Hi, Gents,
I am planning on bracing the 16T X 10W X 14D two way cabinets I've just built with one side to side brace and one baffle to back brace. The cabinets are built of 3/4' Baltic birch and are of 100% mitered construction, including the top and bottom. My question is: Is it better to tie these braces to each other where they cross in roughly the center of the cabinet or keep them seperate from each other? (I know that they should be slightly off center where they butt against the panels). My hunch and one experienced speaker builder says, tie them together, but another very experienced speaker builder I respect says to keep them independent of each other. I wonder if folks who have practical experience can share their views and rationale. I'm still a beginner, this is the biggest cabinet I've built and I'd like it to be the best it can be given that it is using a very basic bracing scheme.
Best,
Jay
I am planning on bracing the 16T X 10W X 14D two way cabinets I've just built with one side to side brace and one baffle to back brace. The cabinets are built of 3/4' Baltic birch and are of 100% mitered construction, including the top and bottom. My question is: Is it better to tie these braces to each other where they cross in roughly the center of the cabinet or keep them seperate from each other? (I know that they should be slightly off center where they butt against the panels). My hunch and one experienced speaker builder says, tie them together, but another very experienced speaker builder I respect says to keep them independent of each other. I wonder if folks who have practical experience can share their views and rationale. I'm still a beginner, this is the biggest cabinet I've built and I'd like it to be the best it can be given that it is using a very basic bracing scheme.
Best,
Jay
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No slamming from me, Mark100, and I think you are right, broadly speaking.
In my opinion: Of primary importance is getting well matched drivers connected with a well designed crossover that takes into account the diffraction and baffle step. Next in importance is low diffraction in the horizontal plane and a smooth, uniform off axis response. Only after getting all of these factors right will the differences in cabinet rigidity and structural damping matter.
On the other hand, it is very difficult to predict how much bracing or damping will be needed to make the cabinet inaudible. Bracing is cheap and easy when the box is being constructed, but it is difficult to add after the system is fully assembled. So it is perhaps better to err on the side of "more is better" while the box is being built.
In my opinion: Of primary importance is getting well matched drivers connected with a well designed crossover that takes into account the diffraction and baffle step. Next in importance is low diffraction in the horizontal plane and a smooth, uniform off axis response. Only after getting all of these factors right will the differences in cabinet rigidity and structural damping matter.
On the other hand, it is very difficult to predict how much bracing or damping will be needed to make the cabinet inaudible. Bracing is cheap and easy when the box is being constructed, but it is difficult to add after the system is fully assembled. So it is perhaps better to err on the side of "more is better" while the box is being built.
Cheap (or free) Finite Element Modelling software is not the hurdle. A valid FEM is difficult to construct. Most FEMs are optimized for strength and stiffness analysis. Modelling structural resonance is an added level of complexity.
Most enclosure design is focused on minimizing the effects of structural resonance. It is complicated because there are six walls in a box, and they mutually couple. Cabinet stiffening can transform the six cabinet walls into many more virtual surfaces.
Anyone who claims that this is simple engineering or basic physics is blowing smoke. It is very difficult to model and simulate. The fixity of the edges are difficult to predict, and the effectiveness of the bracing is hard to know. Are the cabinet walls under pre-load (clamped during glue-up)? What is the effectiveness of the adhesive bond line (i.e. how stiff is the adhesive)? How much internal damping is there in the cabinet walls? All in all, a tough modelling problem.
It has been shown that the effectiveness of bracing to stiffen a panel varies with the stiffness of the braces themselves (link). This is especially true as you increase the number of braces too. Tying braces together will increase their stiffness so that seems like the correct solution.
If the purpose of bracing is to stiffen the cabinet panels and thereby raise their primary resonant frequencies where their is less energy in music to excite them, another way to look at the problem is to ask what is the minimum subpanel primary resonant frequency you want to achieve?
Using the formula for panel resonances with the panels supported on all 4 sides (meaning full on window braces) that diyAudio lists here as a simplistic rough ballpark guide, I come up with the following results for quality 3/4" plywood:
16" x 5" = 1955Hz
14" x 8" = 923Hz
8" x 5" = 2477Hz
14" x 4" = 3010Hz
5" x 4" = 4564Hz
Again, ballpark figures not expected to be exact since cabinet resonant behavior is much more complex than can be summarized with this relatively simple equation but the formula has shown excellent consistency with the panel resonant frequencies that augerpro has been measuring in his experiments in this thread - A Monster Construction Methods Shootout Thread.
The 14" x 8" is the one that looks like it could be a problem and that would be your 2 side panels with 1 brace splitting it in 2. For me, I would opt for 2 horizontal braces on the side panels (also connecting to the front and back panels) and 1 vertical brace connecting the top and bottom panels (also connecting to the front and back panels). And all the braces would be matrixed together.
Note that if you add damping to the panels (which you should), resonant frequencies will drop some. And of course how many braces you use and how 'holey' they are or actually what type they are will be depend on how much gross volume you have to play with in your design since your cabinets are already built.
If the purpose of bracing is to stiffen the cabinet panels and thereby raise their primary resonant frequencies where their is less energy in music to excite them, another way to look at the problem is to ask what is the minimum subpanel primary resonant frequency you want to achieve?
Using the formula for panel resonances with the panels supported on all 4 sides (meaning full on window braces) that diyAudio lists here as a simplistic rough ballpark guide, I come up with the following results for quality 3/4" plywood:
16" x 5" = 1955Hz
14" x 8" = 923Hz
8" x 5" = 2477Hz
14" x 4" = 3010Hz
5" x 4" = 4564Hz
Again, ballpark figures not expected to be exact since cabinet resonant behavior is much more complex than can be summarized with this relatively simple equation but the formula has shown excellent consistency with the panel resonant frequencies that augerpro has been measuring in his experiments in this thread - A Monster Construction Methods Shootout Thread.
