Passive radiators

[vinylnvalves]

Been kicking a few ideas around for LF. One looks at passive radiators instead of vents. Passive radiators seem to be quite expensive for what they are. Was looking at just using car subwoofers, which has naturally heavy metallic cones. I then got to thinking should I chop the magnet assembly off them. I had an idea ( probably something I have read so not innovation) of using the voice coil as a means of damping. Adding resistance across the coil should allow me to control Q. Am I being stupid.... and what’s your experience of using blown drivers, for a passive resistor etc.

[steve s]

I had a pair of kefs with their passive radiator, I thought it was a similar sound to a port but less tuneable if your into diy.

[IslandPink]

I think the main advantage is avoiding port chuffing, but I agree Steve(1) that having a coil and some resistance could give you more options. Do you have an idea how to select a suitable driver though ? - are there guidelines ? Presumably looking at Fs is the first step.

[vinylnvalves]

Only guide rules is that the PR’s should have 2xSd of the bass driver minimum. The weights added allow you to tune it as you would a port. There are some car subs with an Fs of around 20Hz which is similar to my drivers. You can slit the spider to tune then I suppose. But yes in uncharted waters. Other whacky ideas I have read about is someone using a bicycle inner tube, pumped up to support a panel with a groove around it many times the Sd of the driver, adjusting the pressure to tune the Q.

[IslandPink]

I'm trying to remember, did Purifi have a 'serving suggestion' using a passive radiator for the 6.5" midbass unit, so was it an 8" P.R. then ?

[Toppsy]

There seems to be loads of cheap small passive bass radiators on ebay. So if this is for a experimental project, what about a line array (or two) of something like these:
https://www.ebay.co.uk/itm/374119125104 ... 2e236896ce

I'm sure there are other larger sizes on ebay?

[rowuk]

I tried using a woofer as a PR and hoped to have joy with resistors or shorting the voicecoil. It didn‘t work.
I ended up painting the paper cone to make it heavier and harder - 50 - 100 grams heavier. That worked well. Rule of thumb: twice the area of the woofer. Add weight until you get the desired response. I think that AJ horn can calculate them.