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post #301 of 337 Old 08-29-2016, 10:52 AM
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Originally Posted by ronny31 View Post
Even after I showed that the difference in strength was extremely minor,[/URL] yet used half the material
It's unclear what you showed.

How is the model restrained?

Is it static or modal displacement?

If the former, what loads were applied and where?[/QUOTE]

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post #302 of 337 Old 08-29-2016, 03:50 PM
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Originally Posted by noah katz View Post
It's unclear what you showed.

How is the model restrained?

Is it static or modal displacement?

If the former, what loads were applied and where?
Blue side is constrained by anchor (no movement in any axis, directionally or torque).

Static pressure on external faces, no gravity. 0.02Mpa (20 000 pascal, or 180db). To make the results tangible given the small area for the pressure to work on, on the cutout profile.

Will be showing some other sims where I use frictionless anchors so that all pieces can move freely in the relevant directions (out/in). But the simulation takes time at the complexity level of an entire enclosure, especially one as complex as mine. Not sure how relevant it is to diagonal bracing however, the enclosure is so over-engineered that I'd wager its like concrete. Anyone else who don't mind using fiber filler and whatnot would be able to get away with way less bracing. Especially if its a subwoofer, not a 50-1000hz enclosure.
Here's the version I'm simulating:

As such I need quite the level of detail in the mesh, and that takes time to get right. Enough detail without overdoing it (it only uses 2 cores of my 8 core CPU so sim times aren't stellar, it is a free software after all).

Btw, if someone here wants to beat me at my own game, feel free to install Autodesk Fusion 360, its free for 1 year, and make your own bracing schemes and show us precisely how much they move with this and that pascal from this and that driver.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #303 of 337 Old 08-29-2016, 03:57 PM
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... Lets do a simpler example: 2+2 has an answer that is correct regardless of what the CV of either party is. No one can argue that the answer is this or that on the basis of their own authority, or the lack of authority of the other part..

Yes one can, especially when the other party thinks that answer is three.

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post #304 of 337 Old 08-29-2016, 06:08 PM
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Yes one can, especially when the other party thinks that answer is three.
If someone says diagonal bracing beats cross-bracing, which one is the one that thinks the answer is 3, metaphorically speaking? They both consider the other obviously wrong. Perhaps as obviously as claims that the answer to 2+2 is 3.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #305 of 337 Old 08-30-2016, 06:34 AM
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Originally Posted by ronny31 View Post
If someone says diagonal bracing beats cross-bracing, which one is the one that thinks the answer is 3, metaphorically speaking? They both consider the other obviously wrong. Perhaps as obviously as claims that the answer to 2+2 is 3.

Perhaps you should re-read this thread. I don't think anyone said that diagonal bracing doesn't beat cross-bracing. All that was said was that it was more involved and that it wasn't necessary for the OP's build. I think everyone who has posted in this thread knows that a triangle is the stronger shape, it's just that cross-bracing is plenty good enough, and easier to implement.


If I wanted to waste my time and/or my money, I personally would window-brace something like this:
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post #306 of 337 Old 08-30-2016, 08:06 AM
 
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Originally Posted by Augerhandle View Post
I think everyone who has posted in this thread knows that a triangle is the stronger shape
It's the stronger shape in terms of resisting torque, but as there are no forces in a speaker that create torque it's of no consequence.
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post #307 of 337 Old 08-30-2016, 10:04 AM
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I forget if I said this earlier in this thread or somewhere else, but cross-bracing only addresses modes where the panels move in opposition to each other, which is fine for sides and top/bottom.

Cross-bracing front/back will just take the rear panel along for the ride with baffle modes driven by driver reaction force.

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post #308 of 337 Old 08-31-2016, 12:10 AM
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Originally Posted by Augerhandle View Post
Perhaps you should re-read this thread. I don't think anyone said that diagonal bracing doesn't beat cross-bracing. All that was said was that it was more involved and that it wasn't necessary for the OP's build.
I would argue that diagonal bracing is way easier than window bracing. Any ape can cut some scrap pieces with a 45 degree end and glue them in after finishing the enclosure. All the pieces fit through the hole in the baffle you know. Whereas a window brace scheme has to sort of fit together like a puzzle.


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Originally Posted by noah katz View Post
I forget if I said this earlier in this thread or somewhere else, but cross-bracing only addresses modes where the panels move in opposition to each other, which is fine for sides and top/bottom.

