In World War I carbon microphones were still used as sound receivers. The individual receivers were mostly placed in the front part of the vessel along the hull sides to have enough distance from the screw and the noise they emitted. The individual microphones were arranged in groups and each was oriented in a different direction. The individual microphones had to be connected manually to take bearings. They were not very reliable, so other transducers were experimented with. Dynamic microphones were also discarded. At the end of the process, the piezoelectric principle was deemed the most suitable. This was discovered by Pierre Curie in 1880. The quartz crystals generate electric voltage depending on the pressure acting on it.
In collaboration with the Imperial German Navy, Atlas Werke AG in Bremen and Electroacustik (ELAC) in Kiel worked on piezoelectric transducers and the development of detectors and amplifiers in general. They experimented with different kinds of crystals, or combinations of several of them. The best result rendered the Seignette crystal, which is formed from a mixture of different salts. From 1935 crystal receivers were permanently installed on all German submarines. Modern submarines still use electrostriction and barium titanate converters today.
The group listening device (''Gruppenhorchgerät''), abbreviated "GHG", consisted of two groups of 24 sensors (one group on each side of the ship). Each sensor had a tube preamplifier. These 48 low frequency signals were then routed to a switching matrix in the main unit. The sonar operator could determine the ship's side and the exact direction of the sound source. To improve the resolution, there were three switchable crossover with 1, 3 and 6 kHz center frequency. A disadvantage of the side mounting, was a dead zone of 40 ° to fore and aft. Range: 20 km to individual drivers, 100 km against Convoy
Search area: 2 × 140 ° Resolution: <1 ° at 6 kHz, 1.5 ° for 3 kHz, 4 ° for 1 kHz; without crossover 8 °
The GHG fitted to early U-boats could not be used effectively at periscope depth. To solve this, a new listening device, known as Balkon ('balcony'), fitted to a second, lower hull, was successfully tested on U-194 in January 1943.