Sentence:  The device used to perform HFCWO consisted of a <WSD>percussor</WSD> that was driven by a modified sine wave signal amplified by a standard audio amplifier. The <WSD>percussor</WSD> consisted of an electromagnetic vibration generator (LDS Dynamic Systems, Model 520) that was coupled by a steel piston to a 13-cm diameter, flexible rubber diaphragm. 
Meaning of the word marked with <WSD> tags:  A part of a device that is intended to impart a striking action.