Gunshot detection system with encrypted, wireless transmission

BACKGROUND OF THE INVENTION

0001Shooting incidents, involving active shooters shooting victims within buildings such as schools or malls, are increasingly a concern. Identifying and reacting quickly to such incidents can reduce loss of life. For example, first responders need to know if gunshots have actually been fired and the location and number of the fired shots.

0002In confined areas, such as in a school or a private or public building, detecting and locating the source of gunshots is a complicated problem. A gunshot typically generates several sounds including the gunshot itself, the bullet's bow shockwave, noise from bullet impacts and noise of reflected gunshot sounds. In addition, numerous noises are generated in buildings that may be mistakenly identified as gunshots.

0003The broad concept of detecting gunshots utilizing acoustics is known. More specifically, it is known to provide a gunshot detection system including an array of acoustic sensors positioned in a pattern which enables signals from the sensors to be employed to not only detect the firing of a gunshot but to also locate the origin of the shot. One main requirement of such a system is the need to accurately distinguish between the sound produced from a gunshot and a host of other ambient sounds. In at least one known arrangement, a microphone is used to detect each sound, which is then amplified, converted to an electrical signal and then the electrical signal is compared with a threshold value above which a gunshot sound is expected to exceed.

0004<patcit id="pcit0001" dnum="WO2010039130A"><text>WO 2010/039130</text></patcit> relates to systems including audio download and/or noise incident location system features.

0005Recently, gunshot detection systems with improved accuracy, dependability, and effectiveness have been described. One such system is described in International Publication Number <patcit id="pcit0002" dnum="WO2018044553A"><text>WO 2018/044553, published on March 8, 2018</text></patcit> and entitled "System and Method for Acoustically Identifying Gunshots Fired Indoors." This system provides for low false alarms or false positives and high detection rates by employing two microelectromechanical microphones (MEMs) having different sensitivity levels. Acoustic signals from a first microphone with lower sensitivity (for example, making the anomaly detection microphone essentially deaf to routine sounds) are first analyzed for a peak amplitude level large enough to be a potential gunshot. Then acoustic signals from a second microphone having a higher sensitivity are then analyzed further to confirm that the sound was a gunshot.

0006Gunshot detection methods have also been proposed that can count the number of gunshots fired, particularly from an automatic or fast acting weapon. One such method is described in International Publication Number <patcit id="pcit0003" dnum="WO2018044556A"><text>WO 2018/044556, published on March 8, 2018</text></patcit> and entitled "Method for Acoustically Counting Gunshots Fired Indoors." In this method, an acoustic signature of captured noise is analyzed to accurately count how many shots are fired. The method can be employed to identify that the gun is an automatic or rapid fire weapon, which information can be provided to emergency personnel.

0007Additionally, gunshot detection system that can accurately determine where sensed events are located have been proposed. One such system is described in International Publication Number <patcit id="pcit0004" dnum="WO2018185723A"><text>WO 2018/185723, published on October 11, 2018</text></patcit> and entitled "System and Method for Identifying and Locating Sensed Events." Here, a sensor network is employed to detect an event in the form of an audible signal. The event is time stamped and sent to a controller, which evaluates the event as either unique or a multiple detection using the sensor's time of alarm to determine which sensor activated first and to suppress subsequent alarms for the same event. This process is known as de-confliction.

SUMMARY OF THE INVENTION

0008Especially as gunshot detection systems become more prevalent, security concerns must addressed. It is important to prevent third parties from intercepting and/or forging and/or downloading captured audio information.

0009Such concerns exist in a number of different topologies. Certainly, it would be desirable to encrypt the audio data stored on the sensor units and/or transmitted by the sensor units and/or stored by the control panel. This applies to wired systems as well as wireless/wired hybrid systems and all-wireless systems. Specially, wireless-equipped gunshot sensor units would allow greater flexibility in installing and maintaining the units while minimizing costs. But, the wireless aspect exacerbates existing security concerns. In particular, there is a need to prevent third parties from intercepting and/or contaminating and/or spoofing the signals exchanged between the gunshot sensor units and/or a control panel during or after a gunshot detection event.

