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Kanga 31st March 2003 11:08 AM

Cross brace not working
 
Hi all

I am reaching the end of construction of my bass enclosure, and have hit a bit of snag with the bracing.

The enclosure is made of 25mm MDF, and is 425mm x 275mm x 700mm high (19" x 11" x 28" in old measurements), with a shelf brace about 1/3 of the way from the top. All the panels ring fairly high when tapped except for the lower section of the side panels, which are 425mm wide x about 425 high. These have a fundamental frequency of around 200 Hz (by ear), which is a bit low, given that the speaker will start rolling off at around that frequency.

The plan was to fit a cross brace to break up the resonance of this panel, and I made one consisting of two all-thread 10mm rod which tightens against the enclosure wall via nuts agains two pieces of MDF. I can temporarily insert this brace by screwing the nuts so that the enclosure walls are pushed outward and the brace is in compression.

The only snag in this otherwise brilliant brace design is that it seems to have almost no effect on the frequency of resonance of the panel (tested by tapping), despite trying a number of different orientations and locations.

I calculated that the stiffness of the brace should be much higher than the wall (maybe 100x more), so I don't know why it doesn't work. The only theory that I have is that if you tap on one side, then the brace will transfer force across to the other wall, which will move in the same direction. ie when one wall moves in the other moves out.

Any thoughts on why this is happening, and maybe some other brace designs that I could try, without being permanent? :scratch:

I would prefer not to add too much thickness to the wall, to preserve the volume of the enclosure. The enclosure is sealed with a volume of around 50 litres, and will be stuffed with fibreglass.

Thanks

Mick

qwad 31st March 2003 06:57 PM

HI, BBC research years ago showed that merely bracing the walls only shifts the ringing of panels higher in freq where it is more audible, hence their solution of thin wall enclosures heavilly damped with bitumen impregnated felt, A/LA LS3/5A, SPENDOR BC1, ROGERS, HARBETH et all....
GO TO YOUR LOCAL AUTO CRASH SUPPLY OUTLET,(YOU WILL FIND THEM IN THE YELLOW PAGES!) AND BUY SOME BITUMEN SHEET DAMPING AND STICK THAT ON THE WALLS WITH CONTACT ADHESIVE... THAT SHOULD DO THE TRICK:bigeyes: :nod: CHEERS,
TJB

qwad 31st March 2003 07:05 PM

HI, BBC research years ago showed that merely bracing the walls only shifts the ringing of panels higher in freq where it is more audible, hence their solution of thin wall enclosures heavilly damped with bitumen impregnated felt, A/LA LS3/5A, SPENDOR BC1, ROGERS, HARBETH et all....
GO TO YOUR LOCAL AUTO CRASH SUPPLY OUTLET,(YOU WILL FIND THEM IN THE YELLOW PAGES!) AND BUY SOME BITUMEN SHEET DAMPING AND STICK THAT ON THE WALLS WITH CONTACT ADHESIVE... THAT SHOULD DO THE TRICK:bigeyes: :nod: CHEERS,
TJB
AND OH BTW, you would have been better off doing the boxes out of 16mm MDF TOO, the money you save on the cost of the MDF you could have spent on the bitumen sheets for a more inert box.... of course that is had you known the difference this would make:bawling: CHEERS & CARRY ON REGARDLESS AS SID j WOULD SAY....for all is not lost yet
TJB:goodbad:

Kanga 31st March 2003 09:33 PM

Hi Qwad

Because these are bass speakers only I don't mind about shifting the resonant frequencies higher. If I can get them up around 500 Hz + then they should be largely out of harms way.

As for adding damping material now, I thought about this, and I have doubts that I will be able to damp such heavy panel at low frequencies. I don't know the theory here, but my feel is that without doing constrained layer damping (ie adding damping between another layer added to the inside of the box), just attaching damping material to one side won't make a great deal of difference.

Another shelf brace would be best, but it would be difficult to get the fibreglass filling betwen the shelf braces, and very difficult to add it in now. Maybe a couple of pieces of angle iron screwed and glued to the wall would be the best option.

I am planning to use constrained layer damping on the plinth, and I bought some 3mm wool felt yesterday that I'm going to impregnate with car underbody bitumen sealer for this purpose.
I'll probably have some left over, so I could try that on the side wall, but I'm worried about solvents in the sealer attacking the drivers.

What kind of bitumen sheet damping have you tried Qwad, and how did it work?

