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.

Spawn boost

Zombies spawn in waves over a time function. Extremely loud sounds reduce the spawn timer for the next wave, in addition to drawing in all of the already spawned zombies littering the area. Note that spawn times and spawn amounts are directly proportional. This means that if you spawn and draw a lot of zombies, and slaughter them quickly, you will get a small break before more zombies are spawned in (This can be magnified by killing the "wave" with silent weapons). You can easily, however, get almost infinite zombies spawning if you choose to eliminate "waves" with a loud weapon. Any volume above 60 lowers the spawn timer by a formula based on how much over 60 the volume is.

Common sources of noise

• Character's footsteps. By default moving creates 6 volume noise. It can be decreased to 2 with the Light Step trait, and with the leg tentacles mutation, or with a powered RM13 armor, movement is completely silent. The Clumsy trait increases this to 10, and the Squeaky Ankles CBM increases it to 12.
• 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 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; the Canine Ears mutation multiplies it by 1.5, and Poor Hearing halves the initial volume. Obviously, Deaf characters cannot hear any sound, no matter what.

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 most likely to deafen you. Enhanced Hearing, as advertised, filters that out.

If the character is sleeping when a noise happens nearby, they have a chance to wake up. The noise's volume minus its distance must be bigger than a 2d20 roll (3d20 with Heavy Sleeper) to wake up the character. However, the character will not know what sound woke him or her up.

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

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

Volume of footsteps is based on the critter's size; digging critters are much more noisy. 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 (it depends on distance), though the bionic's overall effect is fairly small. (Its bonus is the same size as the penalty for Poor Hearing.)