Cross-bracing front/back will just take the rear panel along for the ride with baffle modes driven by driver reaction force.
Well yeah, but I must add that if you connect to opposing panels you get the stiffness from both. There is a problem with long thin braces though, they can easily have more resonance and create more noise than any of the panels.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #309 of 337 Old 08-31-2016, 06:21 PM
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Well yeah, but I must add that if you connect to opposing panels you get the stiffness from both.
And the mass of both.

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post #310 of 337 Old 08-31-2016, 08:11 PM
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Originally Posted by Augerhandle View Post
Perhaps you should re-read this thread. I don't think anyone said that diagonal bracing doesn't beat cross-bracing. All that was said was that it was more involved and that it wasn't necessary for the OP's build...

I would argue that diagonal bracing is way easier than window bracing. Any ape can cut some scrap pieces with a 45 degree end and glue them in after finishing the enclosure. All the pieces fit through the hole in the baffle you know. Whereas a window brace scheme has to sort of fit together like a puzzle....



You would. But my comment was about cross-bracing being easier, not window bracing. Also, there's a lot of people in the world without the means or desire purchase the equipment to cut a mitered end.

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post #311 of 337 Old 08-31-2016, 08:50 PM
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Originally Posted by Augerhandle View Post
Also, there's a lot of people in the world without the means or desire purchase the equipment to cut a mitered end.
There's this thing called a hand-saw, if you saw something you can pick any degree between 1 and 90 that you want. Same can be done with all the electrical saws.
"Oh no, we can't build the pyramids, there won't be stone saws for mitered ends for another 5000 years".

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #312 of 337 Old 08-31-2016, 11:02 PM
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Originally Posted by ronny31 View Post

This enclosure is probably at the border between when it may be less work and cost to just plaster the inside with polyester and glassfiber. But this particular person knows wood-working and had all the tools for it so it was easier for that person to do it this way with complex bracing than to learn glassfiber composite techniques. If someone fixed his canoe and then built this, he would probably end up just laying a quarter inch of glassfiber and resin on the inside of the plywood like this:

black is resin and glassfiber, orange is sticks for adding thickness to the glassfiber without having to fill it completely in (a cost saving technique which costs a little time in laying the mats). Grey is the plywood.
But this guy did it right. He uses plywood not mdf, he even laminates it further, he uses evenly spaced plywood bracing, he bends two of the largest surfaces twice in an S shape. Its essentially a wooden composite enclosure, wood sheets instead of (glass)fibers, with glue holding it together (epoxy/polyester resin is just fancy names for glue). But, given the time he spent making the reinforcements it may have been as quick and easy for someone equally skilled in glassfiber, to reinforce equally much (or even more) with glassfiber. So if he was a modern boat-builder he would probably have used glassfiber, but since he knew wood-working he used (the correct) wood.
With such a driver, you could probably not make it weaker, or it would flop like a plastic bag. Don't forget, relative to the cone size, its as if we built a 10" subwoofer with something like 4mm plywood. Without all that bracing, it wouldn't work. If it was mdf, it probably wouldn't work.

EDIT: FYI, this method in the picture does strengthen the material even in the direction perpendicular to the sticks. Because the glassfiber is thicker where the sticks are and that reinforces the thin bits between the sticks. You may of course run sticks along the short length instead of the long length so that you have more reinforcement, but then you spend more time on it and may ultimately get more reinforcement in a days work by working with bigger pieces (longer sticks, larger diameter sticks, more space between the sticks, thicker glassfiber mats)(glassfiber mat thickness is proportional to the sharpness of the shapes you want to push the glassfiber into and around).


Followed by this

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Originally Posted by ronny31 View Post
What is so bleeding difficult about it? Its the practice in your house, your offices, your stores, your roof, your floor, your bridges, skyscrapers, ships and airplanes.

