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post #31 of 337 Old 02-04-2016, 08:39 PM
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Quote:
Originally Posted by ronny31 View Post
I find it funny every time someone tries to brace mdf with mdf, as well as trying to brace mdf with other materials.
It has the properties of wet toilet paper, the only way to achieve true rigidity is to completely coat (dope) the inside surface with for example polyester resin and glass fiber mats. Even then the glassfiber enclosure with and without the mdf on the outside is practically identical in rigidity (might be 25% MPa more tensile strength with the Mdf sheet on top of the 100+ MPa tensile strength of the glassfiber composite itself). Mdf is used for practicality and price, not db output nor sound quality. Every time you increase the rigidity of the enclosure you increase db output and lessen noise, as well as make the difference smaller between the simulated enclosure and the real thing because the simulation software assumes infinite rigidity. In a perfect world we'd use glassfiber composite sheets. But the rarity of the use of such material in this way means its expensive to find pre-made sheets of that material.

Tensile strength is different than flexural strength. The braces address the flexural strength problem.
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post #32 of 337 Old 02-05-2016, 12:34 AM
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Quote:
Originally Posted by Augerhandle View Post
Tensile strength is different than flexural strength. The braces address the flexural strength problem.
Sorry for incorrect term usage. But to put it in a more obvious light. Imagine if you make a very nicely braced cardboard subwoofer enclosure. Then judge how much bracing will do. If you take a brush and dope the inside surface with polyester resin, you would do more for the strength of the box than tons of bracing ("tons" being a relative term signifying an amount of bracing). If you in addition to the coat, do some 3-dimensional reinforcement with resin (like using strips of cardboard or even just plastic straws to paint resin over them to get a 3D structure of layers of resin), then you would do even more than massive amounts of bracing. Even the contact patch of the bracing to the mdf, has lots of "give" in it. You can only do so much with mdf itself without "doping" it with resin.
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post #33 of 337 Old 02-05-2016, 05:21 AM
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Quote:
Originally Posted by ronny31 View Post
Sorry for incorrect term usage. But to put it in a more obvious light. Imagine if you make a very nicely braced cardboard subwoofer enclosure. Then judge how much bracing will do. If you take a brush and dope the inside surface with polyester resin, you would do more for the strength of the box than tons of bracing ("tons" being a relative term signifying an amount of bracing). If you in addition to the coat, do some 3-dimensional reinforcement with resin (like using strips of cardboard or even just plastic straws to paint resin over them to get a 3D structure of layers of resin), then you would do even more than massive amounts of bracing. Even the contact patch of the bracing to the mdf, has lots of "give" in it. You can only do so much with mdf itself without "doping" it with resin.
It's this why you see so many car audio boxes that look like they have no bracing? Are they "doping" the boxes instead?
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post #34 of 337 Old 02-05-2016, 06:16 AM
 
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Originally Posted by Augerhandle View Post
Tensile strength is different than flexural strength. The braces address the flexural strength problem.
+1.

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It's this why you see so many car audio boxes that look like they have no bracing
If it's a commercial box they have no bracing because that keeps the cost down, and therefore the profit margin up. If it's a DIY it's because the builder didn't know any better.
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post #35 of 337 Old 02-05-2016, 06:31 AM
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Quote:
Originally Posted by ronny31 View Post
I find it funny every time someone tries to brace mdf with mdf, as well as trying to brace mdf with other materials.
It has the properties of wet toilet paper, the only way to achieve true rigidity is to completely coat (dope) the inside surface with for example polyester resin and glass fiber mats. Even then the glassfiber enclosure with and without the mdf on the outside is practically identical in rigidity (might be 25% MPa more tensile strength with the Mdf sheet on top of the 100+ MPa tensile strength of the glassfiber composite itself). Mdf is used for practicality and price, not db output nor sound quality. Every time you increase the rigidity of the enclosure you increase db output and lessen noise, as well as make the difference smaller between the simulated enclosure and the real thing because the simulation software assumes infinite rigidity. In a perfect world we'd use glassfiber composite sheets. But the rarity of the use of such material in this way means its expensive to find pre-made sheets of that material.
Cheap MDF is crap and the big box stores often sell it, but suppliers of plywood and other materials for cabinetry is very different. Plum Creek is one brand that's hard, stiff and doesn't absorb moisture the way the garbage from Menard's and Home Depot does. I don't know what they use for glue, but it doesn't work. If the MDF is darker and flexible, it should be avoided. MDF works fine if it's decent quality.
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post #36 of 337 Old 02-05-2016, 06:39 AM
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Quote:
Originally Posted by ronny31 View Post
Sorry for incorrect term usage. But to put it in a more obvious light. Imagine if you make a very nicely braced cardboard subwoofer enclosure. Then judge how much bracing will do. If you take a brush and dope the inside surface with polyester resin, you would do more for the strength of the box than tons of bracing ("tons" being a relative term signifying an amount of bracing). If you in addition to the coat, do some 3-dimensional reinforcement with resin (like using strips of cardboard or even just plastic straws to paint resin over them to get a 3D structure of layers of resin), then you would do even more than massive amounts of bracing. Even the contact patch of the bracing to the mdf, has lots of "give" in it. You can only do so much with mdf itself without "doping" it with resin.
A piece of MDF that's 3/4" x 3/4" x the distance between the panels will provide far more resistance to the forces than it will be subjected to unless some extreme conditions are met. A well-braced MDF speaker cabinet that's dropped on a concrete floor will bounce, receiving little damage. As long as the brace can't pull away from the surface it's attached to, it will be fine.

