuming that a possible gunshot sound has been identified, step 70 is reached in which operational and nominal threshold values are established for upcoming calculations. At this point, it should be noted that these threshold values can actually be preset based on at least the acoustic characteristics of the particular building or structure in which sensor 5 is employed. However, for at least versatility reasons, it is desirable to enable these threshold values to be adjustable, such as based on changing acoustic characteristics or sensor layout. In addition to the trigger threshold, other established threshold values include: a Mic threshold (TH_1), a Mic2 RMS threshold (RMS_2_Thresh), a time window (Win_1), an enhanced autocorrelation window (EnAuto_Win_1), an enhanced autocorrelation threshold for an established frequency range between 15 kHz and 25 kHz (EnAuto_15_25_Thresh_1) and a maximum enhanced autocorrelation threshold for the established frequency range (EA_Max_15_25_TH). Byway of example, the following nominal threshold values can be employed: Trig_1=18000; TH_1=5000; RMS_2_Thresh=−13 dBFS (or an equivalent digital output of 7336); Win_1=0.30 seconds; EnAuto_Win_1=0.075 seconds; EnAuto_15_25_Thresh_1=1.25; and EA_Max_15_25_TH=325.

(8) With these nominal threshold values being established, step 80 is entered wherein the maximum amplitude for each of microphones 15 and 20 is determined (Max_1 and Max_2). Next, the time at which the acoustic signal crosses the threshold is determined in step 90. Basically, there is a time lapse between first microphone 15 sensing the sound and outputting the signal which has been identified as a potential gunshot. Here, it is desired to determine time zero (T_Win_1) for the potential shot and use this time for future calculations. Although other formulations could be employed, for purposes of a preferred embodiment of the invention, T_Win_1 is set equal to the time at which the first microphone amplitude exceeds TH_1 minus a predetermined time period, preferably 10 ms, wherein T_Win_1 is required to be less than Win_1, i.e., 0.3 seconds, from the point at which the amplitude is greater than Trig_1. This same calculated time zero is also used in connection with second microphone 20 (T_Win_2=T_Win_1).

(9) Next, step 100 is entered wherein an enhanced autocorrelation is calculated. At this point, it should be recognized that enhanced autocorrelation is known based on harmonics. Here, a known method is employed to filter data by determining pitches based on frequencies. As enhanced autocorrelation methods are known, further details will not be provided here. By way of example, reference is simply made to the article “A Computationally Efficient Multipitch Analysis Model” by Tolonen et al., IEEE Transactions on Speech and Audio Processing, Vol. 8, No. 6, (November 2000), the contents of which are fully incorporated herein by reference. With the invention, the preset operational enhanced correlation window (EnAuto_Win_1) is employed.

(10) In step 110, a maximum value of the enhanced auto correlation is determined. For this purpose, values in a first frequency range or band between 15 kHz and 25 kHz are relied upon for microphone 15. Here, the process is looking to establish a peak in this frequency range (EA_Max_15_25_1). Next, all amplitudes in a slightly larger, second frequency range, preferably 10 kHz to 25 kHz, are summed in step 120 (EA_10_25_Sum_1). Thereafter, all amplitudes in a third, distinct frequency range, preferably frequency bands between 2 kHz to 5.5 kHz, are summed in step 130 (EA_2_55_Sum_1). These two summation steps in distinct ranges are performed in connection with avoiding a false positive identification based on knowing that sounds from a gunshot have a broad range as compared to many other potentially sensed sounds.

(11) With all the above calculations, the algorithm moves to step 140 wherein a ratio of the summation values determined in steps 130 and 120 is determined, i.e., Ratio_EA_1=EA_2_55_Sum_1/EA_10_25_Sum_1. In this step, the denominator cannot equal zero. Therefore, if EA_10_25_Sum_1 equals zero, the Ratio_EA_1 is set to a predetermined value, such as 3.0. Finally, in step 150, the RMS of microphone 20 is calculated. More specifically, the RMS of microphone 20 (RMS_Full_2) is calculated using Win_1 and starting at T_Win_2. Basically, these steps are performed to see how the sound dissipates over a relatively short period of time, say 0.3 seconds, for microphone 20. Here it should be noted that the sound associated with a gunshot takes a fair amount of time to dissipate versus, say, tapping a microphone. Therefore, it can be verified here that the RMS stays high for a requisite period of time. Additionally, it should be recognized that signals from microphone 20 can be used for further verification, e.g., sensing sounds of screaming versus laughter or minor chatter.

(12) Once the calculations associated with the FIG. 2 algorithm are performed, it can then be determined if the detected sounds were actually from a gunshot. In accordance with a preferred embodiment of the invention as represented in FIG. 3, it is only determined that a gunshot has been detected if multiple requirements are satisfied, i.e., each of the requirements of steps 200, 210, 220 and 230 are satisfied. Specifically, to move past step 200, it must be determined that the maximum amplitude sensed by microphone 15 is greater than the trigger value (Max_1>Trig_1). Of course, this is just a verification step based on the requirements of step 60. In addition, RMS_Full_2>RMS_2_Thresh (step 210), EA_Max_15_25_1>EA_Max_15_25_TH (step 220), and Ratio_EA_1<EnAuto_15_25_Thresh_1 (step 230). If any one of these determinations cannot be made, it is determined that a gunshot has not been detected (step 240). On the other hand, if all of these verification steps are satisfied, step 250 is reached to verify that an actual gunshot has been sensed. If a gunshot is detected at 250, this is signaled via port 35 to a networked computer that can be used for alert purposes, such as alerting emergency personnel, such as building or local jurisdictional personnel) of the occurrence of the gunshot and, based on the particular sensor used in making the determination, the location of the gunshot.

(13) Although described with reference to preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. Overall, it has been found that employing two microphones with low and high sensitivities and making a detection decision based on at least certain threshold, root-mean-square (RMS), time window, and auto correlation frequency values, provides for very low false alarms or false positives while, at the same time, provides for high detection rates. In any event, the invention is only intended to be limited by the scope of the following claims.