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 Post subject: MDF vs Plywood
PostPosted: October 23rd, 2016, 9:13 pm 
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Joined: October 10th, 2016, 10:23 pm
Posts: 55
All,
After hearing what I consider to be wonderful bass coming out of a Klipschorn, I've decided to include one, or something similar, when I finish my man-cave. I'm considering the option of building my own, as there is no shortage of plans out there on the old interwebs. If it didn't shut down in my 4-year absence, the Walter Reed annex has a fantastic wood shop. Paul Klipsch made his from plywood, but I've seen someone building a pair from MDF. What are people's learned opinions on using MDF on such a large, oddly "braced" project? It's been my experience that while MDF is strong and inert in compression, it is quite weak when a bending torque is applied.
V/R
James

p.s. if you have any personal experience with bass horn projects that have similar performance as the K-Horn, but offer easier construction, please let me know. There's a good deal of info on the Klipsch boards.


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 Post subject: Re: MDF vs Plywood
PostPosted: October 23rd, 2016, 9:24 pm 
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Posts: 1682
I have the actual pair of Edgarhorns that were in one of the issues of Sound Practices I could let go for a song....

Roscoe

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 Post subject: Re: MDF vs Plywood
PostPosted: October 23rd, 2016, 9:45 pm 
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Joined: October 10th, 2016, 10:23 pm
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Roscoe, what can you tell me about them, performance-wise. Do you have pics? I have a pair of Klipsch LaScallas, so I am looking for lower bass than they can produce.
V/R
James


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 Post subject: Re: MDF vs Plywood
PostPosted: October 23rd, 2016, 10:34 pm 
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MDF has several major strengths and weaknesses (gee what a surprise).

Strengths are:
1. Very strong under both tension and compression.
2. High density to provide more mass. This will not reduce energy impinging on the MDF but will help lower the resonant frequency of a particular piece.
3. With the right tools it produces very clean cuts with no voids at the ends.
4. It is very amenable to construction using biscuits. With the proper applications of biscuits and glue, the material itself will break before a joint.
5. The surface is very smooth and relatively non-porous making it easier to get good finishes.

Weaknesses are:
1. Fair bending strength.
2. It is easy to damage through careless handling.
3. It is susceptible to water so a solvent based primer must be used to seal it.
4. The final precision of the joints when using biscuits is dependent on the precision of the plate joiner used to cut the biscuit slots. The good part is if the slots are precise the joints will be next to perfect. If the slots are not properly aligned there is absolutely no wiggle room to get the pieces into proper alignment.
5. It is very abrasive to work with and will shorten the life of substandard blades and bits.

Regardless of the material used you should not rely on torsional strength to prevent resonances. The only reliable way to eliminate resonances is to use braces that will break up the panel sizes to values whose natural resonance will be outside of the frequencies being reproduced. Density, stiffness, and damping material all have very small effects compared to sizing the unsupported pieces so that they will not resonate at the frequencies involved.

Tom


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 Post subject: Re: MDF vs Plywood
PostPosted: October 23rd, 2016, 10:49 pm 
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Joined: July 8th, 2016, 4:34 pm
Posts: 570
If you use MDF seal it with shellac. Shellac will seal anything. I think the K-horns are made from baltic birch plywood and the panels are cut out (compound cuts required) using jigs on very large radial arm saws.


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 Post subject: Re: MDF vs Plywood
PostPosted: October 24th, 2016, 9:05 am 
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I use Zinsser BIN which is a shellac based primer that you can get anywhere.

Tom


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 Post subject: Re: MDF vs Plywood
PostPosted: October 25th, 2016, 9:07 am 
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Joined: July 3rd, 2014, 7:47 am
Posts: 89
There are a number of methods that can be used to secure MDF cabinets when constructing. One way is a miter joint blade, which is a very strong joint with great surface area for glue up. This joint takes a lot of the fitment issues out of the equation by design. IT is also a joint with a perfect seam that is actually where you would want one. Setup and proper tools to make this joint are vital.

