Noise Level, measured in meters, determines the distance at which an enemy will be alerted by any given weapon or ability. Noise levels can be reduced if there is any obstruction between the source of noise and the target. Weapon noise is a key component of Stealth and must be considered when doing stealth missions. Noise level is independent of the sound a player hears; a weapon can sound loud in the game (e.g. Ogris before Update 16.0 (2015-03-19)) but be perceived by enemies as completely silent.
As of Update 18.1 (2015-12-16), stealth killing enemies will produce sound that travels about 5 meters, and any enemies within that radius will become automatically alerted for about 20 seconds, including melee finishers under stealth.
Enemies who hear noise will become alerted to the player's presence, and will react in one of three ways. An enemy may choose to:
Locate the nearest cover and hide, waiting to ambush players if they get into range;
Run to the source of the noise and then search the area; or
Run to activate a Control Console to sound the alarm in the area.
Enemies who are not killed in one shot or very briefly after being hit the first time will become alerted. Therefore, silent weapons that are not powerful enough to do so, are better utilized to prevent enemies from detecting stealthed or cloaked players who need to destroy objectives, sensor bars or security cameras.
Reducing Noise
Noise level of weapons can be decreased with Hush for rifles, Suppress for pistols and Silent Battery for Shotguns. Melee weapons are silent by default.
Banshee's SuppressorPassive innately silences any weapon she has equipped, and her Silence ability causes all affected enemies to be deafened in an area of effect, preventing enemies from hearing any source of noise even from Banshee's allies. Loki's Syndicate augment Hushed Invisibility acts as a 100% noise reduction for all weapons while using invisibility.
Codex entries and weapon statistics only have 2 listed noise levels; Alarming and Silent. Placing a 25%, 50%, or 75% reduction from Hush or Suppress will not display actual changes in this notation. Many weapons are also listed incorrect or misleading by the codex, such as the Latron Wraith being listed as Alarming, but acting nearly silent.
Note: Ivara's Prowl follows the arsenal listing for a weapon's noise level, regardless of the above statement.
Trivia
Strangely enough, enemies will not react to mid-air melee slam attacks, no matter how close, provided that the AoE will not hit them.
Hush for rifles, 