Only you use polyester resin, glassfiber and "sticks" of proportional size because its cheap and low skill requirement to work with. "Sticks" is anything you have handy that you don't need to expend more money on. Cut-offs from the edge of your sheets, bits and pieces from previous projects, junk from a nearby construction container. The amount of glassfiber you need is microscopic. If you take two 400 gram chopped strand mats and lay them on top of each other on the form of lets say a dinner plate bottom, the finished product is so strong you can stand on it without damaging it. And glassfiber is laid so fast you wouldn't know it. Just out in a sunny day brushing on resin and then applying the glassfiber over some sticks, wetting the sticks and glassfiber with resin with the brush, then let it set. The sticks don't need exact measurements, neither does the glass mats, you don't have to measure twice and cut once you just do it quick and easy. You don't need a plan you just eyeball how much to put on each surface. More on the bigger surfaces and less on the small ones. Then you eyeball where you can have some sticks crossing the span between two big surfaces that you also just glue in place with resin. Don't even have to measure that stick exact, just jam it in where it fits when you have cut it close. it will be set in glassfiber when you're done anyway.
Throughout all the bloody years am I the only person to suggest this way of structural reinforcement? I think some here are too focused on it looking neat during the job. Neat mdf bracing evenly spaced with complex cut-out shapes to get rid of material (that gets paid for and then time wasted to cut it out to be thrown away). Pre-laid plans of how to cut each sheet for the reinforcement, exact measurements and cuts, figuring out where all of them goes and what order to put them in. Each glued in separately with wood glue and secured with staples or screws. Possibly also tools for pressing them together for better glue joint. What a laborious process.
Is the reason I can't really take your design concepts seriously . Do you see a difference in the bracing of the ships hull compared to your concept of bracing ? It's not resin and glass mat . I'm glad to see that you have progressed to bracing that may be useful in cabinet construction , the gluing sticks to the panels in only one axis concept just blew any credibility you might have had out the window for me .

Edit : the day that you can take 2 chopped glass mats , lay them on a dinner plate and have my 240# ass stand on it and it not break is the day I get voted best dress at the Oscars by Vogue magazine .

Turn that $*!# UP!! --Beethoven

Last edited by acras13; 08-31-2016 at 11:09 PM.
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post #313 of 337 Old 09-01-2016, 01:48 AM
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Is the reason I can't really take your design concepts seriously .
That's ages ago.

Since then I did simulations that showed that these two use the same material amount per square foot:
Spoiler!


Even though one is clearly superior to the other in panel movement.

And I also discovered that diagonal bracing uses half the material but achieve nearly the same panel movement:

Spoiler!


Thus if we combine the two, like in my design for example:
Spoiler!

Spoiler!

Then we get darn good results.

Simulation specifics:
Material: MDF.
Internal air pressure: 1000 pascals.
Such internal air pressure achieved at driver wattage: 1800w rms.
Bottom frequency that can achieve such air pressure without exceeding xmax: 120hz (so it is assumed 150hz to be well within safe limits, using 3.5 out of 5mm xmax then).
Power handling of 15" mid that can handle this: 1700w AES (1700w rms pink noise 2 hours).
Noise achieved from panels: 65db at 150hz.
Internal pressure drops off from 150hz upwards, but I will check what the panel movement is at lower internal pressures as well just to be safe that the noise produced by the panels isn't higher than 65db towards 1000hz where the mid will hand over to the compression driver.
Radiation pattern assumed in all db figures: 2.0 X Pi (so floor loaded, no walls or ceiling).
Signal/Noise ratio achieved: 65db (65db noise, 130db output, 130 minus 65 equals 65).
It stands to reason that we can assume the ratio is true for lower putput, so 120db has 55 db noise, 110db has 45db noise, etc.

In day to day use the enclosure will be used with 50hz highpass filter instead of 150hz highpass filter, and instead there will be two woofers on either side. As such the output level will be limited to about 120db a piece (all assumptions about db is in 2.0 x Pi radiation). But it stands to reason that the noise will then be equally far below, so 55db or so from the panels at 120db. Which is darn good I'd say.
For special occasions (read: parties and demos) the mids will be cranked up to 150hz filter and then:


The cross-members in the middle that you can see are cut off in the middle are also held in place with frictionless constraints so they can't move towards the camera or away from the camera, but they can move in the two other axis. But these cross-members show significant resonance so they too must be braced in the final version. Probably vertically in this view.

If someone on AVS forum could just have told me this to begin with, I would never have bothered to share my WRONG idea of how to most effectively brace enclosures. And I also would not have to bloody teach myself this if someone already had designs which could say how much noise they produced from flapping panels. "These window braces cause little enough panel movement" is hardly enlightening.

PS: Does anyone remember what 21" subwoofer thread it was that measured with and without pillow stuffing?
If I remember correctly it was -2db that was the result with two extra pillows in a 21" enclosure with very low tune. I'm betting you will lose more db if you have a higher tune and get higher overall output. I really want to test that on this enclosure.

