A method of determining the content of a desired component in a stream of material including two or more types of component, comprises the steps of: causing or permitting a said stream, comprising plural particles of material, to impact a sensor member (14) located in a path of the material thereby inducing vibration of the sensor member analysing vibrations of the sensor member and from the analysis determining whether individual impacts caused by the stream are one of any two or more types selected from three possible impact types and performing an operation based on the result of the analysis.The invention is of particular use in determining the content of material streams in combine harvesters.
