Passive radiator (speaker)

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A speaker enclosure that utilises a passive radiator usually contains an "active" or the main driver and a "passive" driver (or drone or radiator). The active driver is a regular loudspeaker, and the passive is more or less the same, but without the coil. It is only a cone, not connected to any 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 the cone movements active driver moves the passive radiator.

Passive radiators are used instead of a reflex port in small enclosures, primarily to tune the small volumes to low frequencies, where a port would need to be very long. They are also used to eliminate port turbulence and reduce power compression caused by high velocity airflow in ports. Passive radiators are tuned by their mass (M_mp) and the way their compliance interacts with the compliance of the air in the box. The weight of the cone of the passive radiator should be equivalent to the mass of the air that would have filled the port. Passive radiators add a complication to vented systems which causes a notch in frequency response at the PR's free air resonant frequency and this causes a steeper roll-off below the drone's tuning frequency F_b and poorer transient response than standard vented loudspeakers. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, may be more complex to design and expensive as compared to standard reflexed enclosures.

The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The low frequency rolloff of the passive radiator will be slightly steeper in slope, and will have a notch (dip) in frequency response according to the V_as (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to tune it so that this notch is well below audible levels.

[edit] Misconceptions

In technical support circles, it has been revealed that some owners of speaker cabinets that features a passive radiator have taken the cabinet apart, and think someone (the manufacturer or salesperson) have taken advantage of them. They do not understand the technology, and sometimes think the manufacturer put a fake speaker in the cabinet (since there is no magnet or wire) just to give the visual illusion of a larger, more powerful speaker.