This article may need to be expanded on in the future as better, more exacting data becomes available.
Zombies are attracted by "noise", which is simulated in the game as a numerical value called 'activity'. Within the community, the terms noise and activity are generally used interchangeably. Activity is added to each tile by different events that one would understand logically produce sound. The activity level in an area can be estimated by the player but is not visible directly. The higher the activity in an tile, the further away the game engine checks for infected to 'hear' the activity and come to investigate. Whether an individual infected hears the noise is determined randomly, so essentially for a given level of noise a certain percentage of infected within a given range will come to investigate.
Many higher level infected are more alert and more responsive to noise. An alertness factor modifies how loud a noise is treated by each infected, and consequently from that how far away the infected may be and still hear the noise. While slow zombies multiply base activity by 2, fast zombies and fatties multiply it by 3, venom infected multiply it by 4, and harpies multiply activity by 8.
With very heavy noise, a Harpy can be attracted from up to 72 tiles away (maps 1-3; and even further away on map 4), while other lower level zombies would need to be much closer to have any reaction. This is also why, when clearing out an area, it is not uncommon for many of the walker tier zombies to not react at all, while a few "runner" zombies come bounding in from further away. In general, zombies can hear much better than they can see -- it is much more likely that a zombie comes running over because they heard a bunch of gunfire than it is because they saw one of your units or buildings themselves.
Noise is produced by a variety of activities. Some examples of noise production:
- Attacking with an ranger produces little noise, or 1 activity per attack.
- Attacking with a soldier produces moderate noise, or 3 activity per attack.
- A Ballista firing produces a moderate noise - 5 activity per shot. However, because the ballista can do a relatively large amount of damage per shot, the total noise generated by a ballista to kill a given number of zombies is among the lowest in the game.
- Attacking with a sniper produces higher noise, or 10 activity per attack.
- Attacking with a Titan produces very high noise, or 20 activity per attack.
- Attacking with Thanatos produces extremely high noise, or 500 activity per attack.
- Like units, the attacks of infected themselves also generate noise with each attack. The more dangerous the infected, the more noise its attack generates. Infected attacking a structure, such as a wall, can therefore set off very dangerous noise cascades, where the noise of the attack on the wall and even the attempts of the player to kill the offending infected, themselves produce even more noise that attracts more and more infected at an exponentially increasing rate.
- Zombies infecting a building produces 50 noise per worker or population in the building. Thus the amount of noise generated can be moderate for infecting a Tesla tower, or extremely high for infecting a farm or stone house.
- Infected breaking a wall or building produces a large amount of noise.
Note that the attack speed of a unit affects its effective noise. Thus, a veteran soldier will tend to generate more noise than a newly created sniper. Also note that since the distance over which the game checks for zombies to be alerted by noise depends on the noise level, longer ranged weapons tend to pull relatively fewer adds than shorter ranged weapons by the simple virtue of being further away from any infected that are currently not aggressive.
If a zombie is attracted to an area because of noise, it will walk there and "investigate". If the source of the noise is no longer present (for example, if you were clearing with 10 snipers then moved them away) it will stay in that area and will not be able to chase after the "true" source of the noise, because as far as the zombie is concerned, the source of the noise was that exact spot on the ground.
When noise is produced, the game adds activity to a tile, and a smaller amount of activity to adjacent tiles. The exact amount of activity added to a tile other than one that the noise occurs in is not known. However, the fact that activity is added to both the tile and adjacent tiles is the reason that groups of units working in a small space are much 'louder' than units that are spread over a sufficiently wide area. The noise level that each of these affected tiles has will decay to 0 over a few seconds. Existing activity decays to half of its current value every second. Thus, an ranger firing continuously does not accumulate much noise over time. However a veteran ranger with a higher firing rate can accumulate much more noise, which is observable on map four. Different sources of noise stack up with each other, if the activity adds up faster than it can decay. While 5 Rangers are very quiet, 50 Rangers within the same small area can be quite loud.
Note that kiting a noise producing unit spreads the noise over more tiles, resulting in less activity than if the unit stood in one place and continuously fired.
Noise level can also be used to the player's benefit by using loud units to attract as many zombies in an area as possible. For example, with a band of 20+ snipers shooting off their rifles, or by blasting an area with a Thanatos.
Zombies are much more sensitive to noise on Map 4 but much less sensitive to noise on Map 3. On map four the usual distance that he game checks to see if a zombie hears noise is multiplied by 1.5. For example, if sound might normally only be heard by zombies within 6 tiles, on map 4 it can be heard by infected up to 9 tiles away. This causes even very small sounds, such as a ranger firing, which would normally not be heard from further away than the infected's view range to become significant. For loud sounds, generally more than twice as many infected will arrive to investigate as would normally be expected, which can very quickly create cascading problems as dealing with the adds causes even more noise to be created leading to uncontrollable difficulties.