Sounds (also known as mill level gauge) is a sound sensor with wide audio frequency range and easy operation. The sensor is small in size, light in weight and easy to install. Its sound measurement range covers all frequencies that can be heard by human ears. The measured sound intensity energy range meets all requirements in the national noise standard. The output of the sensor is a voltage signal, which is convenient to form a fine noise measurement system with other measurement and control systems. This product has strong practicability, strong anti-interference ability, low power consumption and strong working stability, and is suitable for field use. Used for measuring the amount of coal stored in the coal mill: the material level measurement method based on the noise of the coal mill is adopted, which is different from the measurement method based on the vibration signal of the coal mill and the measurement method based on the pressure difference at the inlet and outlet of the coal mill. It avoids the problems that the measurement method based on the vibration signal of the coal mill has a real effect when the coal storage is increasing, and the measurement method based on the pressure difference at the inlet and outlet of the coal mill is greatly affected by the system air pressure. The material level measurement method based on the noise of the coal mill is less affected by the coal quality and can only reflect the change of the coal amount in the coal mill.