Posted 18 September 2012 - 11:50 am
Both sides in WW1 used these systems for warnings of gas attack and I believe the distance between each horn was laid down in official instructions. Photos appear on the IWM and AWM collection websites.
I have researched aircraft Klaxon Horns and while most were of the 12 volt electric type a lot of research was going into other types in 1917 and 1918. The research was being undertaken at Orfordness and Butley (which was the acoustic research centre), the items tested included Electric Horns, Mechanical Horns both hand (as in previous photo) and Windmill operated as well as Strombos or trench horns (both large and small) blown by compressed air. In my reading of documents there are mentions that the aircraft 'Klaxon' should not be able to be confused with the sound of the trench 'gas' horn, so they did need different sounds coming out of these devices.
The RFC/RAF used these devices for Contact Patrols, Counter Attack Patrols, Night Flying identification system (also it was tested as an inflight communication device between gunner and pilot on the DH.4), also according to documents from the TNA it also had some uses on the ground at airdromes. This included Fire Alarms, Patrol Warnings and Raid Alarms. So I suspect there were a lot of these sound systems around during WW1.
I hope that is of use.