I agree that resin helps, but plain MDF is a good material for this application unless it's in a high humidity environment.
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post #37 of 337 Old 02-05-2016, 06:42 AM
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Originally Posted by corradizo View Post
It's this why you see so many car audio boxes that look like they have no bracing? Are they "doping" the boxes instead?
Those don't have bracing because some bean counter said that it's not in the budget. Also, a lot of those boxes are made of particle board, which is "the hot dog of building materials" because it has wood chips, saw dust and whatever they can sweep up from the floor at the end of the shift. Nails, staples, screws and other hardware? No problem!
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post #38 of 337 Old 02-05-2016, 07:03 AM
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For a while I joined a couple of Facebook groups for car audio. One was car audio wall's (cuz that's crazy so why not join?) and the other was the Sundown only group (so much has changed since 92'!).
In those groups, it was rare to see any bracing, usually thick box walls (seriously, don't quote those last three words, perv), but very little bracing. When I did see bracing it was almost always a single metal threaded rod connecting the two broadest surfaces. Still can't figure out how these people compete with no bracing or very little.
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post #39 of 337 Old 02-05-2016, 08:17 AM
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I would assume that in car audio, the only thing that matters is max spl. Sound quality is a term that is not even used or considered. Just get the bass as loud and obnoxious as possible. Stock speakers + 140 dB of bass rattling the car apart is just right. Who cares if the cab vibrates...the whole car is rattling anyways.
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post #40 of 337 Old 02-05-2016, 08:32 AM
 
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Quote:
Originally Posted by corradizo View Post
For a while I joined a couple of Facebook groups for car audio...
In those groups, it was rare to see any bracing, usually thick box walls (seriously, don't quote those last three words, perv), but very little bracing.
It comes down to the level of expertise. Facebook car audio groups tend to be populated by younger members who don't have much in the way of engineering education and/or skills.
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post #41 of 337 Old 02-05-2016, 09:32 AM - Thread Starter
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Thinking about all of this I have a few remaining questions,

1) I have read that it is not a good idea to have any of your three outside dimensions of your cabinet being equal. I am not sure this is really a problem if bracing actually breaks up the size of the panel. In addition, a typical square box using butt joints will have two panels of lesser width anyway. Thus it is really the size of the panels with determine the resonance not the actually outside dimension of the box? Am I thinking correctly here?

2) Should braces be staggered at odd intervals inside the box? Are standing waves really an issue in a small box?

3) If bracing moves the resonance outside of the subwoofers passband, that seems good, but will that present a problem higher up with what the mains are producing?
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post #42 of 337 Old 02-05-2016, 09:46 AM
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the quideline about sides not being equal is not just about panel resonance; its also about internal standing waves. If you stuff the box, you'll absorb internal reflections and won't have to worry about standing waves. Then you are free to have equal length sides.
You don't need to stagger brace locations. Instead use enough braces to divide up the panels so the resonances are raised above the passband. (easy for a subwoofer, not so for an L/C/R)
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post #43 of 337 Old 02-05-2016, 09:57 AM
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the quideline about sides not being equal is not about panel resonance; its also about internal standing waves. If you stuff the box, you'll absorb internal reflections and won't have to worry about standing waves. Then you are free to have equal length sides.
You don't need to stagger brace locations. Instead use enough braces to divide up the panels so the resonances are raised above the passband. (easy for a subwoofer, not so for an L/C/R)
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post #44 of 337 Old 02-05-2016, 10:01 AM
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I never used dowels mainly as i wasn't sure what the best way of attaching it to the cabinet. A simple window brace can easily be held together with clamps, but how are folks attaching dowels as braces? Do you show horn it after the opposing sides are glued in place? Or do you attach one end at a time?
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post #45 of 337 Old 02-05-2016, 10:18 AM
 
