February 4, 2012

Bass horns in progress

All sheets now cut for two bass horns - F20 (lilmike's design on AVS) and T20 (dual Rythmik tapped horn). 



What you see is low cost Chinese form ply purchased for the same price as MDF. Not recommended as a rule but my T20 will be installed out of sight - it simply needs to be light and strong. It is quite a bit lighter than MDF and needs less bracing. 


  1. Bunnings for the form ply? Mine seems to have real cheap MDF, but expensive everything else. Any difference between form ply and marine ply?

    On glue, I have read that PVA is already stronger than timber or MDF material, so perhaps there is no advantage other than able to fill voids better for the new cool in glue? I bought a fresh sample of both and might make a quick test to see if indeed there is much difference in a butt join on MDF.

    Great to see more projects in the build!

  2. Adam,
    Nope! This is cheap cheesy Chinese form ply at $35/sheet! I've used formply from Mitre 10 for my bench top, cut in half to give me 4.8m long. Form ply at Bunnings is actually a bit better than this cheap stuff we're using here. It's light and it has some voids.

    Marine ply is completely different. Even cheap Marine ply is better quality and has less voids. It's also quite a bit heavier. This Marine ply was also cheap and it shows in the voids. The real stuff has no voids at all, but you can easily spend $300 - 500 for the really good stuff. This cheap Marine ply in the other bass bins was only $85 per sheet. Bunnings has slightly better for about $120. Also, with the formply, you have the phenol impregnated paper coating that doesn't like paint.

    Glue wise, I use regular PVA, construction adhesive and Polyurethane glue, sometimes all in the one project. They all have their unique uses. By default, I use PVA as it is cheap and easily strong enough. After about 40 minutes I don't have to keep clamping it and for joining timber it's fine. It fails where you are joining formply due to the paper surface. It also doesn't help when you have gaps. That is where the other two come in handy. PU will expand to fill gaps and voids and it is 4x stronger. For butt joins or feet that is nice to have.

    So if building a horn out of formply, construction adhesive is the go. You don't want PVA over the paper and you need to be sure it all seals up. You don't want PU there since ideally it prefers two porous surfaces.

    For building a horn out of marine ply, PVA is fine if you have it all neat and are sure it will all seal perfectly. PU is a safer bet there to ensure it does actually seal, so it is a better although more expensive choice. I still prefer the construction adhesive. I can get a cylinder for only $2, it's quick and seals things up nicely. You can do a big horn with just one of them.

    So all three of those glues are good to have on hand at any time when doing projects like these. I'm also tending to seal up butt joins with PU prior to painting and then butter them up with bog. If you do that the usual crack becomes a very faint line you can see if you look carefully up close. By contrast, no attention at all means a crack you can spot right away.


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