PPS: The simulation is a quarter of the enclosure, in case you hadn't worked that out. The other pieces are equal to this piece, just mirrored across one or two axis.

PPPS: I worked out db noise from using this tool. We can argue a lot about what speaker diameter to use and which average xmax figure to use, of course. I arrived at about 65db from my guesstimate. If we take the wavelength of 150hz and see how much the other four sides can add to each other (sum up) in db we could probably do some more accurate figures.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)

Last edited by ronny31; 09-01-2016 at 02:00 AM.
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post #314 of 337 Old 09-01-2016, 08:55 AM
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With added bracing vertically we stop the excursion in the middle by a lot. But the port still goes out far even though the overall excursion multiplied by area is lower. So I will extend the port bracing next.

Spoiler!

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #315 of 337 Old 09-01-2016, 09:11 AM
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Originally Posted by ronny31 View Post
Noise achieved from panels: 65db at 150hz.
Internal pressure drops off from 150hz upwards, but I will check what the panel movement is at lower internal pressures as well just to be safe that the noise produced by the panels isn't higher than 65db towards 1000hz where the mid will hand over to the compression driver.
Did you account for resonance, which may increase deflection by an order of magnitude, or is that static deflection?

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post #316 of 337 Old 09-01-2016, 09:29 AM
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Did you account for resonance, which may increase deflection by an order of magnitude, or is that static deflection?
Great question, I'm currently looking at decreasing static deflection as much as is practical. Then I'll see what resonance frequencies it has in the 150-250hz spectrum at that point (that's where internal pressure is greatest). And how much deflection it has at those resonance frequencies.
But I need to find another material data for that really, I won't be using MDF.
BUT, we can at least make educate guesses about the noise output at resonance frequencies, even if we don't know precisely what frequencies HDP will resonate at. HDP will likely have at least one resonance frequency below 200hz where panel movement is greatest, even if it will have its resonance frequencies distributed higher up than MDF.

Here's with extended port braces, fantastic improvement. Still haven't checked what db area it is in with this area and excursion however, need to eat before an appointment.

Spoiler!


I wonder if I can apply an internal layer of rubber (or butyl if that's in the program), and then I also wonder if there will be less deflection because some of the air pressure is turned into heat squeezing the rubber against the mdf. Need to test it tomorrow.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #317 of 337 Old 09-01-2016, 07:33 PM
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The models are cool. Not complete, but gives a rough indication that most any reasonable bracing scheme will work well enough. Window, cross, diagonal, some combination, whatever. Each will have some plusses and minuses... less material, stiffer, quicker to assemble, helps hold assembly square during construction or not, amenable to rapid cnc manufacturing and assembly line, looks cool in build pics, whatever.

Need to account for modal behavior. At least as important. But simple modal analysis is no more complete than a simple static stiffness and stress analysis. Resonant amplitude with varying driving force on a realistic model takes, erm, some work... unless things have progressed in the consumer space since I was last in that game. We wrote some custom software to look at these effects on composite surface warship structures under... interesting load conditions. Its complicated.
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post #318 of 337 Old 09-01-2016, 10:06 PM
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But I need to find another material data for that really, I won't be using MDF.
Material data will give a rough, but conservative, estimate of damping; the rest is determined by construction and interactions between panels at joints.

As an interesting aside, bolted joints greatly increase damping because there's relative motion/friction at the interfaces.

I forget if it was KEF or B&W who built some of their speakers with the window braces attached with gasketed joints to increase damping over solid glued in.


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I wonder if I can apply an internal layer of rubber (or butyl if that's in the program), and then I also wonder if there will be less deflection because some of the air pressure is turned into heat squeezing the rubber against the mdf. Need to test it tomorrow.
It would be a lot more material-efficient to use constrained layer damping (CSD).

I've read good things about Dynamat, but it may be intended for much thinner materials, in which case there'd be a stiffness mismatch between your panels and the constraining layer.

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post #319 of 337 Old 09-01-2016, 11:41 PM
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I forget if it was KEF or B&W who built some of their speakers with the window braces attached with gasketed joints to increase damping over solid glued in.
That's an interesting idea.

Quote:
Originally Posted by noah katz View Post
It would be a lot more material-efficient to use constrained layer damping (CSD).

