A passive radiator speaker is a simple device that increases the low frequency response (Bass) of a speaker system*. When used properly, a passive radiator can give a speaker system the comparable performance characteristics of a much larger system; that’s the point, in a nutshell.
A lot of air needs to be moved in order to produce audible bass frequencies. Traditionally, when it comes to bass production, a larger woofer meant louder, clearer bass. Passive radiator technology has broken this tradition.
A passive radiator looks like a normal driver (speaker) from the front, but on the backside, it appears to have had all of its “guts” removed. See the picture on the right. A passive radiator is a speaker without the magnet, and electronic structure attached to it; it is just the cone, suspension, and frame. Really though, a normal speaker can be used as a passive radiator, the “guts” would just be superfluous.
Basically, a passive radiator is a reactionary device as the name suggests. When a driver (such as a subwoofer) is mounted to a sealed speaker box (enclosure), the physical forward/back movement of the speaker affects the internal air pressure of the enclosure. When a passive radiator is mounted to the same speaker box, the internal air pressure fluctuations (caused by the movement of the driving speaker) causes the passive radiator to begin moving forward/back as if it was also a driven speaker. When the passive radiator moves, it creates sound frequencies just as a normal driver does. The diagram below shows how a passive radiator functions.