Passive radiator from old unknown woofer

[rabbitz]

I was trying to find a small passive radiator but the ones available have too low an Fs and Cms which means they couldn't be tuned (Fb) to higher frequencies (70Hz-80Hz) I required with the design.

There's plenty of info around to use an old woofer, remove the magnet and add mass for tuning but there was no info I could find about altering the Cms (make the suspension softer) to increase the SPL. I did buy some woofers with the desired Fs, Mms (only needed additional 5g) and Cms but were too nice to cut up even though the SB Acoustics were cheap PAC models.

So I found a woofer from a Denon speaker that had more than 2x the Sd of the driven woofer I was using, cut off the magnet and added mass until I achieved the bottom end I wanted but the level was way too low. The solution was to increase the Cms by cutting away the spider until the desired level (referenced to a vented speaker of the same design) was reached and ended up with spider remaining in 2 quadrants which still controls the driver and keeps it centred. The back of the cone was coated with LTS 50 to provide extra damping.

Worked very well and pleased with the results. Hope this may help someone in the future.

 

[Cal Weldon]

The only difference I incorporate is to cut the spider into 4 'spokes' rather that 2 in an effort to keep the cone action more pistonic.

 

[Old'n'Cranky]

As the spider is only (mostly) there to help keep the voice coil centred in the magnet gap, do you really need to keep any of the spider ?

 

[rabbitz]

Without the spider and only supported by the surround, the cone can tilt from gravity so it's s not parallel to the baffle.

With the 2 cutouts the cone is stable and pistonic but if I was to do it again I would use 3 cutouts.

 

[GM]

FWIW, cutting it completely out and using string to increase its compliance was popular way back when.

GM