To reduce the noise of the punch press during the punching process, the polyurethane elastomer is used as the damping material. The experiments of reducing noise were made by means of adding the polyurethane elastomer at different positions on the 100 kN and 1 000 kN punch presses. The better effect of reducing noise was obtained. After researching and analyzing a large number of test data, the "equivalent damping hypothesis" is put forward. The hypothesis makes the experiment of reducing noise simpler and also more economical. It is estimated that there is a vast application in the research area of vibration control and noise reduction.