Replies

1. 
   colebearanimals | May 18, 2007 07:48am | #1

   Hi davidpg,

            I built a pair of speaker enclosures about 35 years ago using particle board that was then veneered. My friend still has them. No visible problems with the interior untreated. You shouldn't have any problems.

                                     Paul

   ps   you could, however, coat the interior with a couple applications of shellac if you'd like. It won't hurt anything

2. 
   Kevin | May 18, 2007 09:25am | #2

   I think Paul's right. MDF is quite a bit more stable than particle board too. It can cup slightly under certain conditions. But it's really much more stable than most people seem to realize. I know from having to coat close to 100 full sheets of MDF with a water-based fire retardant that it doesn't absorb moisture anything close to as readily as wood does or even plywood for that matter. I don't know what the glue resin percentage is in MDF but it's ungodly high and the wood particle are really tiny.

   If you wanted to be safe then you could coat shellac or something like that on the interior side and that would act to seal off the inside long enough for you to assemble the boxes. But I doubt it'd be necessary.

3. 
   Knotscott | May 18, 2007 12:43pm | #3

   Hi David - I've never heard that veneering both surfaces of MDF was necessary for stability, though it's good practice to seal both sides of a hardwood surface if one side is treated.  I have heard of treating the inside to help deaden the box acoustically...borosilicate compound was one substance I've used.

   My suggestions are outside the scope of your question but they're free and worth every penny!

   - the front baffle of your enclosure is fairly wide...I assume it's to accomodate a large woofer.  Smaller is better to reduce reflections... it's possible to make the baffle "appear" acoustically smaller by padding the area around the mid/tweeter area.  You can also do some more complex reshaping of the baffle to reduce it but padding is pretty effective and is alot easier.

   - The spacing between the drivers is critical.  The centers of the drivers should be no farther apart than the wavelength of the crossover frequency.  This is a common error in many speaker designs, and it's audible if given the chance to hear the difference directly.  In simple terms, just put them as close together as physically possible. 

   There are a ton of tricks that I'm sure you're aware of.  Some of which are marginally effective and possibly controversial.  My experience has been that if you do several of these barely noticeable tricks, overall it sums up to a pretty signicant performance difference.  Ie:  high grade wire, good capacitors, low inductance resistors, bi-wiring, spiking the speakers to the floor, etc.

   Good luck!  I hope you'll post some pics and comments of your experience.

   Scott

    

    

4. 
   tom77 | May 18, 2007 04:22pm | #4

   David,

   It is customary to veneer both sides to avoid cupping. The reason that cupping occurs is that the veneer absorbs moisture from the glue and initially expands before the glue tacks. The glue then tacks and cures. Afterwards, the veneer dries and tries to shrink back toward its original size, pulling in the process against the now cured glue line. This causes cupping toward the veneer side. It is possible that you could get by because of choice of glue, thickness of mdf, and choice of veneer. Do you feel lucky? I would recommend trying a sample on one piece and see what happens.

   I can't help you with the vacuum tool. No experience. If your panels are of reasonable size, you can clamp them using a system of cauls.

   Good luck, Tom.

5. 
   JohnWW | May 18, 2007 04:32pm | #5

   Technically, if flatness were critical, you should veneer both surfaces.  This is the reason why both the top and back of MDF cored router tables are always covered with laminate.

   For a relatively small box with lots of reinforcement you are probably safe to just veneer the outside.  Shellacing or otherwise treating the interior of the box isn't a substitute for the veneer but it can't do any harm.

   The tongues on your MDF panels will be extremely weak, they will add almost no strength to the joinery.

   John White, Shop Manager, Fine Woodworking Magazine 

6. 
   jptenberg | May 18, 2007 05:32pm | #6

   One of my colleagues builds speaker cabinets for a very high-end boutique speaker line--$40k and up. Rater than apply veneer before assembly, he builds the boxes first and applies the veneers after the assembly.  You cannot use a vaccuum bag when doing it this way, but unless you cave curved or multi faceted cabinets, cauls are usually plenty easy.  You also might try hammer veneering with an iron. 

   Using regular pva glue, you precoat the substrate and the veneer and the the glue "dry"  Then take an iron and basically iron the veneer onto the box. The heat from th iron will reactivate the glue and form a chemical bnd almost instantly.  Use a block of wood or a veneer hammer to apply high pressure to the veneer and smooth out any bubbles.  I know it sounds hokey, but it really works very well.

   Good luck

   Peter

   www.jpswoodworking.com

7. 
   SpeakerScott | May 18, 2007 09:30pm | #7

   Hello David!

   As you might be able to tell from my handle...I build and design speakers. ;-)

   1. It is not necessary to veneer both inside and out of a speaker built from MDF. With bracing adequate for a speaker enclosure and the the use of paper-backed veneers I've never seen an issue. Some of mine are going on 12 years old now. I've done some non-paperbacked veneers and those are fine too, but they aren't as old...only 4 years.

