Abstract: An electrodynamic loudspeaker which comprises a diaphragm which cooperates with a magnet system by means of a moving coil which is connected to said diaphragm and an optical measuring system which comprises two elements--a light source and a detector. At least one of said elements is connected to a central portion of the magnet system and the detector supplies a voltage which substantially corresponds to the displacement of the diaphragm. The other element is connected to the diaphragm and the magnet system is provided with a channel which is disposed in the direct path between the two elements.In order to linearize the luminous intensity characteristic, in accordance with a preferred embodiment, the channel is filled with an optical fibre material. Light weight LED's and phototransistors may be used as the optical elements.