Sound is assumed to be a measure of the breakage conditions at the impact zone of balls (and to a lesser extent, rock) in the mill. High sound levels occur when the ball strike point distribution is high on the liner wall and not overlapping the mill charge toe. Low sound levels occur when the strike point distribution dominantly falls within the toe of the charge (and is damped).
The microphones for the sound level are typically placed as close as possible to the mill shell at the likely strike point of the balls with a fully charged mill. This closeness helps reduce interference by external noise sources such as crane “hooters” etc. In order to emphasise the ball strike component of the sound level, the microphone electronics will often apply a band-pass filter with a centre frequency of approximately 1 kHz to the signal. There are also more advanced sound instrumentation options that can also produce a signal that needs to be kept at a setpoint to maintain optimal breakage conditions for any set of mill states (e.g. load). Holding the sound level constant is, by inference, holding the breakage conditions constant at the desired setpoint. Depending on mill design, the setpoint may be a compromise between best breakage conditions (high sound level) and limiting liner damage due to ball strikes (low sound levels). For any particular mill filling, there is a mill speed which will maintain the setpoint sound level (and therefore grinding conditions).