   2. Most small quantity manufacturers veneer the enclosure after assembly. Only the large shops veneer before, and V-fold or use solid corners. The large shops will veneer both front and back of a panel so that the wood doesn't warp before assembly.

   3. I used all of the following methods to veneer:

   a. Wood glue on both surfaces, wait to dry...iron on.
   b. Rubber cement.
   c. Water based cold press glues...

   And as a teaser...a picture of a pair of speakers I made.

   One other tip...if you veneer the end "grain" of the MDF it MUST be sealed and sanded very very carefully. The end grain will telegraph through even paper backed veneer EASILY. If you see or feel any imperfection in that area...you'll see it in the final product.

   Scott

   
   

   ---

   Edited 5/18/2007 2:33 pm ET by SpeakerScott

   
1. 
   Bruce8287 | May 20, 2007 07:57am | #12

   Beautiful speakers, Scott. 

1. 
   SpeakerScott | May 20, 2007 04:39pm | #14

   Thank you!

8. 
   cadiddlehopper | May 18, 2007 11:46pm | #8

   Humidity changes are what cause panels to warp after veneering. Inside your speaker box, that change will be very s-s-l-l-l-o-o-o-o-w-w. The thick MDF and bracing help, too. Veneering one side only is okay. Shellacking the inside is okay, too.

   However, rather than making complex joints, why not assemble the panels together using cleats? It can save work as well as add some stiffness. I make my panels by gluing solid wood to the exposed edges, applying veneer, trimming, then joining 4 sides at the corners with splined miters which do not exit the edges. Baffle and back are attached via cleats with gasket material between. The grill frames covered with polyester single knit are held in place magnetically.

   I built some speakers using only 1/2-inch MDF veneered on one side only several years ago. They are the same size as your plan. All sides are still flat.

   As another poster suggested, spacing is important. I would recommend having Madisound of Madison, WI design your crossover network for that reason. It has to do with phase of the drivers. Madisound does this design with a computer program at a cost. It is a worthwhile investment.

   Cadiddlehopper

9. 
   davidpg | May 19, 2007 07:33am | #9

   Thanks very much for all of the helpful comments. I was pleasantly surprised to see how many speaker builders there are on this forum. For those who are interested in the audio aspects of this project, the speakers I am building are based on a kit developed in the UK:

   http://wduk.worldomain.net/acatalog/SpkrKits.html

   It's a rather unusual and interesting design, and was designed by a real pro, Peter Comeau, who was the head designer for Mission for many years. The design provided with the kit is a pretty simple MDF box, but I decided to make it more of a woodworking project.

   I'm still not quite sure what to do about veneering the inside surfaces, but I will try a small panel with just a single side. For this project I found some very nice (and pretty pricey) quarter-sawn red oak veneer, and it seems almost criminal to hide an equal amount of it on the inside of a sealed box. Would it be reasonable to use cheaper plain-sawn oak veneer for the inside surfaces?

   With respect to the joints, I did realize that my tongues would not add much strength to the joints. I am using them primarily to keep everything straight while assembling. Is there a better way to do this? I don't have a biscuit jointer, but I'm not totally averse to buying a new tool . . .

   Thanks again for all of the interesting replies.

   David

1. 
   cadiddlehopper | May 19, 2007 06:37pm | #10

   There is really no need for internal veneer, but, if you must, almost any old veneer will do. I have a stock for which I paid between 4 and 15 cents/sq. ft.  Another way to avoid warps is the use of contact cement to apply veneer to assembled cabinets. Solvent-based contact cement contains no water and does not saturate MDF due to its rapid evaporation rate. I have a 40-year-old piece veneered this way. There is a learning curve to the process.

   BTW, my own speaker systems use SEAS drivers though they are 3-way systems. The speakers you are building probably resemble the famous Dynaco A25 with more powerful bass. I assume that you bought the crossover network from the manufacturer. That would be wise for such a sophisticated design.

   Cadiddlehopper

1. 
   davidpg | May 21, 2007 07:02am | #15

   Yes, these speakers are very closely modeled on the Dynaco A-25. World Designs offers two basic versions of the kit, one which is almost exactly the A-25 and is meant to be used on stands, and the tower version that I am building. In principle, these should have all of the virtues of the original A-25 and the benefits of more modern drivers and a more sophisticated (computer optimized) cross-over. I bought the kit which includes the drivers, pre-wired cross-overs and damping materials. I am keeping very close to the original dimensions and layout.David

1. 
   cadiddlehopper | May 21, 2007 11:04pm | #20

   I once participated in a comparitive hearing of several A25-sized speakers. The most remarkable thing I remember about the A25s is that we could understand words that were sung by a chorus far, far better than any other speaker. In fact, I can't remember ever hearing any speaker which performed that function as well. Maybe there is one somewhere though. I haven't heard them all.

