Air acoustics: small, personal listening devices (1930 – 1935)
In the thirties of the last century, the possibility of a surprise attack by many hostile aircraft became apparent. This led to the concept of a permanently operating early warning system by the Luchtwachtdienst (Air Watch Service) consisting of many listening posts spread over a large area. Observers throughout the nations had to report observations of foreign planes. The observed flight direction and estimated speed were transmitted to a central point by telephone. Operations required a device which helped to locate and track the sound of incoming aircraft better than that for the unarmed ear. The Measurement Building was asked to investigate the possibilities. From 1930 onwards various forms were tested. A scaled-down version of the parabolic reflectors achieved the best results, partly because the “sound image” was best preserved. A 1935 report mentions that a series of these devices were produced by the Dutch industry.