Passive Radiator Newbie Q?

I'm thinking about using a passive radiator in a new design rather than a port. I have a lot of experience building ports but never used a PR! One main question - do PRs have unwanted resonances above FS in a similar way ports might have a pipe resonance or let internal cabinet resonances escape? How well do they suppress the unwanted garbage?

I guess it would be important not to excite it where cone resonance kicks in, which would be easy in this case.
 
Elimination of those issues is one of their benefits. From The Loudspeaker Design Cookbook: "Passive Radiators (PR) are a type of vent substitute, and closely follow vented loudspeaker design methodology and performance characteristics. 'Drone cones,' as they are sometimes called, have two important advantages over vents. First, they eliminate vent colorations (such as resonant pipe sounds), wind noises, and the internal high frequency sound reflected out of the vent. Second, they are practical for small enclosures which call for vent lengths in excess of internal box dimensions. PRs are also simpler to deal with, having fewer alignments and less concern with loss calculation. On the downside, PRs have a steeper cut-off (and less transient stability), a slightly higher cut-off frequency and greater overall losses (QL) than vented designs."

I find his statement that they are simpler less persuasive, since you have to deal with the passive radiator's own specs, potential added mass for tuning, etc. To me, there's a little more to think about with passive radiators, but it's not a big difference. With software for design, there are all kinds of designs that could be made, but sticking close to a normal vented box's frequency response/roll-off will typically keep you in a reasonable spot.

I'm not sure what you're referring to when you said "important not to excite it where cone resonance kicks in."
 
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I only meant that a passive radiator will presumably have cone resonances just like its motor driven counterpart, but in most cases we cross the woofer below those so hopefully there will not be much energy to excite them in the PR either.

After simulating, I found I'd need 2 PR with the same SD as main driver to get similar transfer function as a port, so it's less attractive than I originally expected.
 
2:1 is the normal suggested ratio of passive radiator to driver volume displacement. You can often just use one larger diameter passive radiator or a radiator that has 2x the Xmax of the driver if both are the same diameter.

The breakup modes in a normal speaker are more about the cone being driven from a central point (simplifying). In a passive radiator, the motion is being generated by force that's evenly distributed over the cone, so I would expect less of this kind of issue. There may be some suspension related effects, but I would expect those to be small also.
 
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I expected it to be 1:1 if the PR is basically a motor-less version of the main woofer

The enclosed volume of air, driver, and passive radiator form a resonant system though. And at system resonance the passive radiator is normally damping the motion of the driver's cone. I'm sure there's a detailed/mathematical explanation out there somewhere, but I can't point to one at the moment.
 
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The enclosed volume of air, driver, and passive radiator form a resonant system though. And at system resonance the passive radiator is normally damping the motion of the driver's cone. I'm sure there's a detailed/mathematical explanation out there somewhere, but I can't point to one at the moment.
Just musing.. I suppose that the PR must have a greater SD than the main driver in order to provide a better acoustic coupling to the outside air, by which it gives increased efficiency.

A port may not have a large SD but it does have a high air volume if we look at it that way.
 
I don't think it can be that simple to compare PR swept volume with a reflex port volume. The swept volume is only valid at a certian excursion. The reflex port on the other hand always has a volume, even at rest.

I guess this is the root of why PR needs a larger SD than the main driver - because if a passive radiator equal in size to the main driver were coupled 100% efficiently by the internal air cavity to the main driver, we would simply see the same output as having that main driver working. The PR needs to offer an improved coupling to the external air in order to offer a greater bass output.

I think that makes sense?? Let me know if I'm off track.
 
As a matter of fact, the PR cannot increase the efficiency/sensitivity more than a port or any other PR as long as the alignment is the same and the PR can handle the xmax required. Typical added efficiency in the bass range yields a +3dB for any reflex enclosure.
The only way the efficiency can increase is to use an APR arrangement and mechanically increase the output.