damper

(dămˈpər)

1. One that deadens, restrains, or depresses: Rain put a damper on our picnic plans.
2. An adjustable plate, as in the flue of a furnace or stove, for controlling the draft.
3. Music
   a. A device in various keyboard instruments for deadening the vibrations of the strings.
   b. A mute for various brass instruments.
4. A device that eliminates or progressively diminishes vibrations or oscillations, as of a magnetic needle.