The 14" x 8" is the one that looks like it could be a problem and that would be your 2 side panels with 1 brace splitting it in 2. For me, I would opt for 2 horizontal braces on the side panels (also connecting to the front and back panels) and 1 vertical brace connecting the top and bottom panels (also connecting to the front and back panels). And all the braces would be matrixed together.
Note that if you add damping to the panels (which you should), resonant frequencies will drop some. And of course how many braces you use and how 'holey' they are or actually what type they are will be depend on how much gross volume you have to play with in your design since your cabinets are already built.
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Free FEA software has been available since the 70s. There are perhaps a hundred different packages available today with perhaps 2 or 3 worth considering for engineering design studies (aster, elmer, calculix,...). They don't come with masses of manuals, telephone support and slick graphics like commercial codes but they are useable to a fair extent after a bit of study particularly if one is comfortable writing scripts.
For speaker cabinets the only major problem I have found was an inability to efficiently calculate a Frequency Response Function with viscoelastic damping. FRF with constant damping fine, viscoelastic damping at a single frequency fine but not an efficient FRF with viscoelastic damping. Unfortunately if one is interested in a design study for a decently performing speaker cabinet then simulating damping in a reasonably accurate manner is a requirement.
If we take the OPs case as an example one could simulate his proposed drivers and cabinet including a small amount of constant damping to roughly represent the small amount in plywood. Couple it to a BEM acoustic code and get a plot of SPL vs frequency at the listening location of the sound radiated by the cabinet for the cases of no bracing, back-to-front brace, side-to-side brace, both braces, perhaps double thickness baffle and a few others.
This will show that bracing is not making the cabinet quieter just shifting the resonances higher in frequency. Stiffening the baffle though will be useful in reducing the amount of work put into the cabinet so the back-to-front bracing is likely to be seen to be a small benefit. It will leave a few high Q modes, possibly only one, that radiate loudly to the listening location. Given the relevant mode shapes damping then needs to be added at effective places on the cabinet to reduce the problematic modes 10-30 dB in level. The KEF LS50 white paper illustrates this process nicely although constrained layer damping patches are likely to be a sufficient and more practical approach than damping panels.
Curiously I am currently browsing the web in order to take a break from implementing a method to calculate FRFs for viscoelastic materials in an FE code. Sound radiation from speaker cabinets will almost certainly be one of the main worked examples. I will inform the forum when there is something solid to report.
IMO the best bracing schemes involve partitions interfacing 4 sides of the cabinet. You can cut holes in the panels to allow airflow and reduce weight. Usually I start with a panel centered on the woofer magnet with a U-shaped relief cut to clear the basket. Trim to the exact distance between the magnet and rear wall, then add a thin pad of rubber on the edge of the panel. When you install the woofer it will be braced as well, and you can reduce another point of distortion.
A highly regarded member of my Dutch forum posted several results of BEM simulations with ABEC, however he focused not on panel resonances but on the sound field in and around TL’s and BR-speakers. I suspect ABEC could calculate the transfer function of panels too. Not found any time to get into that deeper though.
You could always look at how some of the top end speakers are made, like the B&W honeycomb structure, shown in this video from them:
800 Series: Cabinet bracing to the ultimate level - YouTube
They have put a lot of time and effort into this (modelling and real world testing).
800 Series: Cabinet bracing to the ultimate level - YouTube
They have put a lot of time and effort into this (modelling and real world testing).
That would be awesome if true. My bet is it's more about marketing and product differentiation ....designed to support yet another ridiculous audio price point.
The short answer is that they do, yes they do "manipulate" it a little (like holding back some things that might improve damping so they can create the next generation of speakers with that as an improvement), or at least that is how they used to work before they changed a lot recently. But the basic principle should be sound (pun intended...)
Based on what I read here, I finished up both the cabinets earlier today, tieing the two braces together. At this point I sort of wish that I had left the second one completely unbraced and compared them to see if I could measure or hear a difference.
I'm wondering if anyone here has done an A-B subjective or measured comparison between a cabinet about like mine with no, or minimal bracing and one that is identical in all respects but much " better" braced. I am most interested in whether the "real world" effects of more extensive bracing are worth the effort in a rather small (21L) cabinet built out of 3/4" baltic birch with fully mitered joints.
I'm wondering if anyone here has done an A-B subjective or measured comparison between a cabinet about like mine with no, or minimal bracing and one that is identical in all respects but much " better" braced. I am most interested in whether the "real world" effects of more extensive bracing are worth the effort in a rather small (21L) cabinet built out of 3/4" baltic birch with fully mitered joints.
The joining only has an effect with modes that take the middle point away from its rest position. With the fundamental mode, with each side moving outward together the brace itself doesn't move. When both panels move in a dipole manner, each shifting left at the same time, the brace goes with them. These are the modes that are changed when you connect the braces. It could be argued that dipole modes in general have an equal or lesser effect.
I've done so on at least a dozen different boxes, of widely varying types.
It's a big part of the prototype process.
I never bother to listen subjectively until tuning is done via measurements, so can't comment there.
With measurements, changes in frequency response are easy to see with regard to how tightly constructed the box is...how well it is sealed, even if it's a vented design. Same for lining and fill.... easy to measure.
By frequency response, i mean from a transfer function.
But bracing is very difficult to measure any difference, other than for subs or high power mid-bass, and usually then only at higher SPL levels, looking for harmonic distortion.
Impedance plots are often the best bet for being able to see change in bracing i think. (and anything to do with cabinet construction).
I've concluded it pays to stay sensible with bracing.
Know whether it's to avoid panel flex or to push potential resonances higher.
Do as little as it takes.
My bet is your box is just fine
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