Sentence:  The <WSD>growl</WSD> effect comes from fitting a small straight mute—a cornet mute for trumpet and a trumpet mute for trombone—covering the instrument's bell with a rubber plunger, the kind used by plumbers, and moving it in and out to affect the tone. 
Meaning of the word marked with <WSD> tags:  A low-pitched rumbling sound produced with a wind instrument.