This is the ESU 50348 Passive Radiator Speaker – 18mm x 35mm x 11mm

This speaker module offers extremely good performance data with moderate dimensions. With the help of the built-in passive radiator in conjunction with a high-performance rectangular speaker, the module achieves high quality bass reproduction and achieves a considerable sound pressure even in the highs and mids. The module is particularly suitable for diesel or steam locomotives where contemporary bass reproduction is important. The speaker fits perfectly with our LokSound 5 micro, LokSound 5 or LokSound 5 L decoders.