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

Other things are monsters with additional GOODHEARING flag. They hear sounds at two times larger distance, follow to its source six times longer and they never ever ignore nearby sounds.

Some enemies can be scared with loud enough sounds, others to the contrary, angered more.

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 Light Step trait, and with leg tentacles mutation movement is completely silent.
• 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). And bolts, arrows, pebbles and darts are completely silent.
• Vehicles (generate noise upon thrusting). 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.

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.

There is a chance to become Deafened for up to 40 turns if a sound is loud and close enough. Enhanced Hearing, as advertised, filters that out.

If the character is sleeping when a noise happens nearby, that has a chance to wake. 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 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.)