There are more than 4,600 sirens in Sweden, able to reach most people. The signal that is tested in peacetime is "VMA", (Viktigt meddelande till allmänheten), a remnant of the air raid sirens of the Second World War.
It consists of a seven-second note, followed by 14 seconds of silence, a sequence that repeats for a period of at least two minutes.
They sound as a test at 15:00 hours on the first Monday of March, June, September and December, with the exception of public holidays.
The popular name, "Hesa Fredrik", (hoarse Fredrick), was coined by Dagens Nyheter columnist Oscar Fredrik Rydqvist at the time of the sirens' first test in 1931.
Although largely ignored now, when you hear the alarm you are supposed to go indoors, close doors, windows and vents, and listen to the radio for more information. The all-clear is sounded by a 30-40 second-long note.