Probably the first thing most people heard after Chamberlain's famous speech on the outbreak of war in September 1939 was the air raid siren. This was an essential piece of kit used to warn both the military and civilians of the threat of enemy aircraft and bombing.Two tones were used; Red Alert which meant imminent attack and the All Clear which meant it was safe to leave your shelter (if you were lucky enough to have one/find one).
Some were hand cranked:
|Image found at: http://i1.ytimg.com/vi/9i1QAtET0ok/hqdefault.jpg|
Image found at:http://www.sutton-coldfield.net/ww2/air%20raid%20siren.jpg
The only information about who manufactured them is on this page about the blitz in Liverpool: Liverpool Blitz - the sound of the air raid siren. It says that Gents of Leicestershire made the sirens that were used in Britain throughout the war. Below is an excellent extract from a 1939 film, The Warning, courtesy of BFI.