0010A gunshot detection system according to the present invention is a system as defined in claim 1.

0011The gunshot sensor units encrypt the audio data before storing it and/or sending it to the control panel. The control panel can similarly store the encrypted audio data and decrypt the audio data before presenting it (e.g. via speakers) to an operator of the control panel. Encryption and decryption keys are used by the gunshot sensor units and the control panel to encrypt and decrypt the audio data. Encryption keys might be programmed at the control panel and distributed to the gunshot sensor units or generated by an outside program and used by the control panel. The control panel would also store decryption keys for decrypting the audio data from each of the different gunshot sensor units or request those keys from an outside system or source based on the security privileges of the operator of the control panel.

0012An additional layer of end-to-end encryption is also provided. The gunshot sensor units and the control panel are equipped with encryption/decryption modules for encrypting and decrypting messages exchanged between the devices and/or one or both could be make requests for these services, which are then are processed outside the system. These messages might include the encrypted audio data.

0013Although the encryption functionality can work with existing wired communication infrastructures for gunshot detection systems, in the preferred embodiment, the gunshot sensor units and possibly even the control panel are equipped with wireless network interfaces for wirelessly communicating with each other over a communication network. To address potential bandwidth issues, the gunshot sensor units can monitor the communication network to discern network conditions. Compression modules of the gunshot sensor units are configured to compress the audio data before encryption based on the current network conditions.

0014In embodiments, the gunshot sensor units might store the encrypted audio data in local nonvolatile storage of the gunshot sensor units, along with audio encryption keys that are used to encrypt the audio data. The control panel might store a symmetric decryption key or asymmetric decryption keys associated with each gunshot sensor unit in its nonvolatile memory, the decryption keys being used to decrypt the audio data. The control panel could distribute the symmetric or asymmetric audio encryption keys to the gunshot sensor units, periodically sending updated the audio encryption keys. In addition, the gunshot sensor units can generate messages including the encrypted audio data and further encrypt these messages via encryption/decryption modules of the gunshot sensor units. The control panel would similarly decrypt the messages via its own encryption decryption module before decrypting the encrypted audio data contained in the messages. The gunshot sensor units might wirelessly send the encrypted audio data over a communication network via wireless network interfaces.

0015According to another aspect, the invention features a method for detecting gunshots within a premises as set out in claim 10.

0016The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention as defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0017In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Fig. 1</figref> is a schematic diagram of an exemplary gunshot detection system showing a common context for embodiments of the present invention;</li><li><figref idref="f0002">Fig. 2</figref> is a block diagram showing an exemplary gunshot sensor unit of the gunshot detection system according to the present invention;</li><li><figref idref="f0003">Fig. 3</figref> is a block diagram showing an exemplary control panel of the gunshot detection system according to the present invention;</li><li><figref idref="f0004">Fig. 4</figref> is a schematic diagram of an exemplary gunshot detection system including wireless-capable gunshot sensor units;</li><li><figref idref="f0005">Fig. 5</figref> is a block diagram showing an exemplary wireless-capable gunshot sensor unit;</li><li><figref idref="f0006">Fig. 6</figref> is a block diagram showing an exemplary wireless-capable control panel;</li><li><figref idref="f0007">Fig. 7</figref> is a sequence diagram illustrating a process by which gunshot detection system encrypts and decrypts the audio data and messages exchanged between the gunshot sensor units and the control panel; and</li><li><figref idref="f0008">Fig. 8</figref> is a sequence diagram illustrating a process by which the gunshot sensor units compress the audio data based on current network conditions.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0018The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

0019As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present.

0020Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

0021<figref idref="f0001">Fig. 1</figref> is a schematic diagram of an exemplary gunshot detection system 100.

