'phase-plugs and FR', metal dust

Hi to all,

I have been addicted to the same pr of spkrs for a long time! Silly old me. javascript:smilie(';)')

Australian made Audiosphere Model 3's. Yes, the biggish concrete spheres.

They're NOT QUITE FRangers tho'. they're a 2-way using mid 70's Foster drivers.

BUT they never had any HP filtering on the main 8 inch driver, the FW202, a damping nichrome resistor across them was all.

They do roll off naturally at about 3.5 K!? Yeah?! and b-all raggedness above that on their and the only plot that I have.

- So do I get to stay?

The VC diameter is 38mm, driving a soup-bowl shaped 6.5" paper cone, in a 8" / 200mm frame, 91db/w, FS ~ 35. I think the cones bowl shape may have been a factor, for the clean roll off, but what about the domed cloth dust-cap?!

The big soft-dome tweeter's VC is 35mm, rolled in 3rd order @ 3,500 - includes an FS trap*. Good sounding metallised film plastic caps, in the late 1970's!

Y'see, they were 'voiced' together, carefully, it took weeks for the guys, and the panel, to be happy*.

Now, IF I cut off the fine cloth dust cap, which is almost 2 inches across, and put in a pole piece extension, likely of metal, am I likely to get?

- a gap in FR, this would require that energy levels fall, no that can't be right! if less cancellations.


- an audible improvement in TND, and NO change to FR.

- a small extension of frequency response, still with less TND, but might require some changes to the LP to the tweeter, perhaps?

Your thoughts? I think the last is the most likely from my reading on all this.
wave lenghts at 3 to 4k , AND metal dust?

340m/sec, or 34000 cm/sec, @ 3.5 k lambda is just under 10cm?

So it might well affect reflections, even from the frame's lip? No / yes?

Metal dust, Is it really a BIG risk, I live near a big freeway, in a DRY ecology, and would a magnet on the stand help?