A Dutch sound locator

Between the two world wars, sound locators played an important role in air defence. Radar was not yet discovered and the noise of airplanes was an important aid to detect enemy aircraft.

(from left to right): Ir. van Soest, the prototype, the correctioncylinder

Around 1920, a dozen sound locators were available for military purposes. The Dutch Army however, was not impressed by their quality and instructed Van Soest to develop a better apparatus himself. With some coworkers he started investigations in The Hague-Waalsdorp.

He finally developed a parabolic sound mirror that was cut in half with each part focused directly on an ear of the listener. The half parabolas were joined to a vertical column with a seat. An inflatable ring shaped rubber cushion filled the space between the bearings and the ears of the listener. The adjustment of direction and elevation was by the muscle force of arms and legs.

The sound locator was operated in conjunction with field glasses and a searchlight. It was important that the angle and elevation of the airplane were quickly available and special attention was given to make a good transfer system for this. Because the airplane speed is not negligible with respect to the speed of sound in air, the actual location of the airplane differs. For this a correction cylinder was developed and the data were transmitted with an electrical transport system. With these values the aircraft was located by the field glasses and the angles came directly available at the search light.

(from left to right): the field-glasses, the sound locator during WW2 and in our museum.

The Dutch industry produced nearly one hundred of these sound locators from 1935 on. They were used during the German invasion in May 1940 and later also in Dutch-Indië (Indonesia). Because of increased airplane speeds and the discovery of radar, the sound locators fell out of use and nearly all of them were destroyed. The sound locator in our museum is therefore unique and an example of Dutch military history that has now vanished.

The locator can be seen in our museum beside the impressive French/German sound locator. It was renovated in 2008 but is not yet entirely completed. When a uniform of a Dutch WW2 soldier is available, we intend to complete it.

The information for this article is from Aad van der Voort, Museum Waalsdorp, www.museumwaalsdorp.nl

Leo Steeghs