the thirties the possibility of a surprise intrusion of many airplanes became
apparent. This initiated the concept of a permanently operating early warning
system (Air Watch) consisting of many listening locations distributed over
a wide area. This necessitated the availability of a device to trace the
sound of incoming airplanes with better capabilities than the unarmed ear.
The Measurements Building was asked to investigate the possibilities.
The previously developed listening equipment "Waalsdorp" for the Air Watch Service was too complicated and too expensive for mass-production. A more simple model was required preferably to be worn round the head. From 1930 different versions were tested and it appeared that a miniature version of the parabolic reflectors (see photograph) produced the best results, among other things because these preserved the "sound image" best. According to a report from 1935 a number were taken into production.