Malta was the only overseas possession where acoustic mirror was built, this was the Il Widna Sound Mirror in the North of the Island. After just a couple of years this sound mirror, which was supposed to be first of five, was found to be unreliable. A number of new sound locators (acoustic warning devices) were therefore introduced in an attempt to develop an early warning system for aerial attack.
These new sound locators were produced from 1938 onwards. They were mounted on a four wheeled trailer with three parabolic receivers and sensitive microphone detectors. Operation of these devices was quickly improved with the introduction of a cathode ray tube which converted sound to a visual green screen display.
It was intended that the Sound Locator be deployed with a searchlight, with the later placed 50 yards behind the locator pen. The detachment required to operate the searchlight and locator was nine men. This included two spotters. All communications were by telephone at first, but later radios (No.17 set) were used.
It appears that in Malta specific permanent position were built for the sound locators and their searchlights. These are well constructed, particularly the sound locator pens. The pens appear to have been adapted to function as redoubts. Firing steps have been built inside and in some cases loopholes cut through the original wall. The Sound Locator Emplacement would appear to be unique and the only surviving examples anywhere.