0022In general, the gunshot detection system 100 monitors, detects and reports the occurrence of gunshots or other emergencies within a premises 50 such as a building (e.g. office, hospital, warehouse, retail establishment, shopping mall, school, multi-unit dwelling, government building).

0023In the illustrated example, the premises 50 is a simplified floor example of a building with three areas 52, a lecture hall 52-1, classroom A 52-2, and classroom B 52-3. Two gunshot sensor units 130-1, 130-2 are located in the lecture hall 52-1, while one gunshot sensor unit 130-2 is located in classroom A 52-2, and one gunshot sensor unit 130-3 is located in classroom B 52-3.

0024In the illustrated embodiment, the gunshot detection system 100 includes gunshot sensor units 130, a control panel 116, and a communication network 111. In general, and in one configuration, the gunshot sensor units 130 detect conditions indicative of the gunshots or other emergencies and alert the control panel 116, which takes one or more responsive actions such as alerting building personnel, law enforcement, and/or a monitoring center, or collecting and presenting data pertaining to the detected gunshots to an operator of the control panel 116. The gunshot sensor units 130 and the control panel 116 communicate over the communication network 111.

0025More specifically, the gunshot sensor units 130 are distributed throughout the premises 50, for example, in areas 52 of the premises such as rooms, hallways, lobbies or stairways, to name a few examples. The gunshot sensor units 130 detect acoustic anomalies indicating potential gunshots and generate audio data depicting the acoustic anomalies. The gunshot sensor units 130 also generate event data based on and descriptive of the acoustic anomalies, encrypt the event data, and locally store and/or send the encrypted event data to the control panel 116. The gunshot sensor units 130 send the encrypted event data to the control panel 116 by sending messages containing the encrypted audio data. Whether or not they contain the encrypted audio data, these messages are themselves encrypted by the gunshot sensor units 130 before being sent. The gunshot sensor units 130 also decrypt incoming encrypted messages. Thus, the gunshot detection system 100 provides end-to-end encryption for communication links between the gunshot sensor unit 130 and the control panel 116.

0026The event data includes audio data (e.g. digitized audio clips) depicting the acoustic anomalies, metadata including time information indicating when the acoustic anomalies started and/or stopped, duration information for the acoustic anomalies, the audio data depicting the acoustic anomalies, file information, and identification information for the gunshot sensor unit 130, and sensor data generated by the gunshot sensor unit 130. The event data can be locally stored, collected by the control panel 116, transferred to remote servers, and/or transferred to devices of law enforcement entities for forensic analysis, for example.

0027On the other hand, the control panel 116 directs the overall functionality of the gunshot detection system 100 by sending instructions (e.g. control messages) to be executed by the gunshot sensor units 130, receiving the event data from the gunshot sensor units 130 and taking the responsive actions based on the event data. The control panel 116 receives partial event data ( metadata indicating time and date information) from multiple gunshot sensor units 130 and perform a de-confliction process in order to determine which event data from the different sensor units 130 pertains to the same detected acoustic anomaly and which of the gunshot sensor units 130 is closest to the source of the acoustic anomaly based on, for example, which of the units first detected the acoustic anomaly. The control panel 116 then sends instructions to the gunshot sensor unit 130 closest to the source to send full event data ( including a full audio data sample, environmental data, and other sensor data) to the control panel 116 for further processing and/or to be presented to the operator. Similar to the gunshot sensor units 130, the control panel 116 encrypts outgoing messages and decrypts incoming messages, and if any of the incoming messages contain encrypted audio data, the control panel 116 further decrypts the audio data before, for example, presenting the audio data to an operator of the control panel 116.

0028<figref idref="f0002">Fig. 2</figref> is a block diagram showing an exemplary gunshot sensor unit 130.

0029The gunshot sensor unit 130 includes a controller 200, local nonvolatile storage 202, a wired network interface 204-1, an anomaly detection microphone 206, an audio capture microphone 208, and an encryption/decryption module 213.

