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. 
Opticor) but be perceived by enemies as completely silent.
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.
In the Arsenal, the noise stat will only appear if the weapon is "silent". If the stat does not appear, then the weapon is considered "alarming".
Enemy Reaction[]
Enemies who hear noise will become alerted to the player's presence and will react in one of five 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
- Run to activate a Control Console to sound the alarm in the area
- Run towards the nearest Arctic Eximus when shields are deployed
- Run to a Nullifier Crewman in the area and hide in the shield that was deployed (this only applies to Corpus enemies)
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 cloaked players who need to destroy objectives, Sensor Bars or Security Cameras.
Enemies that have heard an invisible Tenno's gunfire will attack in the area where the sound was made.
Shooting a silent weapon within ~6 meters of an enemy or at the terrain near an enemy will cause them to stop and look around, but will not change their actual alert state.
Reducing Noise[]
Noise level of weapons can be decreased with 
Hush for rifles, Suppress for pistols and Silent Battery for Shotguns. Melee weapons, with the exception of Gunblade gunfire, are silent by default.
Banshee's passive 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 and Arsenal[]
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.
Ivara's 
Prowl follows the arsenal listing for a weapon's noise level regardless.
