least a dozen different acoustic location equipment's for aeroplanes from
different countries were in the 1920's available on the military market.
They served as said as supplement to visual means preceding the use of artillery.
These equipments contained most times four sound pick-up elements such as curved shells or horns used in pairs. Two elements at a fixed mutual distance were adjusted in the horizontal plane for the determination of the chart angle and the remaining two in the vertical plane for the measurement of elevation.
The transport of sound from each element of a pair to a corresponding pair of human ears was carried out by means of metal or rubber tubes. Each pair is adjusted in the correct direction when the two ears receive the sound signals at the same time. To increase the change of the time difference in sound reception the elements of each pair were placed at large mutual distance.
An example of early acoustic listening equipment is on display at the Canadian War Museum in Ottawa. It is a British MK 1 Sound Locator by A.W. Gamage Ltd.