0030The controller 200 executes firmware instructions and an operating system (OS) 201 and generally directs the functionality of the gunshot sensor unit 130. In one example, the controller 200 is small single-board computer. In other examples, the controller is a microcontroller unit or a system on a chip (SoC), including one or more processor cores along with memory and programmable input/output peripherals such as analog to digital converts and digital to analog converters. The operating system 201 interfaces with the hardware components of the gunshot sensor unit 130 for access by an audio data encryption process 207, an audio data compression module 209, and a monitoring process 209, which are processes executing on top of the OS 201.

0031The audio data encryption process 207 encrypts the audio data generated by the gunshot sensor unit 130. In one example, the audio data encryption process 207 encrypts the audio data using an audio encryption key 215 stored in the local nonvolatile storage 202. This audio encryption key 215 can be initially distributed and periodically updated by the control panel 116. In another example, the encryption keys are programmed into the hardware of the units, such as stored in read only memory (ROM). In still other examples the encryption is performed by a special purpose application specific integrated circuit (ASIC) that is programed with both the encryption key and encryption algorithm or programed only with the encryption algorithm.

0032In addition, different kinds of encryption could be used.

0033In one example, a Public-key, or asymmetric, encryption approach is used. Here, the control panel disseminates its public key to the sensor units while retaining its private key. The sensor units then encrypt the audio data using the public key of the control panel, allowing only the control panel to decrypt the audio data using its private key.

0034In another example, a symmetric encryption approach is used. Here, the algorithms for cryptography use the same cryptographic keys for both encryption of the audio data and decryption of the audio data at the control panel.

0035In the preferred embodiment, the audio encryption key 215 is unique for each different gunshot sensor unit 130, and all of the different decryption keys associated with the audio encryption keys 215 are stored by the control panel 116. In an alternative embodiment, the audio encryption key 215 is a "public key" associated with the control panel 116, the public key being known to the gunshot sensor units 130 allowing the gunshot sensor units 130 to encrypt the audio data such that only a "private key" counterpart to the public key known only to the control panel 116 can decrypt the audio data.

0036In another example, the control panel 116 might request audio encryption keys that were generated by a third party program, system, entity, and/or service, such as a cloud service executing on an application server accessible to the control panel 116 via a networks including one or more public network such as the internet. These encryption keys could then be distributed to the gunshot sensor units 130 and the control panel. These keys might be symmetric or asymmetric decryption keys. Similarly, the control panel 116 might request the decryption key(s) from the third party service before decrypting the encrypted audio data from the gunshot sensor units 130 or send the encrypted audio data directly to the third party service to be decrypted remotely. The third party service might be administered by a law enforcement entity, or the third party service might provide the decryption keys and/or decryption service only to law enforcement personnel, or to operators of the control panel 116 with superuser credentials, or in response to receiving a subpoena for the decrypted audio data, in examples.

0037The monitoring process 211 monitors the communication network 111 and determines current network conditions by measuring and/or analyzing network performance metrics such as bandwidth, throughput, latency, jitter and/or error rate. The monitoring process 211 generates and sends network conditions information to the audio data compression module 209 based on the determined network conditions.

0038The audio data compression module 209 compresses the audio data based on the network conditions of the communication network 111. The audio data compression module 209 might request and receive the network conditions information from the monitoring process 211 and compress the audio data based on the network conditions information. The audio compression module 209 uses data compression algorithms to encode the audio data using fewer bits than the originally generated audio data used. In one example, the audio compression module 209 might remove from the audio data or de-emphasize the accuracy of perceptually irrelevant information (e.g. pertaining to sounds that are difficult or impossible for humans to hear). Different compression methods might be used based on the network conditions indicated by the network conditions information. For example, a lossy compression method resulting in compressed audio data with 5 to 20% of the originally generated audio data might be used when the network conditions information indicates a congested or unreliable connection. On the other hand, a lossless method resulting in compressed audio data with 50 to 60% of the originally generated audio data might be used when the network conditions information indicates a reliable connection. The audio data compression module 209 might even send the originally generated audio data without performing any data compression at all, based on the network conditions.

