Passive radiator speaker

A speaker enclosure using a passive radiator (PR) usually contains an "active loudspeaker" (or main driver), and a passive radiator (also known as a "drone cone"). The active loudspeaker is a normal 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 wired to an electrical circuit. The passive radiator is not connected to the power amplifier.

A passive radiator system makes it easier for the speaker system to create the deepest pitches (e.g., basslines). These deep pitches are hard for the woofer to produce. With a PR system, the movement of the "drone cone" passive radiators reduces the amount that the woofer has to move. If a woofer has to move too much to produce very deep pitches, it can damage the woofer (this is called "overextension"). A passive radiator system can provide an improved bass sound. Another benefit is that, given that PR designers usually use a larger "drone cone" than the real woofer (or use two drone cones), this means speaker designers can use a woofer that is not as large.

The passive radiator usually has some means to adjust its mass (e.g., by adding weight to the cone), thereby allowing the speaker designer to change the speaker enclosure "tuning". Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.

Passive radiators are used instead of a reflex port (a tube or vent cut into speaker cabinets to improve the bass response) for much the same reasons—to tune the frequency response, particularly in small volume enclosures and when small diameter drivers are used, for better low frequency performance. PRs are especially popular in situations in which a port would be inconveniently sized (usually too long for practical box configurations).

They are also used to eliminate port turbulence and reduce motion compression caused by high velocity airflow in small ports (especially small diameter ones). Passive radiators are tuned by mass variations (M_mp), changing the way their compliance interacts with motion of the air in the box. The weight of the cone of the passive radiator should be approximately equivalent to the mass of the air that would have filled the port which might have been used for that design.

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Wikipedia