July 17, 2012

Surround speakers

Box built. B&C 8PS21 8" woofer + Dayton round 10" waveguide + DE250.

This surround will sit as shown here as there is just one row of seating, this is why the horizontal mounting. There will be a slight vertical tilt down and the off axis response will cause seats near the surround to still image well.

Bitumin rubber box damping has been applied to the box, so it is quite dead. This box is my trial speaker for a finishing method I have in mind, aimed at eliminating butt join seams which always appear when MDF is painted. I'm using layers of fibreglass reinforced auto filler and fibreglass resin. The result is a very hard and durable skin applied to the MDF. It makes painting much more labour intensive!


  1. I have also had great success getting joins right by using epoxy resin. It's expensive and hard to work with, but gets good results.

  2. Where do you get the bitumin rubber compound from (and what is the product called?) cheers.

  3. Dave, are you saying you have painted over MDF butt joins with epoxy glue on the join and not had a seam appear at all?

    anomalous > Bunnings $124 for a big tub

  4. Yes. Although I glued my panels together using epoxy, so no actual painting over per se.

    Works better on 'HDF' rather than MDF.

    How long do you normally need to wait for your seam to 'appear'?

  5. They can appear from the next day, or maybe a week. I was talking with a sales guy at Super Cheap Auto who said he found the seam can appear 3 years later, that the only long term solution he found was coating the box in fibreglass resin. I have a sample done that way, but I did it in a bit of a rush. I still have a seam!

    So you didn't actually paint over your seam? That is when the problem starts.

    HDF - if you know of a source for 18mm thick HDF let me know!

  6. Paul and Dave,
    I bow to your electronics expertise and have little to contribute but as a woodworker I can offer some advice. I haven't actually tried this myself but many woodworking forums suggect filling end grain MDF with simple fine grain spack filler used for plaster wall repairs. If a glued and screwed butt joint is tight and stable and the end grain filled and sanded smooth then once painted over there is no reason why a joint line should become visible. An upmarket simliarly themed alternative is to use boat building eppoxy thickened with a "fairing" compound such a for making wooden boat hulls smooth. Cheapest source I know for epoxy is FGI in Centre Rd, Clayton. Here is another woodworker's trick, don't hide it, make it a feature. If you rout a tiny rabbet right on the edge, fill the end grain, once painted you create a "shadow line" as a design feature.

  7. This is my 12th attempt! I have not yet tried epoxy glue, although I have tried polyurethane glue, which seems to improve things just slightly, as well as both MDF and ply.

    With my prototype for HE2 I'm using clear finish over ply and butt joins have a groove and black paint. I built a sub with a V groove over the butt joins recently, but I'm not so keen on that solution.

    Are you suggesting this epoxy is applied as a coating?

    1. The epoxy both penetrates and coats. Using it to hide a joint line in MDF, I would do this. Paint unthickened epoxy on the end grain and joint line and about 50 mm of the box sides (so it can be feathered / blended to the sides of the box when sanded). The end grain will suck it up. Continue to apply it at intevals of 5 minutes or so, until it wont suck up any more (it will stay shiny on the surface which is the point it is "wetted out"). Once the substrate is wetted out, penetration stops and surface coating commences.

      For a clear finish, wait 24 hrs, sand it smooth and apply 2nd coat and repeat as many times until you are happy with the surface. Varnish over the top.

      In an extreme version, you can add woven glass fabric and with this I can assure you the joint line will never become visible. The glass weave becomes invisible when wetted. This is a method for water-proofing boat hulls and getting a "bright" finish on decks etc. If you want to try this, further informations is available here: http://www.westsystem.com.

      For a painted finish, after the end grain and joint line is fully wetted out, add some fine fairing compound (powder or wood flour) and mix / thicken to the consistency of peanut butter. Coat the joints, then sand smooth when dry. It will take some effort to sand, epoxy is hard. You may need to add a second coat if any voids or blemishes appear when sanded. This should do it.

  8. Gordon,

    I think I'll have to award this comment as the most helpful ever on my blog!

    I did a little reading and it turns out that epoxy is stronger than polyester although more expensive. Having bought a big container of polyester fibreglass resin, I'm now wishing I had bought epoxy and it seems I could use it as a glue and as a coating.

  9. Paul, just a final comment or two on glue ups and epoxy. Fine Woodworking magazine did a comprehensive analysis on the various woodworking glues a couple of years back. The results were surprising in that Titebond II and other yellow PVA woodworking glues when properly applied are almost as strong as epoxy (but a hell of a lot cheaper and much easier to use). Using epoxy for glue ups can be tricky and not necessary for interior joinery unless you want the speaker box to double as a lifeboat! It's easy to "starve" a joint if not properly wetted out and this makes a very weak joint. Unless you have a complicated glue up and you need a longer open time, I'd stick with Titebond II for gluing up speaker boxes and save the epoxy for use as a surface coating and thickened to "bog" up dings and seal end grain. Re costs, the product known as "Araldite" is simply epoxy and buying it like this is extremely expensive. West System and System 3 Epoxy are the two leading brands in boatbuilding but you pay for the name. FGI in Centre Rd Springvale make eposxy to exactly the same specs as West System. One litre of epoxy and 250 ml of (standard / slow / fast) hardner is about $35. However the best way to buy it is 5L of Epoxy and 1L of hardner for around $130 - lasts for years!

  10. Thanks again, Gordon, that's very helpful.


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