how to brace a speaker cabinet?

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twinter

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Joined 2008
phivates said:
I'm guessing that 2way systems have very different requirements than 3ways in this conversation. It seems that cost-no-object views come to dominate discussions amongst bottom feeders like me. I won't be buying SEAS best any time soon, or ever, because I'm a pensioner working my way through a stack of old speakers that I've saved for this rainy day. Like SpeakerDave's AR4x, there's still a lot of goodness to be derived from vintage pieces that were optimal in their time and which are not junk now.
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The enclosure requirements are similar for 2-ways and 3-ways. As a number of 2-ways are bookshelf size, the enclosure walls require less means to be stiffer. There was plenty of discussions in this thread on extensional damping and constrained layer damping, neither of which are expensive to apply. The BBC paper by Harwood and Mathews on the effectiveness of thin wall damped enclosures was discussed. And references to different damping materials were brought up. I make no slight of earlier loudspeakers, especially vintage pieces that performed well. And, I originally entered this "hobby" to save money, as I had very little in college. I remember using graphite pencils as resistors on my first loudspeaker.

But I make no apologies for myself or others for referencing the best information on enclosure wall vibrations and damping that we could find. We are making efforts to increase our understanding, and increase the performance of our loudspeakers. As new drivers are reaching higher levels of performance, with total harmonic distortion of between -50dB (0.32%) and -60dB (0.1%), better enclosure wall isolation will be more beneficial. Enclosure wall vibrations have not been addressed to the same level as some others fields in DIY loudspeaker building. Due to the complexity of enclosure wall vibrations and damping, slightly off-the-mark preconceptions and extrapolations are pervasive. This thread is one of several with good information that has been on this forum. We touched on a few new areas as far as DIY is concern. For example, I had to dig through engineering books on structural borne noise to list the four factors for good extensional damping, See Post #203. This basic concept information is just not distributed as a whole, yet it should be, as it is necessary for consistent successful results. Crossover design tools and distortion measuring have advanced significantly, high performance enclosure construction should too .

I appreciate B&W and Pioneer TAD for publishing informative articles. They could just keep their information to themselves for a competitive advantage.

By sharing the information in this thread, I hope others don't spend significant efforts in building a loudspeaker enclosure, with stout walls and bracing, that has disappointingly significant vibration modes. See my Post # 179. Several others posting on this thread have had similar experiences, as they also applied the standard rhetoric on building an enclosure.
 
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Philosophil

Philosophil

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Joined 2013
I personally find the ideas being put forward here extremely insightful, particularly the discussion about decoupling drivers as well as the debate about the virtues of overly stiff cabinets. I will definitely try to implement some variation on those ideas in my next speaker build, and want to thank everyone involved for the excellent contributions
 
B

bushmeister

Member
Joined 2012
Hi all. Having achieved excellent results with increased damping in my mid sub enclosure, and by isolating my tweeter and mid with sorbothane gaskets, I now plan to glue a new oak worktop 28mm baffle onto the existing 18mm MDF baffle.

I hope to achieve extra damping and stiffness in the front baffle by doing this with the correct adhesives, so that the drivers and new baffle will only be mounted to the existing cabinet by the adhesive CLD.

Looking at this thread:
http://www.diyaudio.com/forums/multi-way/153419-constrained-layer-damping-mdf-ply-4.html

I am intrigued by this quote:

If you deflect for instance spring tempered carbon steel and release it the energy taken to deflect it a given amount is returned when you release it, and the ratio of the stress to strain is linear, this is known as Hook's law.

In a material that has hysteresis however this ratio is not linear and the amount of energy returned is less than the energy of deflection, the discrepancy being dissipated as heat and / or molecular displacement, and this effect is also frequency dependent.

If you have a very small displacement you need a large resistance to this in order to get enough energy into the material, and the only polymers that have these properties are hard rubbers and plastics and such things as epoxy resins.