0039The wired network interface 204-1 provides connectivity with the control panel 116 and possibly other devices via a wired link to the communication network 111, using a cable (e.g. twisted pair, coaxial, fiber optic) connected to a port of the wired network interface 204-1, for example. In addition, the network also provides power to the device, in many examples. Direct current (DC) is superimposed upon the data that is transmitted between the devices and other nodes on the network.

0040The encryption/decryption module 213 encrypts and decrypts messages exchanged via the network interface 204 by the gunshot sensor units 130, the control panel 116 and/or other devices on the communication network 111. Specifically, the encryption/decryption module 213 encrypts outgoing messages to be sent by the gunshot sensor units 130 and decrypts incoming encrypted messages received by the gunshot sensor units 130. In one example, the encryption/decryption module 213 is implemented in hardware as an intervening chip between the controller 200 and the network interface 204. The encryption/decryption module 213 may include or be part of a secure cryptoprocessor with built in anti-tamper and other physical security measures. In another example, the encryption/decryption module, the operating system 201, audio data encryption process 207, audio data compression module 209, and/or the monitoring process 211 all execute on a secure cryptoprocessor, which also stores the encrypted audio data.

0041The anomaly detection microphone 206 detects the acoustic anomalies, while the audio capture microphone 208 captures ambient sound and generates the audio data. In one embodiment, both microphones 206, 208 are micro electro-mechanical system (MEMS) microphones having different sensitivity levels, and the controller 200 is configured to sample the microphones 206, 208 such that outputs from the microphones can be continuously analyzed in near real time for an acoustic signature. The anomaly detection microphone 206 has the lower sensitivity level and a high clipping level, while the audio capture microphone 208 has the higher sensitivity level. The audio capture microphone 208 continuously captures ambient sound, which is stored in a 9.9 second (for example) loop in a ring buffer of the controller 200. At the same time, incoming acoustic signals from the anomaly detection microphone 206 are continuously analyzed to detect acoustic anomalies, particularly by searching the incoming acoustic signal for a peak amplitude level large enough to be at least preliminarily identified as a gunshot.

0042Once an indication of a possible gunshot has been triggered utilizing the anomaly detection microphone 208, further processing may be performed by the controller 200. The controller 200 analyzes the sound stored in the loop to confirm that the acoustic anomaly is a gunshot. If confirmed as gunshot, the controller stores the captured sound stored in the loop buffer 216, which would include the acoustic anomaly and the previously captured sound (up to 9.9 seconds, in this example) as audio data 214 in the local nonvolatile storage 202 associated with different event files 210 or instances of event data for different gunshot detection events, along with the metadata 212, which includes the time and data information for the events. In embodiments, the local nonvolatile storage 202 could be fixed storage such as flash memory, or removable storage such as an SD card, among other examples.

0043<figref idref="f0003">Fig. 3</figref> is a block diagram showing an exemplary control panel 116.

0044The control panel 116 includes a central processing unit (CPU) 300, nonvolatile memory 302, a wired network interface 304-1, a display 310, and an encryption/decryption module 301.

0045Similar to analogous components on the gunshot sensor units 130, the wired network interface 304-1 provides connectivity with the gunshot sensor units 130 and possibly other devices via a wired link to the communication network 111. In some examples, the control panel may also supply power to the units.

0046Similarly, the encryption/decryption module 301 encrypts and decrypts messages exchanged via the network interface 304 by the control panel 116, the gunshot sensor units 130 and/or other devices on the communication network 111. Specifically, the encryption/decryption module 301 encrypts outgoing messages to be sent by the control panel 116 and decrypts incoming encrypted messages received by the control panel 116.

0047The CPU 300 executes firmware instructions and an operating system (OS) 312 and generally directs the functionality of the control panel 116. The OS 312 interfaces with the hardware components of the control panel 116 for access by a command and control application 316, which is a software process executing on top of the OS 312.

