Originally Posted by zieglj01 /t/1517126/upgrading-an-old-surround-system-advice-needed#post_24460092
The other one is a passive radiator to help enhance the bass, it is not a regular mid-woofer driver.
It has no voice coil.
A speaker enclosure using a passive radiator usually contains an "active driver" (or main driver), and a passive radiator (also known as a "drone cone"). The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil and magnet assembly. It is only a suspended cone, not attached to a voice coil or electrical circuit. The passive radiator usually has some means to adjust its mass, thereby allowing the speaker designer to change the box tuning. Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.