Gunfire Locator - Gunfire Characteristics

Gunfire Characteristics

There are three primary attributes that characterize gunfire and hence enable the detection and location of gunfire and similar weapon discharges:

• An optical flash that occurs when an explosive charge is ignited to propel a projectile from the chamber of the weapon
• A muzzle blast that occurs when an explosive charge is ignited to propel a projectile from the chamber of the weapon. A typical muzzle blast generates an impulse sound wave with a sound pressure level (SPL) that ranges from 120 dB to 160 dB
• A “snap” or “crack” that occurs as a projectile moves through the air at supersonic speeds

Optical flashes can be detected using optical and/or infrared sensing techniques; note that there must be a line of sight from the sensor to the weapon otherwise the flash will not be seen. Indirect flashes that bounce off nearby structures such as walls, trees, and rocks assist in exposing concealed or limited line of sight detections between the weapon and the sensor. Because only optical flashes are detected, such systems are typically only capable of determining the bearing of a discharge relative to sensor unless multiple systems triangulate the shot range. Multiple gunshots, fired from multiple locations, at nearly the same time are easily discriminated as separate gunshots because the sensors generally utilize a focal plane array consisting of many sensitive pixels. Each pixel in the entire Focal plane (e.g. 640X480 pixels) is constantly evaluated.

The projectile generally must travel within 50 to 100 meters of a sensor in order for the sensor to hear a supersonic “snap”. The combination of a muzzle blast and a supersonic snap provides additional information that can be used along with the physics of acoustics and sound propagation to determine the range of a discharge to the sensor, especially if the round or type of projectile is known. Assault rifles are more commonly used in battle scenarios where it is important for potential targets to be immediately alerted to the position of enemy fire. A system that can hear minute differences in the arrival time of the muzzle blast and also hear a projectile’s shockwave “snap” can calculate the origin of the discharge. Multiple gunshots, fired from multiple locations at nearly the same time, such as those found in an ambush, can provide ambiguous signals resulting in location ambiguities.

Gunfire acoustics must be distinguished reliably from noises that can sound similar, such as firework explosions and cars backfiring.

Urban areas typically exhibit diurnal noise patterns where background noise is higher during the daytime and lower at night, where the noise floor directly correlates to urban activity (e.g., automobile traffic, airplane traffic, construction, and so on). During the day, when the noise floor is higher, a typical handgun muzzle blast may propagate as much as a mile. During the night, when the noise floor is lower, a typical handgun muzzle blast may propagate as much as 2 miles. Therefore a co-located array of microphones or a distributed array of acoustic sensors that hear a muzzle blast at different times can contribute to calculating the location of the origin of the discharge provided that each microphone/sensor can specify to within a millisecond when it detected the impulse. Using this information, it is possible to discriminate between gunfire and normal community noises by placing acoustic sensors at wide distances so that only extremely loud sounds (i.e., gunfire) can reach several sensors; this has been termed a ‘spatial filter’ in the first patent issued to ShotSpotter, Inc. Infrared detection systems have a similar advantage at night because the sensor does not have to contend with any solar contributions to the background signal. At night the signature of the gunshot will not be partially hidden within the background of solar IR contributions. Most flash suppressors are designed to minimize the visible signature of the gunfire. Flash suppressors break up the expanding gases into focused cones therby minimizing the blossoming effect of the exploding gasses. These focused cones contain more of the signature in a smaller volume. The added signal strength helps to increase detection range.

Because both the optical flash and muzzle blast are muffled by flash suppressors and muzzle blast suppressors (also known as “silencers”), the efficacy of gunshot detection systems may be reduced for suppressed weapons. The FBI estimates that 1% or fewer of crimes that involve gunfire are committed with silenced gunfire.