Noise

When the game calculates the volume level of a sound, it takes the "noise" and multiples it by 1.5 (for example, an unsilenced base G19 using 9mm ammo would have a noise of 20 + ⌊18 * .8⌋ = 34, which would then be multiplied by 1.5 for a grand total volume rating of 51).

Monsters' alertness

Volume is generally equal to the distance in squares from which the sound can be heard by non-deaf monsters with the HEARS flag, though their urge to investigate that sound is based on the difference between volume and distance — the monster will ignore a 10 volume sound in a 10 square range, but if it is within 5 square range of a 10 volume sound the monster will follow to its source for 5 turns, in 4 range - for 6 turns, etc. Also, monsters ignore sounds in a two-square radius, implying that they rely upon other senses here.

One thing to note are monsters with the GOODHEARING flag. They hear sounds at twice the distance, follow sounds to their source six times longer, and never ignore nearby sounds.

Some enemies can be scared with loud sounds, such as certain wildlife like black bears, while others are angered more by loud sounds.

Common sources of noise

• Character's footsteps. By default moving creates 6 volume noise.
  • Light Step trait decreases noise to 2.
  • Wearing active RM13 armor decreases noise to 2.
  • Leg tentacles mutation makes movement completely silent.
  • Clumsy trait increases noise to 10
  • Squeaky Ankles CBM increases it to 12.
• Size influences the noise made while moving. Tiny beings are totally silent, small ones reduce noise to 33% of normal. Large creatures increase it by 50%, and huge ones double the sound made. Loud monsters (the LOUDMOVES flag) increase sound made by an additional 6. This is important if you mount a horse or something similar.
• smashing things.
• Firearms. Their noisiness is based on ammunition damage ("Noise" volume = ammo_damage * 0.8, +20 if result is greater than 5, plus gunmods modifiers). With exception of batteries, plutonium cells, fusion packs, 40mm grenades (they create noise with fixed 8 volume) and gasoline (4 volume). bolts, arrows, and pebbles are completely silent.
• Vehicles (generate noise upon accelerating). Vehicle noise is equal to 0.25 * engine power, 0.25 * 0.4 * engine power if a muffler is installed.
• Explosions create noise with volume ten times bigger than the explosion power.
• Throwing items causes noise if they hit the ground, either 16 for glass breaking, or 8 "thud" for everything else.
• C: Yelling, if you don't cover your mouth. Mouth encumbrance muffles the noise you make by yelling.

Character's awareness

By default, volume is equal to range for the player, as well; thus, the louder a sound, the farther away a player-character can be and still hear it.

• Enhanced Hearing bionic multiplies sounds' initial volume by 3.5.
• Canine Ears mutation multiplies it by 1.5.
• Ursine Ears and Feline Ears multiply it by 1.25.
• Lupine Ears multiply it by 1.75.
• Rodent Ears multiply it by 2.
• Poor Hearing halves the initial volume.
• Deaf characters are, well, deaf and can't hear anything at all.

There is a chance to become deafened for up to 40 turns if a sound is loud and close enough. Be careful. Extreme and repeated sounds can cause extremely long deafness. Huge explosions and collapsing buildings are the likeliest to deafen you. Enhanced Hearing, as advertised, filters that out.

If the character is sleeping when a noise happens nearby, they have a wake-up chance that depends on the sound's volume and Heavy Sleeper or Very Heavy Sleeper traits. However, the character will not know what sound awoke them.

If the character is busy performing a task when he hears a loud noise, the game will alert the player to the noise and ask whether you want to interrupt the activity. You are never interrupted for sounds that come from your position.

Footsteps

Monsters' footsteps do not produce any actual sound, in contrast to the character's footsteps. These can be heard only by the player and do not attract other monsters' attention. Moreover, footsteps of any volume can be detected by the player from any distance. Volume and distance only affect the sound markers' precision.

Footstep volume is based on a critter's size; digging critters are noisier. Tiny, flying, and already visible enemies do not produce footsteps at all. Markers are never placed on visible tiles.

Without the Enhanced Hearing bionic, footstep marks will be inaccurate by at least one tile (depends on distance), although the bionic's overall effect is fairly small. Its bonus is the same size as the penalty for Poor Hearing.