Sound/Acoustic mirrors in WW1?

[NigelS]

In the BBC World War I at home series the programme 'The Trawlermen' - originally only shown in the Yorks & Lincs region back in June - was shown on BBC4 tonight (23rd September). As suggested by the title, this was mainly about the North Sea trawlermen's conflict with U-boats, with more detailed coverage given to the refusal of a Grimsby trawler captain to pick up the crew of L19 (LZ54) leaving them to drown after she'd ditched at sea, but it also touched on the use of Q ships & Zeppelin air raids on north east coastal towns

Another topic covered very briefly was coastal defences which included a short film sequence showing a concrete sound or acoustic mirror (given as being in the area of Spurn Point) with the commentary saying that it was in use during WW1. I know that sound detection was used for ranging purposes on the Western Front, but had always understood that the static, concrete type shown to be an inter-war, mainly experimental, development that was rendered obsolete by the increasing speed of aircraft making their detection too late to be of any real benefit, and the invention of radar.

Was the programme hugely in error in showing the mirror and linking it with WW1; I haven't seen any evidence to suggest there were, but were any constructed and put to use during WW1 ? From the footage, to me it appeared to be very similar, if not the same, as the smallest of the three at Denge near Dungeness which was certainly an inter-war site.

NigelS

[centurion]

The programme is correct. Tucker microphones were being installed in cliffs etc from 1917 but did not become really effective until 1918 when they could be coupled with newly developed amplifiers using vacuum tubes I posted a link to a Royal Society article on the use of optical mirrors for tracking air craft and AA fire only yesterday. This included a section on sound mirrors

http://rsnr.royalsoc....2010.0090.full

[NigelS]

Thanks for that Centurion; an interesting link. I have since found the English Heritage listing for the one that was shown (Click) which confirms the date of construction and also gives that some of those on the Kent coast also dated back to WW1, but those at Denge to the 1920s; my understanding is that Denge was a research site, so possibly the one there that resembled that shown in the programme was built to the same design in the early days as a starting point for the project there. Strangely, although the listing gives that all the remaining acoustic mirror sites 'are considered to be of national importance' the site at Denge (near Greatstone, Hythe), which has the largest surviving example in the UK (an impressive 200ft version), appears to be unlisted . I thought a copy of the Echoes from the sky. A story of acoustic defence (published by the Hythe Civic Society in 1999 & given in the references of the link) might be interesting, but unfortunately it's out of print with silly money (£100+) being asked for used copies.

NigelS

[centurion]

The first experimental sound mirror was built at Binbury Manor, near Maidstone, Kent in 1915 and operational mirrors began to be built from 1916 to 1930. There was a series of mirrors built to protect the N East during WW1. These were at Kilnsea, East Yorkshire, Boulby, North Yorkshire, Redcar. Hartlepool, Seaham and Sunderland The Redcar Mirror was built in 1916 and still exists.Others were built on the South Coast. Typically these were of concrete construction with an inclined wall about twenty feet long into which the bowl was embedded. Two wings about sixteen feet long were placed one at each end, these shielded the reflector from wind and extraneous noises coming from the side. A sound collector was placed in front of the dish; this looked something like an old fashioned gramophone horn mounted on a post. In the early versions the operator sat in a trench below the sound collector connected to it by a stethoscope but later the sound collector was replaced by a Tucker microphone (see previous post) and operators sat in a warm(er) hut nearby.

The use of one of these detectors effectively required the operator to have had considerable training; even then and with the Tucker microphones aircraft would not normally be detected before they were 25 miles away. This gave a warning period of about ten to twelve minutes (longer in the case of the slower airships)

[Tunsilk]

Nigel

The "Sunderland Sound Mirror" is still there.

http://sunderlandcitycouncil.com/acousticmirrorgallery.html

David

[centurion]

By 1930 the sound mirrors had advanced from their WW1 rather crude detection apparatus as microphones and amplifiers improved a process started during WW1 by Tucker and Bragg however they could never overcome their fundamental limitation - the speed of sound itself. Even in 1918 by the time the sound from an aircraft 25 miles away had reached the operator that aircraft was already a couple of miles closer and closing (and there was a latency in sounding the alarm, scrambling aircraft etc as the operator need to make sure that he really was hearing an enemy aircraft). Post war as aircraft speeds increased the warning time became less and less

[NigelS]

An article on the Independent's website about two sound mirrors on the Kent coast (with a rather good photo of one in use) which were believed to have been destroyed in the 1970s, but in fact had only been buried, and are now being excavated Click

NigelS

[Phil Eyden]

An article on the Independent's website about two sound mirrors on the Kent coast (with a rather good photo of one in use) which were believed to have been destroyed in the 1970s, but in fact had only been buried, and are now being excavated Click

NigelS

I visited these sound mirrors at Fan Bay at Langdon, Dover last weekend. Excavation of the site by volunteers working for the National Trust is virtually completed and they will be open to the public to visit.

The mirror in the foreground is the 1917 dish, the one in the background is early thirties, and both are in good condition. They have been buried for almost forty years so it is remarkable to see these again.

The walls in front were 1940 toilets. By then the mirrors were redundant so building in front of them really didn't matter...mind you they may have amplified certain straining noises.

A little operational history for you:

The 1917 Dover mirror was a fixed 15ft reflector cut from the chalk and covered in concrete and was focused on the mid point between Dunkirk and Calais. The sound was reflected into a 3ft trumpet which was pivoted in the centre, connected to stethoscopes. Later experiments were made with two trumpets and later still with resonators instead of trumpets. The device was first used during the raid of 1/2 October 1917 and it successfully picked up the sound of enemy aircraft flying down the Channel, 12-15 miles out to sea. The sound was inaudible to the Dover gun crews. The bombers raided a French coastal town but the sound of the heavy gunfire at Boulogne was 'unbearable' for the Fan Bay listening post operators.

Lt. Col. W.M. Thompson, the Dover Anti-Aircraft Defence Commander, was enthusiastic and predicted the device would be invaluable in obtaining early air raid information. On the night of the 22/23rd December 1917, the Fan Bay listening post detected a 'Giant' bomber (Zeppelin-Staaken Riesenflugzeug - a four engined tractor biplane with an enclosed cabin and a wingspan of 42m!) heading down the Channel to bomb Boulogne. It was first detected at North Foreland at 19:36 but Fan Bay then picked it up and tracked it until 20:05.

On the night of the 19/20th May 1918 a huge bombing raid was launched on London and aerodromes in Kent en route utilising 38 Gothas and 3 Giants. 88 fighters attempted to intercept them with some success. Lt. Col. W.R. Thompson stated that the reflectors at Fan Bay and Joss Gap at Broadstairs proved invaluable in picking up enemy aircraft five to ten minutes in advance of the first warnings received at the gun stations.

Source: Cole, C., and Cheesman, E.F., The Air Defence of Britain 1914-18, Putnam (1984),

Edited 20 November , 2014 by Fossil Phil

[NigelS]

Thanks for that Phil. Interesting to read the operational notes

NigelS

[Phil Eyden]

To keep you updated, the National Trust will be opening up the Dover mirrors to the public in May. I will let you know exact dates as soon as I hear.

[Tunsilk]

The Sunderland mirror has been restored

http://www.bbc.co.uk/news/uk-england-tyne-33059534

[Gareth Davies]

More details here: http://www.historicengland.org.uk/news-and-features/news/first-world-war-acoustic-mirror-sunderland

[NigelS]

That's good to see, thanks for the links Gareth & Tunsilk

NigelS

[trajan]

And well done those who managed to get the site properly conserved and displayed!