In the 1930's the Dutch set up an 'Air Watch Service', similar to the British Observer Corps, to detect incursions of foreign aircraft. Several 'personal sound locators' were developed by the Dutch military research station at Waalsdorp. Apparently this version was the most effective.
This beauty consists of two parabolic sections, presumably made from aluminium for lightness. They are mounted a fixed distance apart, but the size of the human head varies somewhat. To accommodate this, it appears that the instrument is fitted with inflatable ear-pads, to which the parabolic sections are mounted by a star-shaped arrangement of strings; tubes from each ear-pad join in a T-piece and a further tube runs down to what appears to be some sort of valve which may or may not be connected to something in our hero's top pocket. It seems likely that the ear-pads were inflated by mouth. "You can top up your air at any time..." From his pained expression, it looks as if this chap may have overdone the inflation.
According to a report dated 1935, this device was put into at least limited production.
