A method for the determination of the value of a property of the material within a mill during the comminution of solid material within the mill comprises the steps of:(a) detecting the noise intensity of milling by means of an analog electrical signal, the intensity of which is proportional to the intensity of the noise in the audio frequency range,(b) converting the analog signal to a digital signal,(c) passing the digital signal through a digital band-pass filter to select at least two frequency bands, and(d) analyzing the data from the bands by means of a multivariate statistical technique to obtain the value of the property. The method is particularly suitable for determining the particle size distribution of material being ground in a ball mill.