This early listening device named “van Soest” after the engineer who created it was developed in 1929. It was in service with the Dutch Army in May 1940 at the start of the Second World War in the Netherlands. It accousicly dected incoming aircraft at a greater distance than normal hearing and it was used together with searchlights and anti-aircraft guns. After acoustic detection of an aircraft a nearby searchlight took over and guided the anti-aircraft gun to a target.