Thanks

Mick

Steve Eddy 31st March 2003 10:18 PM

Quote:

Originally posted by qwad
HI, BBC research years ago showed that merely bracing the walls only shifts the ringing of panels higher in freq where it is more audible, hence their solution of thin wall enclosures heavilly damped with bitumen impregnated felt, A/LA LS3/5A, SPENDOR BC1, ROGERS, HARBETH et all....
This doesn't make a whole lot of sense.

First, while bracing will increase the frequency of the resonance, moving it higher in frequency also moves it into a region where there's less acoustic energy to stimulate the resonance in the first place. And higher frequencies attenuate more rapidly in air (as well as any dampening material used inside the enclosure) than lower frequencies so there's less acoustic coupling to the panel. Also, the higher the resonance, the easier it is to damp.

Lowering the frequency places the resonance where there's not only more energy, but also where there's more efficient acoustic coupling. And trying to damp a low frequency resonance is far more difficult.

And speaking of damping resonances, that's not the same as eliminating vibration. Damping a resonance simply means to reduce subsequent ringing. Even if the panel is well-damped and not ringing, it will still flex. And a thinner panel will flex far more than a thicker panel which means that the enclosure will be far more lossy in the bass region.

And moving it to a lower frequency where it's "less audible" as per Fletcher/Munsen, that's rather a specious argument. The resonance will still maintain the same level relative to the stimulus frequency regardless of where you put it.

Also if you look at the range of resonant frequencies you're likely to acheive in a reasonably sized enclosure, say between 100 Hz and 1,000 Hz, there's really not much difference in the F/M curves at typical listening levels.

se

OMNIFEX 1st April 2003 12:32 AM

Hey Kanga.

I build large bass cabinets (12 cubic feet and up) and, can tell you even if you just line the backwall with fiberglass, it will help.

There comes a point when too much bracing kills the internal volume of the box.

If your box is ringing at certain frequencies, you have a better chance killing the ringing with a damping material, than more
braces.

Kanga 1st April 2003 12:47 AM

Thanks for your comments Steve

Having a stiff enclosure for bass frequencies seems like a good idea to me, as the internal pressures that can be generated by the enclosure are quite high, and so a flexible wall box will move a lot more, requiring more damping to return vibration amplitude to inaudible levels. A stiff wall will not vibrate with much amplitude, and so may need less damping to bring amplitude to inaudible levels.

Do you know the theory of why it is easier to damp high frequency vibrations? I've seen this written elsewhere, and I'm interested in why.

Omnifex - when you mention fibreglass are you talking about insulation type or fibreglass-resin mix applied to the wall? BTW why do you need such a large box? Are you using monstrous size or multiple drivers?

At the moment I'm thinking of having a stiff MDF batten (maybe 25mm thick x 50mm high x 300mm long) running along each wall, with the all thread rod pushing the battens outward against the enclosure walls.

I'll let you know how that goes.

Mick

OMNIFEX 1st April 2003 01:03 AM

Quote:

Originally posted by Kanga


Omnifex - when you mention fibreglass are you talking about insulation type or fibreglass-resin mix applied to the wall? BTW why do you need such a large box? Are you using monstrous size or multiple drivers?

At the moment I'm thinking of having a stiff MDF batten (maybe 25mm thick x 50mm high x 300mm long) running along each wall, with the all thread rod pushing the battens outward against the enclosure walls.

I'll let you know how that goes.

Mick


I'm reffering to the insulation type (The itchy yellow/pink stuff :bigeyes: )

I use 15 and, 18 inch drivers for bass applications. Dual drivers in each box. Hey! the 12 cubic foot box is only 9.883 internal.

Maybe you can use something less irritating than fiberglass. I buy them by the rolls so, it works for me! :nod:

Are you bracing from the front to the back? By the front I mean where the driver is mounted on. The back I mean the rear wall.

Kanga 1st April 2003 01:17 AM

Quote:

Originally posted by OMNIFEX

Are you bracing from the front to the back? By the front I mean where the driver is mounted on. The back I mean the rear wall.


I'm bracing the side panels. Because the front and back are fairly narrow (275mm external) these ring fairly high.

Mick

OMNIFEX 1st April 2003 02:08 AM

You generally brace front to back. Considering, its the woofer that's causing the vibration in the first place.(Woofer shaking the front baffle board) The rear wall needs bracing also. Since gets the most punishment. (Woofer's back wave hits the rear wall)

That is the most vital brace of all.


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