Abstract: In the present invention a refiner (108) of a mechanical pulping line is supplied with a controllable acoustic source (14), arranged for emitting acoustic signals into the pulp fluid. The controllable acoustic signal is allowed to interact with the fibres in the pulp, and the acoustic (pressure) signals resulting from such an interaction is measured. At least one spectral component is measured. The measured spectral component is used to predict properties, content and/or size of the fibres. These properties may then be used to control the refiner (108) process. The used acoustic signal has preferably a wavelength that is large compared with a typical site of the particles.