Modding dome mids to damp FS spike.

[johnnyrichards]

So I have those MB Quart 2" dome mids and am working with them on my current project. I have a question or two about modifying the rear chamber.

With the factory chamber, they have a highish Fs and concurrent high "Q" impedance spike. Now, I am not interested in crossing them any lower than 800 Hz anyways, but to do so I think I am going to have to comp that Fs and that adds a lot of money to "El Budgetero".

So, assuming I do not plan to cross any lower than a stock driver (~800 Hz), and assuming I want to create a larger chamber for the sole purpose of mechanically damping the impedance spike what potential for problems will I run into?

I know one of the members here did some extensive modifications on the Dayton "silkie" and now, for the life of me, cannot remember who - maybe David Ralph? A link to that page would be most welcome.

I have a donor driver to experiment with, but to avoid really extensive testing would like to bypass any of the "stupids" and go straight to the "tweaks" part of things.

1. Shape of new chamber... If I find a tupperware that fits my driver, would that be horrible? Resin reinforced pop bottle? They sell plastic drinking glasses in all shapes and sizes - maybe that will be better?

2. What to stuff it with? The factory damping treatment is a foam plug and a wad of polyfil.

This will be my first serious foray into driver modification. In the past I have doped cones and did some frame treatments, stuff like that.

[DjDisturbed]

A tupperware cup would probably work with a little sound deadening material on it. The taller the cup the better,that way you can always add filler if the volume is to large.

Don't have answers to all of your questions. But I have one suggestion of using a good wool for the lining of the chamber. I like using f-7 grade because it uses 80% wool and it's not extremely compacted. You can get small sheets of it cheap or buy it by the linear foot. 1/4" or 1/2" thickness both work very well.
Then just stuff lightly with poly-fil and you should be fine.

I'm interested in seeing what you come up.
Are you going to post the before and after of your efforts?

[Whitneyville1]

Johnny, I remember that thread too, but darned if I can find it. (note to Moderators, "Search" needs some work) It was in the Project Showcase at one time, but not now.:( As I remember, he added a piece of thin felt and X grams of
wool/acousti-stuf (??) I think I read it about Aug-Sept last year, if that helps.

[Zach C.]

Under "Tweeter Tests and Modifications." I'm almost sure there used to be some other pics of some modded silkies around somewhere. I though that was DLR too. Hmmm.

http://www.speakerdesign.net/

Zach

[lhwidget]

I'm pretty sure the tweeter mods write-up was in the Project Showcase, but I can't find the complete article any longer...

[kevinb]

Is this it?

http://www.speakerdesign.net/tweeter_tweaks/lambs_wool_tests/tweeter_tweak_lw_dayton.html

[Sidi]

Make an adjustable chamber.

Get a piece of appropriately sized pvc, make a rubber gasketed plug that fits into the end, and adjust the internal volume. allows you to change stuffing from the back, volume, length. hell you can change out the rear wall profiles to have odd shapes.

[johnnyrichards]

First bump in the road... There is a brace right behind the midrange cutout lol. My bad. Silly bracing anyways.

Second bump in the road, tried three different approaches and they all led to a much smoother bottom end, but also a spike at ~2k of between 3 and 5 db, and a concurrent spike in 3rd order distortion.

I will approach again at a later date.

The three different cups were a basic 1" ring, with the factory cup glued to that. A small, hard plastic bowl I found in the bathroom. I replaced my wifes ear rings before she noticed the bowl was missing. Third was a 3" PVC cap I found at work. I didn't get too crazy with stuffin materials since all I have on hand is polyfil.

No idea where the 2k nonsense is coming from, and given that the way I built my enclosures is only going to give me another inch of chamber, I am not worried about it.

Another time :)

[JRT]

So I have those MB Quart 2" dome mids and am working with them on my current project. I have a question or two about modifying the rear chamber.

With the factory chamber, they have a highish Fs and concurrent high "Q" impedance spike. Now, I am not interested in crossing them any lower than 800 Hz anyways, but to do so I think I am going to have to comp that Fs and that adds a lot of money to "El Budgetero".

So, assuming I do not plan to cross any lower than a stock driver (~800 Hz), and assuming I want to create a larger chamber for the sole purpose of mechanically damping the impedance spike what potential for problems will I run into?

I know one of the members here did some extensive modifications on the Dayton "silkie" and now, for the life of me, cannot remember who - maybe David Ralph? A link to that page would be most welcome.

I have a donor driver to experiment with, but to avoid really extensive testing would like to bypass any of the "stupids" and go straight to the "tweaks" part of things.

1. Shape of new chamber... If I find a tupperware that fits my driver, would that be horrible? Resin reinforced pop bottle? They sell plastic drinking glasses in all shapes and sizes - maybe that will be better?

2. What to stuff it with? The factory damping treatment is a foam plug and a wad of polyfil.

This will be my first serious foray into driver modification. In the past I have doped cones and did some frame treatments, stuff like that.

I have not seen the specs. If Qts is high enough to be suitable, consider an MAPD alignment (link to George Short's white paper (http://www.northcreekmusic.com/MAPD1.htm)).

The chamber on the back of the midrange would be stuffed and sized (probably resized) to 1/3 Vas and vented through a flow resistance to a second stuffed chamber (maybe just the sub enclosure that the mid is mounted into) sized to approximate Vas, important point being that the second chamber has similar compliance as the driver's mechanical suspension. The combination of compliance provided by the air suspension of first chamber damped by the flow resistance of chamber to chamber air flow is supposed to improve linearity over that of a simple sealed chamber. The resulting acoustic highpass rolloff is 2nd order like a sealed alignment.

Siegfried Linkwitz has suggested using a sintered metal fluid filter as flow resistor for improved linearity (link to Linkwitz's suggestion (http://www.linkwitzlab.com/frontiers.htm#A)).






.

[Wolf]

I would use a resistor in parallel with it, value of about half the Zmax of the Fs spike. This may be enough damping to get decent results.

If not- then use caps from Apex Jr, or a set of polarized caps in series for cheap. The coil is always salty. Erse-direct 19AWG I-core or Jantzen P-core is cheapest generally.

Later,
Wolf

[Wolf]

I'm pretty sure the tweeter mods write-up was in the Project Showcase, but I can't find the complete article any longer...

'Rusty's Silkie mod', might get you there. This was for the older Gen2 Silkie with a felt pad over the pole-vent.

Later,
Wolf