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kelticwizard 8th February 2003 07:33 PM

Why Do Passive Radiators Require Spiders? And Has Anybody Made Their Own?
From the LDSG Speaker Selection Guide on Passive Radiators: "Sometimes, the rear spiders are also omitted, but PR's with spiders are highly preferred."

Why? What is wrong with a piece of material of the proper weight and a rubber or foam surround?

I am assuming that the Passive Radiator is at least twice the cone area.

Anybody with advice on how to make your own Drone Cones, (Passive Radiators) is extremely welcome to post. Yes, I know about the surrounds at Parts Express and Madisound, but let's see if we can get cheaper. I am thinkng several Passive Radiators for a total area several times the speaker's cone.

jcarr 8th February 2003 07:49 PM


Rather than using a passive radiator, I prefer to use standard woofers, complete with motor. I can apply weights to the diaphragm to alter the f0, but I can also place networks across the electrical terminal to affect the damping behavior. Variable resistors, capacitors, R-C networks etc. are all worth trying, and give you much more freedom to adjust the sound than a straight passive radiator.

hth, jonathan carr

kelticwizard 8th February 2003 09:29 PM


Thank you for this reply. I had only just become aware of this phenomenon a few months ago from the following discussion:

Tuning the box with a second speaker does have some disadvantages, mostly cost. One thing though, I could think could make it worthwhile.

If you take a speaker and build an appropriately sized and tuned vented box and a Passive Radiator box of equal volume, the vented box will roll off at 24 dB/octave after the tuning frequency is reached. The Passive Radiator will roll off at 30 dB/octave.

Can you tune a full speaker, one you are not hooking up to an amp, to change that situation? Have you heard of a network you can attach to the voice coil wires of the second (passive) speaker that will restore that order of rolloff to the system?

I am thinking of perhaps adding weight to the cone of the passive speaker to tune it too low, then adding a network to the voice coil that would tune the passive speaker higher.

Just a thought. :)

jcarr 9th February 2003 12:00 AM

Thank you for pointing out an informative thread.

The speaker that I had directly in mind was the current AudioPhysic Brilon loudspeaker (it was created at our request, and we provided some design input). However, I believe that its goals and your goals may differ.

The Brilon is a compact speaker designed for high-quality sound in a small room (it can be considered to be the "head" section of the current AudioPhysic Virgo, sans bass section). The bass and mids in the Brilon are handled by an approximately 5-inch driver, which in concert with the compact cabinet, is inadequate to produce much bass on its own. Hence the desire for some sort of of bass augmentation system. However, room acoustics are a big variable, especially in small rooms. The bass response from woofers, passive radiators and vents changes substantially according to the proximity to room surface(s). We felt that some sort of tuneable bass augmentation system was the appropriate choice, and this led to the choice of an unpowered woofer operated as a passive radiator, and with the voice-coil leads brought out to a separate pair of electrical terminals for bass tuning purposes.

As you can see, the reasons for why the Brilon uses a tuneable passive radiator system are different from what you had in mind. I imagine that some sort of complex impedance compensation network may get you in the ballpark of where you want to be. However, the passive radiator impedance should be measured mounted in the cabinet, and the stiffness of the air in the cabinet will change as you alter the damping. This will affect the bass response and impedance curve of the main unit in addition to those of the passive radiator. Definitely sounds like a situation where trial-and-error and multiple iterations are called for.

regards, jonathan carr

7V 9th February 2003 01:04 AM

Hi Kelticwizard

I did once make a passive radiator without a spider exactly as you suggested - a piece of material of the proper weight and a rubber or foam surround.

Disaster! Instead of moving backwards and forwards like a piston, the thing wobbled all over the place and sounded awful. I did think that perhaps I could cure this with another piece of material with another surround, connected to the first a little distance away. The passive radiator would then be a cylinder.

Never tried it though. Sustituted the passive cone for a tuned port and, finally, dumped the whole design and started again.

One can stumble many times on the road to Audio Nirvana.

Good luck with yours

KenP 9th February 2003 02:50 AM

The old celestion ditton line used passive radiators but they didn't have spiders at all. In the 12" pr's it's a plastic tube 12" wide about 4" deep with a rubber roll on both ends and a sandwich construction in between, imagine a piece of hard foam supported on both ends by the rubber roll.

kelticwizard 9th February 2003 03:09 AM

2 Attachment(s)

Something like this?

LOL, MS Paint doesn't circles. which leaves me to do it Freehand, with all too predictable results.

KenP 9th February 2003 03:15 AM

That pretty much sums it up, there is also a layer of peg board on the front and back (probably for added rigidity) and the plastic tube has vent holes in it. If you need a pic I have a pair of them sitting on a shelf.

kelticwizard 9th February 2003 03:18 AM

Pic would be appreciated.


KenP 9th February 2003 03:30 AM

Side back and front.

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