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 turn, 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, in 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 create 6 volume noise. It can be decreased to 2 with Light Step trait, and with leg tentacles mutation movement is completely silent.
• Smashing.
• 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, fusion cells, 40mm grenades (they create noise with fixed 8 volume) and gasoline (4 volume). And bolts and arrows 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 in ten times bigger than explosion power.
• Throwing items cause 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.

Initial volume of sound can be increased by Enhanced Hearing bionic in 3.5 times, by Canine Ears mutation in 1.5, and decreased by Poor Hearing trait in half.

There is a chance to become deaf up to 40 turns if sound is loud and close enough. Enhanced Hearing bionic protects from that.

If sound can be heard and character is sleeping, there will be noise-check, where difference between volume and distance should be bigger than 2d20 roll (3d20 with Heavy Sleeper) to wake up character. Though character will not know what sound woke him up.

If character is busy, then game will ask whether player wants to cancel current activity or not. You are never warned about 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 to precision of markers placing.

Volume of footsteps based on monster's size and digging enemies 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.)