Sentence:  In trumpets for assisting the hearing, all reverbation of the trumpet must be avoided. It must be made thick, of the least elastic <WSD>materials</WSD>, and covered with cloth externally. For all reverbation lasts for a short time, and produces new sounds which mix with those which are coming in. 
Meaning of the word marked with <WSD> tags:  (matter)Matter which may be shaped or manipulated, particularly in making something.