Determining the fineness of a ground material, in particular an ore, in a mill

The invention relates to the determination of the fineness of a ground material (120) in a mill (100), wherein the acoustic spectrum of the mill during the grinding process is used for concluding the fineness. The mill (100) comprises a drum (110) in which the ground material is present during the grinding process, said drum being driven by a magnetic drive (130) having at least one magnetic segment (131/1, 131/2, 131/3) during the grinding process, so that a rotating magnetic field generated with the aid of the magnetic drive causes the drum to rotate. In order to determine the acoustic spectrum, a voltage (Vind) induced in a coil (133/1) of the magnetic segment (131/1) is determined at least intermittently. On the basis of a model, using the detected time-dependent induced voltage (Vind), conclusions can be drawn about the acoustic spectrum and therefrom about the fineness.