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Old 10th January 2015, 02:45 PM   #1
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Default How to brace MY speaker cabinet?

Hi guys,

Been using my speakers for a while - PA/party use. They're a pair of old EV 15" Deltamax cabinets - Beyma 15P1200Nd and a big horn with an EV DH1a compression driver. There's often feedback problems in the 200Hz range, and, knocking on the sides of the cabinet, that would appear to be where the panels resonate. I suspect the panels are throwing lots of lower-midrange energy sideways.

So, I'd like to add some bracing.

The cabinets are trapezoidal, ~750mm tall, ~250-450mm wide (back and front), and ~300mm deep. The compression driver takes up a significant chunk of the top half, and touches the back panel (part of which is removable).
The 15" drivers aren't that deep, but there are side-handles to contend with - see below.

Click the image to open in full size.

I don't want to add masses of bracing because, at the end of the day, I've gotta hoik these on to stands. So, keeping it simple, what would you do?

I'm thinking a couple of bits of 12mm ply glued for an x-shaped cross-section joining the two side panels would give good results - that'd give some support against lots of different bending modes of the panels.

Cheers
Chris
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Old 10th January 2015, 04:22 PM   #2
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Patent US7270215 - Loudspeaker enclosure with damping material laminated within internal ... - Google Patents

Maybe somnething like Fig. 7 , but oriented to srut the driver's magnet against the "smaller" rear baffle or the joints of the rear baffle if the cabinet is sufficiently rigid there.

That strut should be able to take "push" an "pull" forces and have inherent damping.

Two (rigid) cardboard (or aluminum?) cylinders sticked together and the inside gap inbetween them filled with some dampening glue could be a "model" for that strut.

Outer cylinder glued intimately to the driver's magnet, the inner one glued intimately to the rear baffle or vice versa. Of course a kind of "adapter" will be needed at both sides for glueing.

Both cylinders only have contact via dampening glue in their common gap. Gap may also be filled by some kind of viscoelastic material, to save glue.

If area of adapters is large enough, glueing (magnet, rear baffle) may even be using double sided adhesive tape (e.g. acrylate) of sufficient thickness and quality.

Just thinking ... no guarantee
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Last edited by LineArray; 10th January 2015 at 04:31 PM.
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Old 10th January 2015, 05:15 PM   #3
Pano is offline Pano  United States
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Does look like the handles are right where you need to brace. Is that why you are thinking X brace?
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Old 11th January 2015, 11:15 AM   #4
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I think I could sneak the bracing in between the horn and handles, or even butt the bracing against the handles if necessary.
Looking at the side of the cabinet, a brace that looks like x (not +) along its length seems to me to support a lot of material with fairly minimal work, except for getting the angles right.


Oliver, I like the cylinder idea, though I'm not so keen on putting anything against the driver's magnet. There are lots of vents on the back of the magnet, and these drivers see some power - I've nudged into the limiters on a NU6000DSP (~1.2kW/ch) for hours at a time, so I'm willing to bet the drivers get warm.

Another option that I'd consider less effective would be some triangles, joining adjacent panels. While individually less effective, if they let me brace areas that wouldn't normally have any support, that can't hurt, right?

Would there be any interest in measurements of this project?
I was considering getting a cheap acoustic guitar pickup and taking frequency response measurements of the cabinet walls with REW.

Cheers
Chris
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Old 12th January 2015, 04:24 PM   #5
Pano is offline Pano  United States
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I did not have much luck with guitar pickups for this. Mainly because my sound card just didn't play well with the pickup.

Interested to see the results, if you can get them, Don't see much published on this.
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Old 12th January 2015, 04:46 PM   #6
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Default Cross-Brace?

I'd consider using wooden dowel rod, maybe 25mm, if you can find it. A single one across the box close to the handles. If you're lucky, you could use the handle fasteners to attach it, instead of drilling holes through the outside of those nice boxes.
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Old 12th January 2015, 06:31 PM   #7
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Quote:
Originally Posted by Pano View Post
I did not have much luck with guitar pickups for this. Mainly because my sound card just didn't play well with the pickup.

Interested to see the results, if you can get them, Don't see much published on this.
Not a problem, I've got a mixing desk - can have as much gain as I like. I need to make sure the EQs don't do anything to the signal, though - there's no hard bypass.


PKNJ, do you mean a single dowel, or several?
I think a single one would only damp out a few vibrational modes of the panel (picture standing waves) depending on its position, but a few of them would get a lot more. Very minimal work, too...
To the internet!

Cheers, all

Chris
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Old 13th January 2015, 05:28 AM   #8
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Let me know if it works, Chris. The so called "Instrument input" on my M-Audio card would not do it. Never tried it on a mixer.
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Old 13th January 2015, 10:20 AM   #9
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Ordering a little piezo pickup now. Since I'll be looking for differences in the graphs, I won't be needing a specialised pre-amp.

Chris
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Old 13th January 2015, 10:31 AM   #10
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Hi chris661,

I use either a broom handle or better still, a shovel handle which are about 1" to 1 1/4" diameter. I get them from my local hardware/gardening store, but you may be able to get some from B&Q. Attached is what I did on a sub, but I've also used them very effectively on my floor standing main speakers. Attached is a pic of the sub.

Peter
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