0048The command and control application 316, in general, generates a graphical user interface (GUI) 318 that is rendered on the display 310 (e.g. touchscreen display) of the control panel 116. In one example, the GUI 318 might present gunshot sensor unit information to an operator of the control panel 116 and receive input indicating selections of various options for controlling the gunshot sensor units 130 such as programming the encryption and/or decryption keys for the control panel 116 and/or the gunshot sensor units 130. In one example, the command and control application 316 might also be configured to decrypt the encrypted audio data received from the gunshot sensor units 130 and present the decrypted audio data (e.g. via speakers) based on input from the operator received via the GUI.

0049The nonvolatile memory 302 stores the audio decryption keys 305 for decrypting the encrypted audio data received from the gunshot sensor units 130. In the illustrated example, the audio decryption keys 305 are stored in an audio decryption key table 303. As previously described, in one example, the audio decryption key table 303 might include a series of audio, symmetric or asymmetric, decryption keys 305, with each of the keys being associated with a different one of the gunshot sensor units 130. The encrypted audio data received from the gunshot sensor units 130 are decrypted using the associated audio decryption keys 305. On the other hand, in an alternative example, there might only be one audio decryption key 305 used to decrypt the encrypted audio data received from all of the gunshot sensor units 130.

0050<figref idref="f0004">Fig. 4</figref> is a schematic diagram of an exemplary gunshot detection system 100 according to another embodiment including a hybrid network with wireless-capable gunshot sensor units 130-w. Here, a wireless access point 105 of the premises 50, which might be, for example, a wireless router, connects to the wireless gunshot sensor units 130-w via wireless links. The wireless access point 105 provides connectivity between the wireless gunshot sensor units 130-w and the communication network 111.

0051<figref idref="f0005">Fig. 5</figref> is a block diagram showing an exemplary wireless-capable gunshot sensor unit 130-w. Here, instead of the wired network interface 204-1, the wireless-capable gunshot sensor unit 130-w includes a wireless network interface 204-2 and an antenna 205. The wireless network interface 204-2 provides connectivity with the control panel 116 and possibly other devices via a wireless link to the communication network 111 via the antenna 205.

0052The wireless network interface 204-2 implements IEEE 802.11 standards, in one embodiment. It can use the 2.4 gigahertz UHF or 5.8 gigahertz SHF ISM radio bands.

0053In another embodiment, the wireless network interface 204-2 implements IEEE 802.15 standards, such as Bluetooth or Bluetooth Low Energy. In this case, the wireless network interface 204-2 could be used by the gunshot sensor units 130 to transmit encrypted audio data to handheld units (e.g. mobile computing devices) of law enforcement and/or other authorized entities.

0054In another embodiment, the wireless network interface 204-2 implements one or more standards associated with cellular technologies, such as Global System for Mobile communications (GSM) technologies in the range of 380 to 1900 MHz, Universal Mobile Telecommunications System (UMTS)/3G technologies in the range of 700 to 3500 MHz, Long Term Evolution (LTE) technologies in the range of 450 to 5900 MHz, and/or 5G technologies in the ranges of 600 to 4700 MHz or 24 to 39 GHz. In one example, the gunshot sensor units 130 communicate via a distributed antenna system (DAS) implemented locally within the premises 50 or across a wider geographical area using the wireless network interface 204-2. The DAS might also be implemented in conjunction with an in-building cellular enhancement system for extending and distributing cellular signal of a mobile network within the premises 50.

0055<figref idref="f0006">Fig. 6</figref> is a block diagram showing an exemplary wireless-capable control panel 116-w. Here, instead of the wired network interface 304-1, the wireless-capable control panel 116-w includes a wireless network interface 304-2 and an antenna 307. The wireless network interface 304-2 provides connectivity with the gunshot sensor units 130 and possibly other devices via a wireless link to the communication network 111 via the antenna 307.

