swell box

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A chamber of an organ that houses a pipe or pipes and has shutters for controlling volume.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A box or chamber in an organ that contains the reeds or a set of pipes and has shutters that open or shut usually by means of a pedal in order to regulate the volume of musical tone.