The Electric Ear System, an intelligent digital mill sound measuring instrument, is used for monitoring the material level in a ball mill by measuring the sound caused by the grinding ball hitting the mill lining and material in the mill. Base on the output signal from the electric ear system, the mill operator will be able to control the material level in the ball mill on a constant level. The electric ear system uses a microphone probe as an ear. The microphone ear converts the mill shell sound into an electrical signal. The output signal from microphone ear correspond to degree of the material filling degree in the ball mill and should be connected to process plant control system for further signal treatment and display and recording, e.g. to be used as supervision in connection with automatic control of the mill feed.