A single-chip silicon condenser microphone with a sandwich diaphragm and an electroplating perforated copper backplate has been developed. The sandwich diaphragm is designed to have a slight tensile stress. The backplate is made using copper electroplating technology and perforated with circular holes. Perforated circular holes on backplate are laid out as hexagon to adjust air-gap damping between diaphragm and backplate to critical damping to enlarge frequency bandwidth of microphone. Fabrication is simple enough and only needs five photolithography marsks. Measurements show the open-circuit with 9V bias voltage reaches 5.45mv/Pa in 1KHz, pull-in voltage is 14V, and the flat frequency response within 3Db is from 100Hz to 20KHz. The performances of microphone meet the requirement of industrial use. Thereby, it has promising prospect for industrial and commercial field.