An unpowered loudspeaker cone, placed in the port of a vented enclosure system. Also called a drone cone, or a slave cone.
"Just a note to tell you how happy I am with the laser transfer of the three-78 rpm album of Fibber McGee and Molly on the Night Before Christmas. This dates from 1945, and has been part of every Christmas celebration in my family ever since, so it has seen some use. Cassette copies are played in my sons' homes on Christmas Eve, and enjoyed by ..."