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Quote:
Originally Posted by JackNC View Post
use enough braces to divide up the panels so the resonances are raised above the passband. (easy for a subwoofer, not so for an L/C/R)
Not particularly necessary for an L/C/R either. If the cab walls are sufficiently stiff they're not going to vibrate enough to cause a problem, no matter what their resonant frequency. If they're not sufficiently stiff they will vibrate enough to cause a problem, also no matter what their resonant frequency.
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I never used dowels mainly as i wasn't sure what the best way of attaching it to the cabinet.
Read Post #6.
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post #46 of 337 Old 02-05-2016, 10:19 AM
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Originally Posted by Bill Fitzmaurice View Post
It comes down to the level of expertise. Facebook car audio groups tend to be populated by younger members who don't have much in the way of engineering education and/or skills.
Bill, can you weigh in on coating mdf with polyester resin?
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post #47 of 337 Old 02-05-2016, 10:35 AM - Thread Starter
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Originally Posted by JackNC View Post
the quideline about sides not being equal is not about panel resonance; its also about internal standing waves. If you stuff the box, you'll absorb internal reflections and won't have to worry about standing waves. Then you are free to have equal length sides.
You don't need to stagger brace locations. Instead use enough braces to divide up the panels so the resonances are raised above the passband. (easy for a subwoofer, not so for an L/C/R)
I will stuff but can a standing wave even form in the subs passband given the small size of the cabinet? Thanks
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post #48 of 337 Old 02-05-2016, 12:01 PM
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My understanding is that the subs passband is not a brick wall, if you cross at 80hz, you will get sound above 80hz that rolls off at the rate of 12db or 24db per octave typically. So 160hz will be 12db quieter... But still there. That's why you dampen the enclosure..methinks.
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post #49 of 337 Old 02-05-2016, 01:33 PM
 
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Bill, can you weigh in on coating mdf with polyester resin?
I don't see the point of it. It certainly won't make the cab panels any stiffer. I didn't directly comment on the posts you're referring to, but this was what came to mind:

Quote:
My understanding is that the subs passband is not a brick wall, if you cross at 80hz, you will get sound above 80hz that rolls off at the rate of 12db or 24db per octave typically. So 160hz will be 12db quieter... But still there. That's why you dampen the enclosure..methinks.
There's that, but there's also the harmonic distortion created by the driver. Those above bandwidth harmonics radiate both directly into the room and backward into the enclosure. If you don't damp them, as well as above bandwidth content that exists because the low pass filtering isn't brickwall, they will reflect back to the cone and cause higher audible levels above the desired pass band.
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I will stuff
Stuffing is used to lower Q, not to damp internal reflections. How much stuffing to use, or whether it should be stuffed at all, would be determined in the design stage.
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post #50 of 337 Old 02-05-2016, 01:58 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
Those above bandwidth harmonics radiate both directly into the room and backward into the enclosure. If you don't damp them, as well as above bandwidth content that exists because the low pass filtering isn't brickwall, they will reflect back to the cone and cause higher audible levels above the desired pass band.
Stuffing is used to lower Q, not to damp internal reflections. How much stuffing to use, or whether it should be stuffed at all, would be determined in the design stage.
I thought I read stuffing would absorb some of the reflections. I am thinking about being in a hollow room where I hear lots and echo when I speak but fill the room with furniture and the room becomes quiet?

So the best way to damp the internal reflections is to stiffen the cabinet? Thanks
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post #51 of 337 Old 02-05-2016, 03:41 PM
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So the best way to damp the internal reflections is to stiffen the cabinet? Thanks
Here's my preferred method to stiffen the cabinet.

Grey is the wood material, black is polyester/epoxy resin, orange is sticks. First the grey surface is coated with resin, then the sticks are applied, then ideally I also use some chopped mats glassfiber and add it on top of the whole thing over the sticks and between them when I apply the resin on top of the sticks. The sticks go the long axis of the surface they reinforce. For db drag I would use a lot of glassfiber and the distance between the sticks would be smaller than a home-stereo speaker. Home speakers may do just fine with only one layer of glassfiber, and on the smallest surfaces no glassfiber at all.
This works even on LCR speakers, because you don't need tons of stuffing to avoid high frequency sounds coming out of the ports, just a 1 inch or 5/6 inch layer of stuffing on all the internal surfaces.
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post #52 of 337 Old 02-05-2016, 04:14 PM
 
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I thought I read stuffing would absorb some of the reflections. I am thinking about being in a hollow room where I hear lots and echo when I speak but fill the room with furniture and the room becomes quiet?
So the best way to damp the internal reflections is to stiffen the cabinet?
You damp internal reflections with absorbent material. Damping reflections might take an inch or two of material, which is totally different than stuffing the cab. Of course stuffing will also damp reflections, but if you stuff when you shouldn't it could make things worse rather than better.

