A gunshot is the discharge of a firearm, producing a mechanical sound effect and a chemical gunshot residue. The term can also refer to a gunshot wound caused by such a discharge. Multiple discharges of a firearm or firearms are referred to as gunfire. The word can connote 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. It is better to be a bit more specific while writing however. "The sound of gunfire" or "we came under gunfire" would be more descriptive and prevent confusion. In the latter phrase, in particular, "fire" is more commonly used (i.e. "under fire"), as both words hold the same general meaning within the proper context.
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) of 140db or louder.
- A “snap” or “crack” caused by the sonic boom that occurs as a projectile moves through the air at supersonic speeds.
Gunfire can be confused with other sounds that can sound similar, such as firework explosions and cars backfiring. Gunfire noise propagation is anisotropic. The sounds may be heard at greater distances in the direction of bullet travel than behind or beside the firearm.
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). Firearm muzzle blast may be masked by ambient noise during daytime; but may be detected at greater distances during the quieter hours of darkness. A popular urban gunfire locator system typically uses six to ten audio sensors per square mile for trilateration. (two or three per square kilometer)
A silencer can be attached to the muzzle of a firearm to decrease the audio signature of high velocity gas released from the muzzle when firing the weapon. The sound of firing is only decreased and is still considerable. Silencers attached to the muzzle will not reduce the sound of high velocity gas released from other locations like the gap between the cylinder and barrel of a revolver. A muzzle silencer is similarly ineffective in reducing the snap of a supersonic bullet, or the noise produced by the mechanical action of a self-loading firearm. Use of silencers is rare in United States crimes. A 2007 study estimated unlawful silencer possession was involved in only 0.05 percent (1 in 2,000) federal criminal prosecutions; and the silencer was unused, but simply in the possession of the defendant for 92% of prosecutions involving a silencer.
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