Driver modification query

[Moondog55]

First off I have to make a confession, I have a problem which drives my better half insane, I collect old broken speakers found on the kerb and bring them home.

One of the speakers I brought home tother day just happened to include a small dome mid-range from Jaycar, a driver that I have on my shelf.

I used the trusty DMM and found the driver open cicuit so I decided to pull it apart to see what was inside, like a lot of Jaycar drivers this one is a Dynaudio clone, 32mm voice coil and textile dome, the old catalogues give a resonant frequency of 1450Hz.

Thinking about tweeter tweeks I got to thinking about adding a chamber to the rear, I found a small plastic container amongst the rubbish on the bench and started cutting.
Remember that the driver I'm working with in the attached pictures is OPEN CIRCUIT, I'll be a lot more careful if I do this with a working driver.

http://farm4.static.flickr.com/3499/...f5992f.jpg?v=0

http://farm4.static.flickr.com/3338/...a927bf.jpg?v=0

http://farm4.static.flickr.com/3428/...a76bc3.jpg?v=0

http://farm4.static.flickr.com/3486/...96632b.jpg?v=0

http://farm4.static.flickr.com/3314/...36e3c9.jpg?v=0

http://farm4.static.flickr.com/3325/...e35503.jpg?v=0

http://farm4.static.flickr.com/3387/...b56d39.jpg?v=0

http://farm4.static.flickr.com/3532/...cde5ed.jpg?v=0

http://farm4.static.flickr.com/3468/...060aac.jpg?v=0

http://farm4.static.flickr.com/3468/...410199.jpg?v=0

Lots of photo links so you can follow my train of thought, chamber is about 500cc, high carbon foam and some BAM.
Any thoughts on how low the resonance will drop?/ in other words is it worth while going ahead??
I would probably also add a little lambswool to the hollow voice-coil and perhaps add a little collar of felt, it is the same size as a Doctor Scholl corn pad after all

[Moondog55]

Whoops, wrong resonance frequency given.

Does any-one from OZ have an old Jaycar cataloque??
And check the Fs of the smaller of their mid domes for me??

1250/1450 is for the 1inch tweeter.

[Moondog55]

I just found an OLD Jaycar catalogue from 1994 in an Op-shop, this driver was in it as a new driver the catalogue lists the Fs as 750 Hz.

Is expecting a 20 or 30% improvement downwards unreasonable?? It would be nice if I could use these crossed around 1700

[SimontY]

750hz sounds quite high for a midrange, good luck with the mods, looks interesting!

Simon

[Moondog55]

When I bought these I was considering using them as a low crossing tweeter.
With a 32mm voice coil really that is all they are IMO.
But if I can reduce resonant frequecy a little ( or a lot ) then I might find them easier to work with.
The graph from the catalogue shows a steeo natural roll-off above 12k, and I have a small ribbon tweeter to use above them.
Price was right but I don't have a design in my head at the moment, perhaps used with my 8inch Tandy/RadioShack mid-woofers??

[SimontY]

And a 15" woofer under that

Whilst you're here Ted, might I ask a thread-jack question? How thick is the felt you advocate placing on the inside of a driver frame? I'm trying to source some on Ebay...

Thanks,
Simon

[Moondog55]

The Feltac self stick heavy duty white
#23108 is about 3mm thick
The black #17039 is a soft medium duty and about 1.5mm thick, the combination of the two thicknesses seemed to work well.
The thick denser felt seems to stop the thin stamped frame from ringing and the softer seemed to stop very high frequencies from bouncing back thru the cone material, mind you; this is a subjective opinion with no measurements to back my opinion.

This combination had very little effect on the bass drivers, I think
blue- tac may be better for those as it is applied to the outside and can be laid very thick

[SimontY]

Thanks a lot, I found some 2cm stuff and lots of 2mm stuff on Ebay, but not much inbetween... now I know what I'm looking for

[GM]

Originally posted by Moondog55

Is expecting a 20 or 30% improvement downwards unreasonable?? It would be nice if I could use these crossed around 1700

If it's a properly design tweeter, then its Vas is so low that its tiny rear chamber is acoustically ~ an IB, i.e. its Qts is ~1.0 to get a flst response, so there shouldn't be any useful drop. If this isn't the case, then without T/S measurements there's no way to know, but reduced power handling becomes a major concern if not horn loaded to 'pump it up'.

GM

[Moondog55]

Yes, but as it is a copy of the Dynaudio 32mm; they have cheated and left all of the stuffing out of the rear chamber and where the original used a foam plug in the hollow VC centre these have none.

I was looking for a copy of the specs for that particular Dynaudio tweeter but without a model number I had no luck