Sealing joints can be difficult but there are a number of methods used to eliminate the issue. With a miter joint, there isn't really an issue, but with butt joints like rabbet edges, the valley method works best. Using a router you essentially trench the joint about halfway through the material and halfway between the joint and the edge, along with the same distance on the opposite side of the joint. Then fill with a good stable body filler, sand and seal. This method ensures no telgraphing through finishes or veneers. This has been used in long term testing, and I have yet to see one done properly telegraph through the finish.

Dust is one of the largest downsides to MDF. Even without formaldehyde, MDF is still a chemically concentrated product, and respirators are not merely suggested but necessary when working with it. Proper dust collection is a must. Also a downside is due to its construction, it literally can peel easily. This is a plus when recessing drivers, but for joints can result in an ugly joint pretty quick. Again, proper tooling, and very sharp tools are a must.

When it comes to subwoofers, with the amount of pressure and air movement, I have always been a big supporter of both glue and mechanical fasteners. There are a number of fasteners that work incredibly well in MDF (powerhead screws are my favorite) and when properly installed and countersunk, as well as filled properly, the cabinets are truly indestructible. Dowels make for great braces in subwoofers, and there are a few designs that are pretty ingenious when it comes to proper bracing. If you look up the stonehenge subwoofer enclosure on diysoungroup.com you will see it. Pretty amazing design and application.

I personally prefer working with plywood most times, and find that when properly dampened it works better for the types of enclosures I make. If I was doing things that required complex contours and smooth transitions, I would definitely use MDF. It sands better and is a better product for that type of application.


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 Post subject: Re: MDF vs Plywood
PostPosted: October 25th, 2016, 9:53 am 
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Joined: July 8th, 2016, 4:34 pm
Posts: 570
Here are good screws for MDF -

https://www.amazon.com/HighPoint-Cabine ... B00YP4ZYTE

I also prefer plywood (baltic birch) for construction and I would rather finish plywood than MDF. However, the Parts Express kits I have are made of MDF. I was told at the Woodworkers Club that General Finishes milk paint (synthetic milk paint) works very well on MDF -

https://generalfinishes.com/retail-prod ... A9jxbXaZQs

even though it is water based (they also make sealers).


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 Post subject: Re: MDF vs Plywood
PostPosted: October 25th, 2016, 10:49 am 
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Screws also work well at providing the structural strength needed when working with MDF. I do not recommend unsupported butt or mitered joints with MDF because as Jeff said the surface can peel easily. Without screws or biscuits only the surface of the MDF is in contact. With screws or biscuits the supporting material goes down into the center of the MDF which is a much stronger method.

I have tried screws and they suffer from the same problem as biscuits and in addition have the requirement to fill the holes after the screws are used. Those fills are just another thing to sand. The pilot holes provide the same locating properties as biscuits but have the same drawbacks. If they are not perfectly aligned when you draw the screws down tight there is no way to correct for a misalignment. With biscuits, since they fit into a slot you have no movement capability at all across the slots but some very limited movement in the direction of the slots. The other advantage of using a plate joiner is that the ability to precisely control the position of the slots is far greater than with a drill. There are both coarse and fine adjustments built into the plate joiner to precisely position the locating fence on the work. Both methods work and are better than unsupported joints. All of my speaker construction including large sealed box subs have been made with biscuits and I have never had a mechanical or containment problem.

If anyone would like to try biscuits I have a Porter Cable plate joiner they can borrow. I have also fashioned a hose connection from it's exhaust port to a standard shop vac hose. As Jeff said the dust is a problem and all of my tools have vacuum connections to my shop vac. That even extends to my sander. I'm enclosing a photo of two MDF pieces that have had slots cut into them and are being glued while being positioned with corner clamps and a PDF file of some of the adjustments of the plate joiner.

Tom


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 Post subject: Re: MDF vs Plywood
PostPosted: October 25th, 2016, 12:00 pm 
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Joined: July 8th, 2016, 4:34 pm
Posts: 570
What I did with the Parts Express 18 inch subwoofer kit was to use glue and Confirmat screws for all the joints and then cover five sides of the enclosure with flat black Formica (cover all the screw holes). The front was covered with speaker bezel finished with black shellac.


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