Gunfire

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Gunfire is the action or sound a firearm makes when discharged, or "the fire of a gun or guns". The word can connotate either the sound of a gun firing, the projectiles that were fired, or both. For example, the statement "gunfire came from the next street" could either mean the sound of discharge, or it could mean the bullets themselves.

Usage:

• "The sound of gunfire"
• "we came under gunfire"

Gunfire characteristics

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

1. An optical flash that occurs when an explosive charge is ignited to propel a projectile from the chamber of the weapon
2. 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
3. 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. Because only optical flashes are detected, such systems are typically only capable of determining the bearing of a discharge relative to sensor.

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.

Gunfire 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 correlated 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 and the knowledge that most urban crimes are committed with handgunsTemplate:Citation needed, 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. [Showen and Dunham 1997].

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.

Locating gunfire

Gunshot location devices generally require one or more sensing modalities to detect either the fact that a weapon has been fired or to detect the projectile fired by the weapon. To date, only sound and visual or infrared light have successfully been used as sensing technologies.