0056<figref idref="f0007">Fig. 7</figref> is a sequence diagram illustrating a process by which gunshot detection system encrypts and decrypts the audio data and messages exchanged between the gunshot sensor units 130 and the control panel 116. The following process could apply to both the wired and wireless embodiments of the gunshot sensor units 130.

0057First, in step 400, the control panel 116 is programmed with the encryption key(s) for encrypting the audio data, which the control panel 116 distributes to the gunshot sensor units 130 in step 402. But, in other examples, the sensor units are preprogrammed with the keys. In step 404, the audio encryption keys are stored by the gunshot sensor units 130 in the local nonvolatile storage 202, for example.

0058After a period of time (for example, a year or two after initial programming of the units), in step 406, the gunshot sensor units 130 detect gunshots, and, in step 408, the gunshot sensor units 130 generate and store in local nonvolatile storage 202 event data including audio data depicting acoustic anomalies (e.g. the gunshot sounds). Preferably, this audio data is first encrypted prior to being stored.

0059In step 410, the gunshot sensor units 130 generate alert messages indicating that the gunshots were detected. These alert messages include partial event data including metadata indicating the time and duration of the detected shots. In step 412, this alert message is encrypted by the encryption/decryption module 213. The gunshot sensor units 130 send the encrypted alert messages to the control panel 116 in step 414.

0060In step 414, the control panel 116 decrypts the encrypted alert messages via the encryption/decryption module 301. In step 420, the control panel 116 generates a request message requesting the audio data depicting the gunshot sounds. The control panel 116 then encrypts this request message in step 420 and sends the encrypted request message to the gunshot sensor units 130 in step 422.

0061In step 424, the gunshot sensor units 130 decrypt the encrypted request message via the encryption/decryption module 213. In response to the request message, the gunshot sensor units 130 then encrypt the audio data, if not previously encrypted, using the stored audio encryption key 215 via the audio data encryption process 207. The gunshot sensor units 130 then generates a reply message containing the encrypted audio data in step 428 and encrypts the reply message in step 430 via the encryption/decryption module 213. This encrypted reply message is then sent by the gunshot sensor units 130 to the control panel 116 in step 432.

0062In step 434, the control panel 116 decrypts the encrypted reply message via the encryption/decryption module 301. The control panel 116, in step 436, also decrypts the audio data contained within the decrypted reply message using the audio decryption key 305 associated with the gunshot sensor units 130 from which the messages were sent. This audio data can then be presented, for example, via speakers of the control panel 116.

0063In step 438, the control panel 116 periodically sends updated audio encryption keys to the gunshot sensor units 130, and the gunshot sensor units 130 store the updated audio encryption keys in the local nonvolatile storage 202.

0064<figref idref="f0008">Fig. 8</figref> is a sequence diagram illustrating a process by which the gunshot sensor units 130 compress the audio data based on conditions of the communication network 111. As in <figref idref="f0007">Fig. 7</figref>, this process could apply to both the wired and wireless embodiments of the gunshot sensor units 130.

0065Steps 400 through 424 proceed as previously described, as the gunshot sensor units 130 detect gunshots, alert the control panel 116, and receive request messages to send the audio data.

0066Now, however, in step 502, the gunshot sensor units determine network conditions of the communication network 111 via the monitoring process 211 and compress the audio data based on the network conditions via the audio data compression module 209.

0067In steps 426 through 436, the compressed audio data is encrypted and sent to the control panel 116 as previously described.

0068In step 504, as appropriate based on the compression method used, the control panel 116 might then decompress the audio data.

0069In another example, the gunshot sensor units 130 might encrypt the audio data, generate a message including the encrypted audio data, encrypt the message via the encryption/decryption module, and send the encrypted message to a third party cloud service (for example, executing on a service accessible to the gunshot sensor units 130 and/or control panel 116 only via a public network such as the internet). The third party cloud service might then process requests for the encrypted audio data from the control panel 116 or other devices.

0070While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.