Confirmed: No-one has actually tested hordes. Here is my proof:
TL;DR: Zombies from at least 76 map tiles away, and possibly more, will travel towards your position, never wavering, travelling at about half of your walking speed, whenever you fire a gun, or make a noise more than 100 sound. Not tested: Making sounds that are more reasonable, such as 20 or 30. Zombies will also occasionally travel to a completely different position than the sound you made, and will also initially head towards the top-right corner of the map. Since there are no naturally occurring hordes above the top of the map, heading north-west initially gives you the greatest chance of survival.
Initially I gave myself clairvoyance, revealing the map, and 99 stats for character creation. I then looked at my horde overmap.
Bug #1: All horde groups already had a point of interest, all within (-10 to 10),(-10 to 10), and an interest value of 20, meaning the hordes were already moving towards the top-right corner of the map.
Then, I waited, taking snapshots of a horde as it moved towards its target.
Fact #1: When a horde is in motion, it will travel at about 1 overmap tile every 15 minutes[Edit later, more structured, tests show that the actual speed is exactly 1 tile every 10 minutes]. If you sleep for 8 hours, then every horde that is travelling towards you from 32 overmap tiles away will reach you.
After about 2 hours I noticed another bug. While the interest value initially decreased by 1-2 points every 30 minutes for the first 2 hours, at 2.5 hours it was stuck at 15.
Bug #2: Zombies never stop heading towards sounds. Their interest drops to a minimum of 15, and they always head towards the sound.
OK … next experiment. How does player sound effect things? Well… first I fired a Ruger 22 loaded with rat shot. 112 sound later, a horde that was around 16 over map tiles was heading towards my position with 100 interest, while a horde that was 40 overmap tiles completely ignored me. I then moved to another position, and …
Bug #3: Zombies with 100 interest do not appear to move towards a sound.
Info #2:I found out that PCs can move, at a walk, about 1 overmap tile every 5 minutes or so, [Edit]2[/Edit] times faster than hordes.
Finally, I wished for a mininuke launcher and a modified mininuke. One deafening explosion later (which kept me deaf for about 2 hours) … and there was no change compared to the Ruger.
Unconfirmed Bug #4: I theorize that any sound over a certain limit attracts zombies from a certain distance, after a certain distance, no amount of noise will attract a horde. In addition, any firearm has the same effect: It will attract all zombies within this ‘cutoff’ distance, as all firearms produce over 100 sound.
(PS. While I was looking at this, I found that a horde with 99 interest was moving towards me. 30 minutes later, they moved 3 diagonal tiles, and lost 6 interest. 30 minutes later, they move another 3 diagonal tiles, and lost 6 more interest.)
(PPS. Holy … OK, so unconfirmed bug #4 is false. I was attacked by a bear, so I shot it with rat shot to keep it away. About 80% of the horde that I was looking at [I had teleported 55 tiles to the west] was now interested in me, with interest 50.)
(PPPS. OK, so it turns out that the initial zombies, after hearing either the mininuke or the 22 bullet, decided to go to completely the wrong coordinates. They decided to travel to -37,66; but my position was likely more like 284,66; . The second group (who were attracted to the second gunshot) continued to travel at 1 tile every 10 minutes (this is the actual speed, and as such, zombies move about half the speed as regular people do), losing 6 interest every 30 minutes.)
(Final PPPPS. So it seems that the experiment has failed after the horde encountered a horde of fungi. This scattered the group and some lost all but 15 interest. Another horde that I hadn’t seen (but, assuming that they travelled at 1 tile every 10 minutes, they were 52 + (4 hours / 10 minutes = 24) = 76 tiles away has 15 interest, and is still marching towards my position.)
