Passive radiator (speaker)

Passive radiator enclosure with front mounted passive radiator; back or side mounting is also used

A speaker enclosure using a passive radiator 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 small Logitech computer speaker cabinet. The middle speaker is a real loudspeaker. The top and bottom cones are passive radiators.

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.[1]

Design considerations

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).[2]

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 (Mmp), 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.

Passive radiators do not behave exactly as their equivalent bass-reflex designs in that they cause a notch in system frequency response at the PR's free air resonant frequency; this causes a steeper roll-off below the system's tuned frequency Fb, and poorer transient response. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, are more complex to design and likely to be more expensive as compared to standard bass reflex enclosures.

The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The system low frequency roll-off in a passive radiator design will be slightly steeper, and will have a notch (dip) in frequency response due to the Vas (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to adjust the tuning so that this notch is below audible levels.

Passive radiators are considered to be similar in design to bass reflex systems, which use a port or vent cut into the speaker cabinet. Both bass reflex and passive radiator approaches are used with the same goal: to obtain better deep pitch sound from the system.[3] Bass reflex is the most widely used technique for obtaining deeper bass notes from a speaker enclosure. Passive radiator designs are the next most-used approach to obtain deeper bass notes from a cabinet[4]One metaphor used to explain how PR systems work is to think of the air in the speaker enclosure like a spring.[5]Sound on Sound Technical Editor Hugh Robjohns states that PR systems seem to be sealed up, like a sealed speaker enclosure, but in fact, he argues that because the drone cone(s) move in and out, a PR system is not really sealed. Robjohns states that the effect created by installing a PR in a speaker cabinet is basically creating a bigger tube, as with a bass reflex system.[6] AE Speakers states that a PR is not a sealed box enclosure.[7]

Applications

Passive radiators are used in home stereo speakers, subwoofer cabinets and car audio speaker systems, particularly in cases where there is not enough space for a port or vent system. While most studio monitor speakers are closed-back, without a vent or passive radiator, Mackie sells one small monitor speaker that has a passive radiator installed on the rear of the cabinet.

References

  1. ↑ http://audiojudgement.com/passive-radiator-speaker-design/
  2. ↑ http://www.subwoofer-builder.com/passive.htm
  3. ↑ http://www.diysubwoofers.org/prd/
  4. ↑ http://www.psaudio.com/pauls-posts/passively-radiating/
  5. ↑ http://www.subwoofer-builder.com/passive.htm
  6. ↑ http://www.soundonsound.com/sound-advice/q-what-are-advantages-passive-radiator-design
  7. ↑ http://www.aespeakers.com/PRFAQ.php
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