The RRH (Ringtrichter-Richtungshörer) is a circular acoustic locator developed in Germany in the 1930s. When developing acoustic locators for the Wehrmacht and Luftwaffe in the 1930s, German engineers found that using stereophonic headphones and arranging the acoustic locator horns in a circular pattern provided a more accurate audio perception of an approaching target compared to conventional horn-type acoustic locators. Using several RRHs in combination, it was even possible to roughly triangulate the position of an incoming aircraft before it was within the reach of the searchlights of the anti-aircraft units. In good weather, the operational range of an array of RRHs was up to 12 kilometres.

From the 1930s, there were thousands of RRHs manufactured in Germany. They were also sold to Finland and other friendly countries. The development of acoustic locators continued until 1944. In autumn 1944, the German Air Defence had more than 5,500 RRHs (various models) in service. In Finland, acoustic locators were phased out in the early 1950s.

References: Palmu Pentti, Yön yli päivään – Suomen Ilmatorjunnan Vaiheita 1925–1989, Ilmatorjuntaupseeriyhdistys, Helsinki 1989; Werner Müller, Horchgeräte – Kommandogeräte und Scheinwerfer der schweren Flak (= Waffen-Arsenal. Sonderband. 21). Podzun-Pallas, Friedberg/H. (Dorheim) 1991