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Dear all,
I have put together small two way with a 2206H and a 2450SL/2332 in a SR4722X cabinet.
These basreflex Cabinets are original JBL and made for the 2206H.
When I measure I get a huge dip att 1Khz, -15db dip.
Any idea what that can be? Resonance in the cabinet?
Regards
//RoB
Try to "invert" cables connecting 2450SL driverQuote:
Originally Posted by sebackman
Regards
Ivica
Hi,
It's only the 2206H. No filters, no driver. MLS measuring Does not mater if the gate is long or short so it is not from the room.
Can't see how the cabinet could interfere so much. Both boxes behave the same.
Can it be reflektions from the recessed baffle or the sides? Should not JBL have seen that?
Thx
//RoB
Half a wavelength at 1000 Hz is only 17 cm. Could be reflections from the floor if cabinet is situated directly on the floor.
It could also be the horn working as a kind of helmholz. Try stuffing it with rags.
Hi,
Thank for good input.
The box is wired for active XO. Only the 2206 is connected. Horn and driver is not connected at all beyond the rear terminal. I have checked the measuring gear on other speakers and all works as it should.
After measuring again today it appears that it is reflections from the cabinet sides that stretch out on the sides of the 2206. Se picture above.
When measuring the dip is worst in right in front of the speaker. If I move the mike to either side, it reduces significantly. The measuring distance does not seem to matter much. The dip is apparent from 30cm and out to approx. 2m.
I use MLS and can gate down to 10ms or less which would take out any reflections from floor or ceiling. Dip is still there. The box sits on a speaker stand half way to the ceiling (280cm total height).
The only thing I can think about is that for whatever reason this cabinet must give strong reflections from the inside sides on each side of the driver and the bottom that interferes with the direct sound at just 1kHz. Weird.
I tried to put some wooden pieces on the sides of the 2206 to change the geometry and see if that changed the results. It did. It must be reflections and just bad luck that it is below the XO point 1,1Khz… Above would not matter…
Now the question is;
Do I do a new baffle on top of the old to move the 2206 forward and reduce the overhang from the sides/bottom? Drawback is that it adds weight..
Or do I make a 20mm thick MDF distance ring just below the 2206 to move it out from the existing baffle and hence reduce reflections?
Or do I mount 1" dampening foam on the areas where the reflections come from?
What do you think?
It is probably a badly designed JBL PA cabinet as they are original for the 2206H. JBL should have seen this when they made the box I guess. Original it runs with QASR/2417 and maybe the combination with those drivers and the dip in the box design worked. A bit hard to believe but I have no chance testing now since I rebuilt for 2332/2450SL.
I have not seen such narrow dip comming from refelctions before.
Best regards
//RoB
You are probably better at engineering cabinets than the original designers. ;)
Add some kind of lightweight material as wedges towards the edges.
Interesting find that the sides interfere so much. Must be quite some worst case distances coming together here. Wedges would help I'd say. Or filling the space between the driver and the sides with a dense foam would also help I think.
Keep in mind that cabinets like this ( SR4722 ) are meant to be used ( & are measured ) with a metal ( protective ) screen in place ( that is sometimes foam-backed ) .
I agree ( the deep notch ) is most likely ( caused by reflections ) from the high edges of the cabinet .
I would remeasure , with ( 1/2" thick ) acoustic foam that covers the full baffle-board ( plus a metal screen that has 60 to 80% transparency ) . I'm pretty sure your notch will be quite minor after this is done .
:)
Hi,
Thank you.
I do have the original metal mesh fronts so I will try to do a measure with them in place. I don’t think that only will reduce the dip enough. There is no foam on the inside.
What kind of foam would you recommend? Would ordinary open cell foam of the same type used in furniture do it? Or are there special types with better function?
Best regards
//Robert
- Furniture foam is usually "closed-cell" type / so no / it's not usable .
- Here's the actual foam product ( that I've used for decades ) for my custom SR cabinets ( click the pic ) . It is definitely "open-cell" .
http://www.tchweb.com/tchresource/im...-4805905-l.gif
- An added benefit ( of laying in some foam behind a metal grill ) is that if the foam is "lightly compressed" by the metal screen , the metal screen will be damped a bit ( & help stop ringing at higher output levels ) .
- Also, it's a pretty good look .
- I imagine you can find acoustic grill-foam ( in Germany ) from someone like Thomann .
- Here ya go ;
http://a2.images4.thomann.de/pics/prod/182269.jpg
:)
I just bought a couple of Auralex Studiofoam sheets. It's time to see what happens.
http://www.audioheritage.org/vbullet...yingsmiley.gif
Attachment 61191
Ok, tested the foam sheets and it clearly showed that these are reflections. However I can't get rid of all of it.
I will try to make some MDF distance rings with the router to move th 2206 out of the box to change the geometry.
Best regrds
//RoB
Pictures with foam sheets? Foam will not direct all energy away from the edges.
Same problem here- and I've been working out figuring a similar solution. I keep wanting to just cut the lips off and be done but the cab is fiberglassed.... will be tough to do it well. If anyone has seen these before, I've got a thread in the mktplace, I'd be interested to get some feedback on how a 5525 evolved into a coated 2206h w/2426h and what seems to be a DDS eng90 waveguide. I'm missing the R and L values, which appear to have been intentionally obscured. http://www.audioheritage.org/vbullet...raigslist-Deal
Attachment 61210
Attachment 61209