Specification for underground loudspeaker communication systems in coal mines|Acoustic couplers, Communication equipment, Electric cables, Communication cables, Speech transmission systems, Electrical protection equipment, Audio equipment, Underground, Overcurrent protection, Audio systems, Overvoltage protection, Input impedance, Mining equipment, Signal level, Network parameters, Telephone systems, Mines, Safety devices, Output impedance, Electrical safety, Coal mining, Loudspeakers, Voltage, Circuits, Communication networks