For these small displacements you simply cannot get enough energy into such things as the green substance, and if you put it between panels the damping you get is due to mass damping and decoupling at these small displacements, not constrained layer damping, since you are simply not pushing the stuff far enough into its dissipative hysteresis region.

If you design specifically for these damping schemes, you can come up with a design that
is really effective an not some ersatz non optimal combination of them.
Rcw.
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So I am now planning to glue the oak onto the MDF with a two part epoxy resin.....any comments before I commit?!
 
T

twinter

Member
Joined 2008
I would be cautious, and read some more on CLD "adhesives". I agree with RCW stance completely for extensional damping material, I'm not so sure for CLD shear layer adhesives. Gedlee doesn't agree. He's a boffin (PhD), and has good practical experience too.

The recommendations I have seen for CLD adhesives have not been very stiff.
Green Glue, Sikaflex Polyurethane, NoiseKiller, polyurethane caulks in general

The stiffness of epoxy resin is in the same ballpark as MDF, 1/2 as stiff as Baltic Birch, and about 1/3 to 1/4 stiff as oak. See Post #132.
Is that enough difference in stiffness between the layers to consume a lot of energy in shear in the mid-layer?

You may want to call E. A. R., or run your own simplified tests with several pair of boards and different adhesives.
E-A-R Specialty Composites
 
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Mark Whitney

Mark Whitney

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Joined 2013
You could split the baffle into two parts. One part optimized for the woofer and fastened with epoxy. The second part optimized for the tweeter and mid driver which can be fastened with adequate decoupling to the old baffle.

I also advise to give the tweeter its own small chamber and not to leave the rear of the tweeter exposed to the pressure created by the woofer.
 
Greebster

Greebster

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Joined 2010
bushmeister said:
One concern I have floating a heavy oak baffle on green glue is that it could sag over time, green glue never seems to fully dry so I would worry that the baffle could shift with gravity and vibration.
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That would be a good question to ask the manufacture. Possibly only requiring a few changes made to accomidate. For example if the heavy oak baffle was over pinning (like that of a concrete foundation eg capping) the inner baffle would prevent sagging. Additional cure time for ample dryout of the GG prior to actual use would also prevent slippage.
 
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lousymusician

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Joined 2005
I just read through a long thread in the Construction forum on cabinet materials. This chart was posted there (without provoking any comments), and it seemed appropriate to this discussion:

153952d1263482029-discussion-what-materials-build-speakers-out-transmission-loss-figure.gif


I think it drives home that different approaches are needed for different frequency ranges. Does anyone know the source? Google image search was no help.

What would "Shear controlled" mean in this context?

Bill
 
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bushmeister

Member
Joined 2012
Thanks Greebster - a few useful options - but unfortunately the baffles are flat, and I don't fancy machining a lip or overhang into them.

I am intrigued by the 'Titebond melamine glue' as suggested by Gedlee as an excellent damping adhesive he uses. Does anyone have any experience with this? I will PM him regarding his experience.
 
T

twinter

Member
Joined 2008
lousymusician said:
I just read through a long thread in the Construction forum on cabinet materials. This chart was posted there (without provoking any comments), and it seemed appropriate to this discussion:

153952d1263482029-discussion-what-materials-build-speakers-out-transmission-loss-figure.gif


I think it drives home that different approaches are needed for different frequency ranges. Does anyone know the source? Google image search was no help.

What would "Shear controlled" mean in this context?

Bill
Click to expand...

A link to a similar chart was given in Post #174
Noise insulation case
 
T

twinter

Member
Joined 2008
bushmeister said:
Earl Geddes kindly replied to my pm, he still recommends the melamine glue as one of the best commercially available glues for cld. So I am going to buy some and experiment.
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Bushmeister, did Gedlee give any other instructions?
How thick a layer? Use spacers, or just apply layer and stack then let set, or allow compressed attachment of board-to-board on edges? Screw or pin in places?
 
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