   Cadiddlehopper

2. 
   SpeakerScott | May 20, 2007 04:37pm | #13

   David,The joinery I use 99% of the time is a simple combination of dado joints (internal braces) and butt joints for cabinet sides. I will often use cleats on long edges to be joined....Complex internal bracing can be achieved using these techniques. With the design you are looking at I would make sure that your baffle dimensions and driver spacing are exactly what is called out in the plans. The large diameter woofer/dome tweeter two way is a challenging crossover and often entails using the woofer at or above it's upper edge of usable frequency range. I've tried biscuits..but they don't work as well in MDF as hardwood IMO. ScottP.S. The hole cutting for internal braces is a scientific process...can't you tell? ;-)

   
1. 
   davidpg | May 21, 2007 07:14am | #17

   Scott,Thanks very much for your suggestions and pictures. Your speakers look very nice! As I mentioned in an earlier post, I am, as you suggest, keeping very close to the original design dimensions.David

1. 
   SpeakerScott | May 21, 2007 02:35pm | #18

   Whoops missed that part about staying close...sorry about that. I've been doing my own "fine" woodworking project this weekend..retrofitting 21, 2x12x20' into the ceiling of the garage. I'm sore, tired and wanting to go back to work so I don't have to work so hard!Scott

10. 
    JohnH | May 20, 2007 02:38am | #11

    It seems no one else responded to your question about the RoarRocket vacuum system, so I will. I got started in veneering with one. I have no commercial relationship with them. It works fine, but you must follow the instructions carefully. Even so, here are a few hints for success:

    1. Work hard to get the sealing tape into the corner of the bag where top and bottom meet. This is where most air leaks occur.

    2. I had a lot of trouble opening the bag after each veneering session. The sealant tape sticks like grim death; even a new-and-improved version. It takes powerful finger strength to peel the bag apart. And the sealant usually does not come loose smoothly, so it is hard to get a good seal on the next session if you reuse the sealant from the previous session. I ended up cutting the bag open and throwing away the sealed end after every session. I used up more sealant tape, but had no problem with leaks and failed veneer panels.

    3. Their customer service is superb. Even on Saturday, I got prompt, accurate, answers to my inquiry.

    4. Purists may say that without a vacuum gage, you don't know what the clamping pressure is. My experience is that it is not that critical; I have made at least 20 veneer panels. For example, in mechanical clamping of veneer panels, you don't know what the pressure is either.

    My advice is: If you think this veneer job is a one-shot deal, use the RoarRocket. If you are cash-challenged, use the RoarRocket. Once you are sure that you are going to do a lot of veneering, buy a mecanical vacuum pump system.

1. 
   davidpg | May 21, 2007 07:11am | #16

   John,

   Thanks very much for your comments on the Roarockit kit. I went ahead and bought one of these this weekend, and I have to confess that I haven't really come to terms with it. Just as you suggest, I had a terrible time getting a good seal, even after replacing the tape 3 times! And, I think that the problem is at the seam of the bag. I bought the kit from Woodcraft, which has a "satisfaction" guarantee, and I think I am going to return it. I'm sure that it can be made to work, but for flat pieces, it not so obviously worth the trouble.

   Instead, I decided to press the MDF and veneer between two pieces of melamine-coated particle board, with a lot of clamps, as described in a recent FWW article:

   http://www.taunton.com/finewoodworking/SkillsAndTechniques/SkillsAndTechniquesPDF.aspx?id=27820

   I tried this today on a relatively small piece, and it seems to have worked just fine. I'm going to leave it for a week or so to see if there is much curvature, since I only veneered one side.

   Thanks again for your comments.

   David

1. 
   Recounter | May 21, 2007 10:14pm | #19

   Re: RoarRocket ...

   My experience is similar.  It's darn cheap and I was actually able to do some veneering with it.  But that "black tar" sealer is... challenging.  The very first time I tried to open the bag aftger veneering I had to pull on it so hard that when it gave way I tore the seam at the edge of the bag several inches.  :^(   After that I never used that sealer again.  I just fold the end of the bag over a couple of times and then put boards on either side of the folded end and clamp the heck out of it.  That worked. 

   I also found that an old, unused kitchen suck-the-air-out-of-the-food-storage-bag thingie that was gathering dust in our pantry fit perfectly on the valve for the RoarRocket and was much easier to use than the manual pump!

11. 
    scooter1 | May 23, 2007 01:09am | #21

    Evertime I tried to veneer only one side of the MDF, the veneered side cupped in. I have to veneer both sides at once and put the material in a veneer press for a very long time to avoid cupping.

    Regards,

    Scooter

    "I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934

1. 
   SpeakerScott | May 23, 2007 03:15pm | #22

   Scooter,You were veneering a flat sheet before assembly into a project right?If so, then yes, both sides need to be veneered. A loudspeaker has (should have) enough internal braces to keep everything square and flat when veneered after assembly.Scott

1. 
   scooter1 | May 23, 2007 07:49pm | #23

   Yep.Regards, Scooter"I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934