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Passive radiators (PR's), are also known as auxiliary bass radiators (ABR's) and
"drone cones". Simply stated these are woofer/subwoofer chassis with cones but no
motor. The only characteristics of PR's are their excursion limit, compliance, and
their resonant frequency (determined mostly by cone mass). PR systems use one or
more woofers in a sealed box with a PR. Since the box is sealed and the PR has no
magnetic motor, the PR's motion is entirely determined by the air in the enclosure
and the back wave of the woofer(s). System design and modeling are exactly as for a
ported system, except the port tuning is replaced by adjusting the PR's mass to
match the mass of the air in an equivalent vent with the same cross-sectional area
as the Sd of the PR. For best results, the PR should therefore have low mass, a
compliant suspension capable of wide excursions, and a resonant frequency below both
the box and port tuning frequencies.
Any DIY'er can make a PR from an old woofer by simply removing the magnet and voice
coil assembly, leaving only the chassis and cone. This is generally not a good idea.
The PR's "Xmax" will be equal to Xsus of the driver. Without the constraints of
motor design, PR designers can achieve outrageously high vales of Xsus. As might be
expected, along with the PR's typically compliant suspension, this raises Vas
considerably. Furthermore, good PR's typically use much more massive cones than
drivers, resulting in a subsonic Fs and a very high Qms. For more professional (and
cosmetically pleasing) results, there are several commercial sources of PR's listed
below. The currently preeminent source for general purpose PR's is Stryke
Audio. There are several vendors who produce PR's specially designed to work one or
more of their own drivers.
For more information on PR's in general, read
"Passive Radiator
Enclosures: In Theory and In Practice" by Adam Bird.
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