Sound mirrors

Last Denge sound mirror guided walk of 2008

Posted on by Andrew Grantham

Sunday 14 September is the final chance in 2008 to visit the Sounds Mirrors at Lade Pit, near Dungeness.

The walk will be led by Dr Scarth, the world’s expert on sound mirrors and author of the book Echoes from the Sky, who will tell the fascinating story behind them.

The meeting point is at 2pm in Lade car park opposite Taylor Road on Coast Drive, approximately half way between the Pilot Pub and Romney Sands (grid reference TR 085 208).

There is no charge, but donations are appreciated. No booking is required.

See the Romney Marsh Countryside Project website for details.

Microphone array for wind tunnel testing

Posted on by Andrew Grantham

There is a reference to sound mirrors (and this website!) in the University of Florida’s Journal of Undergraduate Research, Volume 9, Issue 2. November/December 2007.

Nikolas Zawodny’s paper on the Design and Fabrication of a Phased Acoustic Array to Analyze Noise Generation of Aircraft Components does what it says on the tin, though there appears to be lots of hard sums involved.

Early techniques of airframe noise analysis involved the concept of an “acoustic mirror,” which consisted of a single microphone positioned in the acoustic far field of a large concave elliptical mirror. The origin of acoustic mirrors can be traced back to the north and southeast coasts of England in the early 1920s, where they were used to provide early warning of incoming enemy aircraft planning to attack coastal towns. These coastal “listening ears” were eventually rendered obsolete with the development of faster aircraft and the invention of radar.

The date is actually slightly out, as some mirrors were built during World War I.

Stone ears teach people to shout through the English Channel

Posted on by Andrew Grantham

A Russian-language page on the Autogena proposal for modern cross-Channel sound mirrors, with some rather familiar looking photos. “????????? ???” seems to be listening ear, and “???????? ???????” is sound wall. Google translates the title of the article as “Stone ears teach people to shout through the English Channel”.

The vodka is good, but the meat is rotten?

Warden Point mirror before the collapse

Posted on by Andrew Grantham

Sound mirror on cliff top at Warden Point
Paul Prior has supplied an old photograph, possibly taken circa 1969-70, showing the sound mirror at Warden Point near Leysdown on the Isle of Sheppey.

The picture was taken before the cliffs were eroded so far that the mirror fell onto the beach below, which is thought to have happened in about 1978-79.

The mirror is on the extreme left of the picture. The design is similar to the mirrors at Selsey and in the northeast, suggesting a First World War date for its construction.

It had to happen – the Ghanaian sound mirror scam

Posted on by Andrew Grantham

Hurrah, I’ve got a [presumably sound] mirror 419 scam e-mail via the website. I’m sorely tempted to try to sell “Mr Curtis Taylor” a 200 foot mirror, slightly used.

Hello,
    Am Mr.Curtis Taylor and will like to place an order regarding some Mirrors from your company to a firm work in Ghana. I will appreciate you email me back with the sizes that you have instock as well as their price ranges and also your terms of payments as well.I will like to be one of your honest customer’s and hope you answer to my request ASAP.Dont hesitate to email me back.Thank you very much and waiting for your prompt responds.

Best Regards,
Mr.Curtis Taylor.

Hythe sound mirror pictures

Posted on by Andrew Grantham

Paul Sheersmith went to Hythe in Kent on May 7, and took these photos of the sound mirror there.

Hythe Sound mirror (Paul Shearsmith)

Hythe sound mirror

Another correspondent writes with news of about a recent trip to the Hythe sound mirror:

I intended to go for a walk by the military canal near Hythe today, but was diverted by the sight of the mirror still standing above the Pennypot estate – so decided to clamber up there.

Very overgrown near the top – could not walk around the back as the stinging nettles were too high. However fascinating to see – so have been hunting for some more information about them this evening. Remember seeing something on ‘Coast‘ sometime ago.

Thanks for the info and the photos – unfortunately we did not have the camera with us today which is most unusual.


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Would the sound mirrors have been much use?

Posted on by Andrew Grantham

Denge sound mirror

Probably not, concludes Brett Holman in a posting on his Airminded blog entitled The widening margin. It is an interesting analysis by someone doing a PhD at the University of Melbourne, examining the impact of airpower propaganda on the British people between 1908 and 1939. The whole site looks worth a poke around.

Lastly, here’s a counterfactual which I’ve long wondered about. Between 1933 and 1935, the Air Ministry put a fair amount of effort into researching the feasibility of using acoustic mirrors as a comprehensive early warning system. The acoustic mirrors were, mostly, concrete hemispheric dishes for focusing sound, which had been used as early as 1916. The biggest ones, at Dungeness in Kent and Maghtab in Malta, were 200 feet long curved walls. Land was actually purchased along the Thames Estuary for the beginnings of a national acoustic mirror system, but work never started because radar came along. But if it hadn’t, then in 1940 Fighter Command might have relied upon a network of these acoustic mirrors all along the coast. How useful would they have been?

The experimental mirrors had a maximum detection range of 22 miles (on very windy days it was a lot less). I’ll be generous and call it 25 miles, which is then added to the 50 miles from the coast to London for a total distance of 75 miles. The Thames Estuary acoustic mirrors probably would have come online in 1936, and so again I’ll be generous, and assume that London at least would have a working early warning system from that year.

Taking all this into account, the results can be seen above [article has a graphic]. And sadly the acoustic mirrors wouldn’t have made much difference — a margin of only about 10 minutes, not much improved on the 5 minutes with no warning system. Of course, even a few minutes’ extra warning was worth having, but the Air Ministry was right to terminate development of the acoustic mirror network in order to concentrate on the far more promising radar.

Read Holman’s full article (and some warnings about the assumptions made).