I've read good things about Dynamat, but it may be intended for much thinner materials, in which case there'd be a stiffness mismatch between your panels and the constraining layer.
Dynamat is just a brand name. The butyl comes in many forms and composition. I think its primary advantage is that it transmits sound very poorly, whereas HDP transmits sound easier than MDF. Even patches of butyl would slow down portions of the sound signal so that it arrives through the HDP at different times instead of at the same time.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #320 of 337 Old 09-02-2016, 06:18 AM
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There's this thing called a hand-saw, if you saw something you can pick any degree between 1 and 90 that you want. Same can be done with all the electrical saws.
"Oh no, we can't build the pyramids, there won't be stone saws for mitered ends for another 5000 years".

As usual, your reply has nothing to do with the post you quoted.

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post #321 of 337 Old 09-02-2016, 09:57 AM
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Originally Posted by ronny31 View Post
Dynamat is just a brand name. The butyl comes in many forms and composition. I think its primary advantage is that it transmits sound very poorly, whereas HDP transmits sound easier than MDF. Even patches of butyl would slow down portions of the sound signal so that it arrives through the HDP at different times instead of at the same time.

None of that has anything to do with how CSD works.

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post #322 of 337 Old 09-02-2016, 08:00 PM
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Originally Posted by ronny31 View Post
I would argue that diagonal bracing is way easier than window bracing. Any ape can cut some scrap pieces with a 45 degree end and glue them in after finishing the enclosure. All the pieces fit through the hole in the baffle you know. Whereas a window brace scheme has to sort of fit together like a puzzle.




Well yeah, but I must add that if you connect to opposing panels you get the stiffness from both. There is a problem with long thin braces though, they can easily have more resonance and create more noise than any of the panels.
ok , aside from questioning your previous comments about the best practices for construction , in this model I'm confused , how is this modeling for a subwoofer box ? It looks like force is being exerted from outside the structure instead of from within . I am not an expert by any stretch but looking at that example I cant see force bowing any panels outward from the original shape , everything is moved toward the center . If theres a link to an explanation , I'd be happy to read it on my own , just haven't found anything that matches this .

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post #323 of 337 Old 09-03-2016, 07:24 PM
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Not that it changes anything... but pressure forces from within will be both positive and negative. But the way this particular model is constrained is weird.

Those who have some experience with FEA just realize that they are investigating a symmetric structure with symmetric forces, and create a quarter model constrained along lines of symmetry. Frees up resources and is more realistic to boot.
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post #324 of 337 Old 09-04-2016, 01:52 AM
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Quote:
Originally Posted by acras13 View Post
ok , aside from questioning your previous comments about the best practices for construction , in this model I'm confused , how is this modeling for a subwoofer box ? It looks like force is being exerted from outside the structure instead of from within . I am not an expert by any stretch but looking at that example I cant see force bowing any panels outward from the original shape , everything is moved toward the center . If theres a link to an explanation , I'd be happy to read it on my own , just haven't found anything that matches this .
... There's this thing around the subwoofer called "air". And the subwoofer produces pressure differences in this air, which pushes on the enclosure faces. The pressure difference at about 130db in my enclosure is 900 odd pascals, so round it up to 1000 and then apply that force to the enclosure sides. I can model the enclosure both with force from the inside out, or from outside in, as that may have different results by a tiny bit.

But then again there are those in this thread that say I'm not an expert on air, and that they don't want to take my word for it. And they may therefore argue that there couldn't possibly be any air around subwoofer enclosures, and that there could not possibly be any pressure differences anywhere near subwoofer enclosures.

@Bigus Yeah I didn't know how to use the other constraints back then, so just used rigid constraint. The model wasn't too complex so cutting it in four bits wouldn't really save me much time (took like 2 seconds to simulate it).

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #325 of 337 Old 09-04-2016, 01:36 PM
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Yeah for a 2d model that simple the mesh size isn't a concern. But planes of symmetry are the correct way to model it so that the mesh can move realistically. Cut along vertical plane, and constrain nodes along that vertical face only for horizontal translation. Cut horizontally, and constrain nodes on that horizontal face against vertical translation only.

Doesn't help with a 3d mesh though. Your constraint is still a bit wonky. No need to contmstrain the while face. I'd probably just pin one front corner for translation in 3 axes, the other front corner for translation in z and either x or y, and the rear corner nodes against z translation only.

