Air acoustics: Early listening devices (1915 – 1940)
At least a dozen different acoustic aircraft location devices from different countries were available in the military market in the 1920s. They served as a supplement to visual means for the artillery.
These devices usually contain four sound-absorbing elements such as curved shells or horns that were used in pairs. Two elements at a fixed mutual distance were adjustable in the horizontal plane for the determination of the map angle. The other two shells or horns could be adjusted in the vertical plane to measure the elevation.
The sound transport of each element from a pair of shells or horns to the human ears took place using metal or rubber tubes. A pair of shells or horns are positioned in the right direction if both ears receive the aircraft engine noise simultaneously. To increase the change in the time difference in sound reception, the elements of each pair of shells or horns were placed at a great distance from each other.
Text by courtesy of the Canadian War Museum, Ottawa, Canada:
The Mk 1 Sound Locator was manufactured by A.W. Gamage Ltd. in Britain during the First World War. In the early days of the First World War, anti-aircraft defence was a new field. The detection of unseen incoming aircraft was a major problem. The only possible solution with the technology available at the time was sound detectors, which could provide a rough idea of an aircraft’s direction and height based on the sound of its engine. Tubes connected the bases of two horizontally mounted gramophone-style horns to a pair of stethoscope earpieces. An operator moved the detector until the sound was heard equally in each ear, at which point (theoretically) it would be pointed in the direction of the aircraft. A second operator used the vertically mounted horns to estimate height. The system was rudimentary at best, however, as the location of the aircraft could only be established for the time that the sound was recorded. After a sound contact was made, laborious calculations were then required to properly aim an anti-aircraft gun, and any deviation in the aircraft’s flight path rendered the system useless. It was, however, the only system available for detecting the approach of unseen aircraft until the development of radar in the 1930s. According to the Imperial War Museums, this device had a maximum accuracy of 1/10 of a degree.