Air Raid Siren Sound
Thanks to electricity and the invention of the motorised siren, this type of siren was widely adopted before and during the Second World War. The undulating, howling sound of the siren was highly characteristic, which made it an efficient warning signal ahead of air raids. It also became synonymous with the terror of falling bombs -- death dealt from the skies. An entire generation was scarred forever by the sound of the motorised siren.
Air Raid Siren Sound
In England, each village and city had its own network of sirens. Some of them were electrical (e.g. the famous brand Klaxon) while others were manually operated (particularly in rural areas).
From Air Raids to Nuclear Alarm
During the Cold War, sirens were no longer used to warn of air raids. Instead, they became alarm systems for possible nuclear attacks. In Germany, the term 'alert' replaced the previous 'air raid warning'. A shortened, one-minute siren alarm was devised to alert citizens of a possible attack.
In post-WWII England, alarm sirens were assigned a new meaning: alerting people to the threat of a nuclear attack. They were also used to warn people of floods and freak weather, as well as escaped prisoners or highly dangerous mental-health patients.
Mechanical Air Raid Siren Sound
Mechanical alarm sirens create a howling noise by rotating a shovel-studded drum within a container that sports openings. The higher the number of the rotor's revolutions, the higher the pitch of the howling. The result is an undulating siren alarm sound. The siren can be operated electrically or manually. These mechanical alarm sirens are widely used all over the world. In the United States, very big and loud mechanical alarm sirens equipped with V8 cylinder motors were used during the Cold War.
In Germany, the mechanical siren is called unit siren E57 (although East Germany and countries of the former Soviet Union used their own type of similar construction during the time of the German Democratic Republic). This alarm siren creates a howling alarm sound that can allegedly be heard across very long distances (it reaches a sound level of 105 dB). In rural areas, the sound of the siren is said to reach 70dB even at a distance of 700m. In cities and industrial areas, these levels can obviously not be reached. One can imagine that the American alarm siren outfitted with a V8 motor was probably quite a bit louder than that.
Mechanical alarm sirens are often mounted on rooftops, high rise buildings and pylons. As these sirens are exposed to the elements and, in some cases, animals (particularly birds), they have to be tested several times a year. The sirens in Switzerland, for example, get tested annually on the first Wednesday in February -- which results in an impressive, country-wide howling concert.