Air acoustics: Industrial production listening devices Waalsdorp (1935 – 1940)
From 1935, the first listening devices to the Van Soest model were produced by the Artillerie-Inrichtingen (Artillerie-Inrichtingen was a Dutch state-owned artillery, small arms, and munitions production company which was established in Delft, the Netherlands in 1679. Due to insufficient contact with the Measurement Building, the first factory delivered devices were poorly squeaking and creaking devices.
In 1937, The Artillery-Institutes produced six sets with listening devices (LTs), viewing devices (KTs) and position data transport systems for the Dutch East Indies. These device sets are known listening devices model Colonies. According to the reports they have functioned well. In 1937, another set of 33 listening devices of the type Air Watch Service (small, personal model) were delivered to the Royal Dutch East Indies Army (KNIL). In 1939 another set of seven ordinary listening devices were delivered to the East Indies.
A report dated April 10, 1940 – just before the outbreak of the war – mentions thirty delivered listening devices by the Artillerie-Inrichtingen in Zaandam to the Sappers Corps of the Army and 80 devices under construction. The Artillerie-Inrichtingen version listening device was mounted on a flat two-wheeled cart. It was operated by a sergeant as unit commander, a listener, and a reader for the cylindrical plane table board, which determined the target’s map angle and elevation.