And lets see a modal analysis. Does fusion 360 do that?
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post #326 of 337 Old 09-04-2016, 02:13 PM
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I'm familiar with "air" , my question was more focused around the thought that the volume of air moved by the driver is give or take the same on both sides , the inside and outside in this case . So to my way of thinking the greater force , and thus deflection of the panels should be generated inside the smaller enclosure (the box) as opposed to greater force generated in the room directed in toward the center of the box. In either case I forgot to mention that I'm confused that only 3 sides are deflected in the model . Again , I am just trying to learn , but my construction background is not allowing me to see how this model makes any sense in the context of box design . The "900 odd Pascals" applied to the sides of the enclosure , deflecting them only inward looks like you are generating 130db with a vacuum pump instead of a subwoofer .

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post #327 of 337 Old 09-04-2016, 03:14 PM
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Quote:
Originally Posted by acras13 View Post
I'm familiar with "air" , my question was more focused around the thought that the volume of air moved by the driver is give or take the same on both sides , the inside and outside in this case . So to my way of thinking the greater force , and thus deflection of the panels should be generated inside the smaller enclosure (the box) as opposed to greater force generated in the room directed in toward the center of the box. In either case I forgot to mention that I'm confused that only 3 sides are deflected in the model . Again , I am just trying to learn , but my construction background is not allowing me to see how this model makes any sense in the context of box design . The "900 odd Pascals" applied to the sides of the enclosure , deflecting them only inward looks like you are generating 130db with a vacuum pump instead of a subwoofer .
The one side is the anchor point in the model.

The subwoofer produces pressure differences, 900 pascals above and 900 pascals below the air pressure of 100 000 some odd pascals, in alternating fashion. Whether you apply it as a negative pressure on the inside, or positive pressure on the outside, is practically the same.

The pascals generated is calculated by Hornresp and we're talking 150hz-ish here. The 15" driver is only moving in and out 3.5mm in respect to neutral position, yet the frequency means the air pressure differences is quite great. And both external and internal pressure is quite high, both negative pressures and positive pressures. You can go on and on about that it could be less because of so and so but I'm still going to use 1000 pascals as a benchmark because it works as a worst-case. Might even use 2000 pascals just to be safe.

@Bigus Yeah fusion 360 can do modal analysis. I have been busy with E-sports for the last few days and that will go on for a little over a week. But I'll be getting back to finish the simulation soon.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)

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post #328 of 337 Old 09-05-2016, 11:20 AM
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Meanwhile, while someone was and is still working on simulations, 2,304 subwoofer boxes have been built with cross-bracing and are thumping away in HTs all across the world.
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post #329 of 337 Old 09-06-2016, 11:15 AM
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Quote:
Originally Posted by Bigus View Post
Those who have some experience with FEA just realize that they are investigating a symmetric structure with symmetric forces, and create a quarter model constrained along lines of symmetry. Frees up resources and is more realistic to boot.
How is it more realistic, unless you mean take advantage of the size savings to increase mesh density.

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Originally Posted by Bigus View Post
Yeah for a 2d model that simple the mesh size isn't a concern. But planes of symmetry are the correct way to model it so that the mesh can move realistically. Cut along vertical plane, and constrain nodes along that vertical face only for horizontal translation. Cut horizontally, and constrain nodes on that horizontal face against vertical translation only.

Doesn't help with a 3d mesh though.
Not sure what the last statement means, but might refer to the below.

Restraints at planes of symmetry must also prohibit out-of-plane rotations.

This won't matter with 3D elements, whose nodes have no rotational stiffness, but will if for example the braces were modeled as beam elements.

Noah
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post #330 of 337 Old 09-06-2016, 11:53 AM
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Not sure how experienced you are with FEM, from your question I assume you are at least to some degree. Proper constraints are of course hugely important to getting realistic results from a model.

The use of symmetry is just the easiest way to get it right IMO. I suppose you could constrain the midpoint node of each face/edge against translation along its length, but them you've just utilized symmetry and the rest of the elements are redundant. And you're right - you do have to constrain against rotation at planes of symmetry... inplane rotation though, right? I don't think out of plane rotation is possible in a 2D simulation inherently.

In any case, constraining a face/edge the way he did artificially stiffens significant parts of the model. Which isn't realistic. Symmetry is, which was my point.

Ad for my comment about 3d models, its simply an acknowledgement that 3d models often get complicated and lose planes of symmetry in trying to accurately model real geometry. That's all. If you still have a symmetric model, by all means, take advantage of it.
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