A round mute used with brass instruments that is held in front of the bell of the instrument to dampen (muffle) the sound. The plunger is often movedback and forth, by the direction of the composition, in front of the bell to provide a range of sounds. This is a common effect in jazz and swingcompositions. The common plunger is most often a standard rubber plunger (plumber's helper) from a hardware store (without the wooden handle).
The device used in a slide whistle to block the airflow in the slide allowing for the lengthening and shortening of the slide tubing.