This is in effect a continuation the "using dead bodies materials for food/construction/etc" series.
The world is suffering from the creatures of fantasy and nightmares which can appear with just a few minutes of warning anywhere, regardless of it being the middle of the street or on top of your bed while you are asleep. It can be a single one, or millions that swarm the area. The world's infrastructure has been degrading, food and supplies are harder and harder to come by. While firearms work well against these creatures, the bodies left behind attract and breed both insects and disease. It is not feasible to hold on to areas with low population as a sudden appearance of many nightmares can wipe them out.
Civilians will be seeking shelter when the alarm is sounded while militia, law enforcement and paramilitaries will move in to try and clear the spawning of nightmare creatures.
There is a problem, however. When a spawning starts all electronics in a semi-random radius are affected, causing electric lights to flicker and computers to fail for several hours. The radius can be as small as 50 meters up to several kilometers.
Before a spawning occurs an otherworldly sound of level around 75 dB (vacuum cleaner) can be heard and spooky light flashes which give an indication of where and how many might spawn, but without absolute certainty until the creatures have fully spawned. If a small group spawns in a remote area of the city, the amount of noise and light might go completely unnoticed. It takes from half a minute up to 10 minutes before the nightmares basically teleport into the area (avoiding any obstacles).
The difficulty here is sounding an alarm that can be adjusted to the threat level. For a small amount of creatures you do not want the entire city to find shelter while every part of the militia goes to the area, but for a large spawning (in the thousands) you do want such a reaction. Further complicating this is the absence of proper electronics to send coherent messages while this is going on.
The question: how would a system look like to sound an alarm with adjustable reaction from the cities' militia and civilians?
Features of the alarm system in order of importance:
- civilians will be the main source of spotting the disturbances before a spawning and have to be the first chain for any alarm system (unless you can find one that really doesn't need them).
- civilians are expected to leave immediately after the alarm is sounded, some way of making sure the location remains detectable after they leave is required.
- the alarm has to be able to signal people both inside and outside
- at minimum, have to have a 99% effectiveness in reaching people and militia nearby the disturbance.
- the alarm needs to be able to guide militia towards the spawning, and warn civilians away.
- the alarm has to scale with the size of the spawning area and eventually the actual amount of nightmares counted when they appear.
- the alarm has to be easily reachable by civilians or portable. Like flare guns.
- the alarm is allowed to use word-of-mouth in its chain, like someone outside warning people inside. However it has to use as few people as possible in jobs as watchmen that warn others.
- people asleep will not be counted, people will sleep in small communities with a designated watcher to wake them up in case of an alarm.
- if it wasn't clear already: I expect this alarm to be a chain rather than a single person being able to do all the work.
PS: I understand this isn't an easy question and has a lot of variables. Try to keep an open mind please.