“Listing of sound mirrors urged: Oliver Gillie reports on the pre-radar detection devices that enthusiasts want to see preserved” from the Independent on 3 July 1993.
HUGE CONCRETE sound mirrors used before the days of radar to listen for the approach of hostile aircraft should be scheduled as national monuments, according to enthusiasts. Already several have been demolished …
The remains of three large concrete structures, formerly an anti-aircraft Acoustic Detection installation. Microphones were attached to the three reinforced concrete structures in order to pick up the sound of approaching enemy aircraft. The smallest ‘sound mirror’ was found to be fairly ineffective so it was superseded by a larger dish, 12 m in diameter. This in turn was replaced by a 70m long ‘sound wall’. The structures were built by the RAF between 1930-4. They were rendered obsolete by the introduction of radar in 1935, and by advances in aerial technology.
Source: Exploring Kent’s Past
… as radar technology was in its infancy, in Malta, the British authorities decided to build an early-warning system, the Parabolic Acoustic Mirror known as Il-Widna (ear), constructed at Ta’ San Pietru, Maghtab.
Il-Widna became outdated when the British government decided to install the first RDF [Radio Directing Finding] in Malta. It is important to say that this was to be the first RDF installed outside the UK.
The Sound Mirrors of Denge at Passing Strangeness. If a technology solves a problem that’s particularly desperate (or it solves one that’s less desperate but more profitable), it’s often prefigured by other, lesser technologies.
The Museum Waalsdorp near Den Haag reflects the history of TNO Defense, Security and Safety at location Waalsdorp (and its predecessors) since 1927.
At the Waalsdorp site, Dutch scientist JL van Soest investigated the use of listening equipment for aircraft observation by the army, developing his own equipment. The Van Soest apparatus was a great success and has led to industrial production for the Army.
The museum’s website has an interesting short history of Dutch sound location. From the first world war until the 30’s air acoustics played an important role in the air defence. Air vehicles carrying a weapon could not be located from the ground e.g. at night time or under cloudy conditions. As radar was still to be discovered, vision had to be supplemented by hearing using the sound of the engines.