Ronny31, your method is better than nothing, but that's about the best I can say about it. Its effectiveness pales compared to panel to panel bracing.
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post #53 of 337 Old 02-05-2016, 05:41 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
You damp internal reflections with absorbent material. Damping reflections might take an inch or two of material, which is totally different than stuffing the cab. Of course stuffing will also damp reflections, but if you stuff when you shouldn't it could make things worse rather than better.
So do you generally recommend Damping with and inch or so of material regardless whether additional stuffing is in the design? Bob
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post #54 of 337 Old 02-05-2016, 08:32 PM
 
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So do you generally recommend Damping with and inch or so of material regardless whether additional stuffing is in the design?
You don't need to use both.
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post #55 of 337 Old 02-05-2016, 11:14 PM
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Originally Posted by Bill Fitzmaurice View Post
Its effectiveness pales compared to panel to panel bracing.
A piece of string running from one panel to the opposite panel, isn't effective. That's where mdf ranks in strength. It just looks effective, it looks specious; apparently strong or plausibly strong. A panel to panel brace with panels of steel braced with steel, that would be strong. But mdf panels braced with anything, isn't. The resin and fiberglass reinforcement is actually a misuse of the term, what really happens is that you build a composite enclosure that happens to have some mdf around it for no reason other than to work as a form for the fiberglass. mdf is like building with moist sand, only you don't get to treat it so as to make it glass (or fiberglass).
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post #56 of 337 Old 02-06-2016, 05:06 AM
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I'd like to see the measured difference in sound quality that proves your position about polyester resin.
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post #57 of 337 Old 02-06-2016, 06:44 AM
 
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I'd like to see the measured difference in sound quality that proves your position about polyester resin.
I'd like to see MDF that's 'like moist sand'.
By his description it's the equivalent of rigid foam sheet. I really have to question if he knows what MDF is.
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post #58 of 337 Old 02-06-2016, 09:16 AM
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Bracing again

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Originally Posted by ronny31 View Post
you build a composite enclosure that happens to have some mdf around it for no reason other than to work as a form for the fiberglass. mdf is like building with moist sand, only you don't get to treat it so as to make it glass (or fiberglass).

If you really believe that, then you should build your speaker cabinets from styrofoam and fiberglass. No reason to waste money on MDF.

For myself, on the other hand, the DO sealed sub enclosure I made from 1/2" MDF braced with tictactoe strips of 1/2" MDF has proven itself absolutely excellent, and demonstrates no panel flex or vibration with a pair of UM18s and all the power an NU6000 can throw at it.
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post #59 of 337 Old 02-06-2016, 09:53 AM
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Quote:
Originally Posted by ronny31 View Post
I find it funny every time someone tries to brace mdf with mdf, as well as trying to brace mdf with other materials.
It has the properties of wet toilet paper,

The MDF in your country must be pretty bad, even worse than what Home Depot or Lowes sell around here.

But the newer CARB 2 compliant refined MDF is much better that anything you're talking about. The dados and rabbet joints on the stuff I use cut so smooth and sharp that the edges can actually slice your skin open. There are no fuzzy edges at all.

If you add resin to it, you're just making 3/4" into 1" because MDF is already put together with resin.

There was an old thread here on AVS from years ago with engineers giving structural data, and cross bracing outweighs thicker material by a significant margin.


I do agree with everything Bill said, but I would debate that a full window brace with built in cross braces will be more effective that just a dowel or stick cross brace. If you put a dowel in the middle of a 16" x 16" panel, it does break that panel at 8" in the middle. But there are still spans of 16" x 8" sections going front to back and top to bottom. A full window brace cuts that in half from front to back. So there is some benefit and I would say that the best brace would actually be a mix of both photos he posted earlier.
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post #60 of 337 Old 02-06-2016, 10:03 AM
 
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Originally Posted by Erich H View Post
I do agree with everything Bill said, but I would debate that a full window brace with built in cross braces will be more effective that just a dowel or stick cross brace.
I wouldn't debate that. I never said I use window braces, I just pointed out the wrong way and the right way to make them if you choose to do so.
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