The percussion hammer sold by instrument makers is made of rubber or has a rubber tip, so that when the pleximeter, which is placed against the side, is struck the impact will not be accompanied by a noise.    From Wordnik.com.  [Special Report on Diseases of the Horse] Reference

Percussion is practiced with the fingers alone or with the aid of a special percussion hammer and an object to strike upon known as a pleximeter.    From Wordnik.com.  [Special Report on Diseases of the Horse] Reference