damp·er 
(dămpər)
(dămpər)n.
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 diminishes vibrations or oscillations, as of a magnetic needle.