Looking for finishing ideas for stereo speakers

[JonW]

I’m in the process of designing some stereo speakers. For acoustic reasons, they’re probably going to end up with an odd shape on the front. I’ll make the cabinets out of baltic birch plywood. Then cut out facets and sand down all the sharp edges. So lots of the plywood lines showing. It won’t look exactly like these (rather unattractive) Thiel speakers, but this pic will give you some idea of what I’m talking about:

I don’t think I’ll have any luck veneering something with such a complex 3D shape. And my understanding is I can’t use, say, 3/4” thick hard wood (e.g., cherry) for the outer layers because there will be expansion/contraction issues with the BB ply underneath.

So what else can I do for finishing? I could try a red dye followed by some polyurethane, but not sure how that would be in the end- you’l still see all the ply lines. There is the classic piano gloss black, although I’m not sure my finishing skills (ahem) are up to that. Could be interesting to try, though. Maybe some automotive paint of some sort and clear coat? Some other type of painting? Idunno. I’ve got the Rockler $100 HVLP sprayer, but have not yet had much luck trying to use it.

Just curious what you folks like or might suggest for finishes. Thanks.

-Jon

[JimD]

I use little 2 way speakers and a 12 inch subwoofer. The subwoofer is in the floor shooting out a fake heat vent so I did not have to finish it. The 2 ways are 1/2 BB with 1/4 inch walnut plywood laminated to it. I found a sheet of 1/4 oak with walnut as the backing veneer at Home Depot and used scraps of that sheet on these speakers. Anyway, the corners are solid walnut so I could hide the plywood laminations and so I could round over the edge of the boxes. I picture framed the front and back and then put the walnut on the short dimension of the sides and then glued the front and back on. It was not real hard and we like the final product but be real careful with the thin veneer. It's easy to sand through flushing the solid wood to the plywood. The solid wood edging pieces are the same cross section as the plywood thickness - was supposed to be 3/4 inch in my case but really more like 11/16 so the edging is 11/16 thick and wide.

If you do not want to go to this much work, I would let the edge of the BB show on the sides but not the front. It is not real unattractive to me and can be finished like solid wood but one quick look will tell you what the speaker is made of. Not entirely a bad thing.

Jim

[LCHIEN]

I think most people are using MDF nowadays for speakers beacuse it is denser and won't flex as much for the same amount of material. Stiffness is very important if you are looking for hi-fi performance.

If you do MDF, the the only thing you can reasonably do is veneer.

[Uncle Hook]

I belive most speaker manufactures use lacquer as the final top finish coat.

I am not sure how a clear finish looks on Baltic Birch plywood. You might experiment with dyes to bring out the grain.

I don't see why you could not use or laminate hardwood plywood to the BB plywood or to MFD. Generally sheet goods are designed to minimize wood movement.

[BT3 WOODOG]

You should use MDF,plywood resonates to much to use for speaker cabnets! Or You could go with a nice hardwood. Later

[Knottscott]

Ditto on the MDF suggestion, which pushes you more towards veneer for any cosmetic wood surfaces, which eliminates the issue of the plywood edge showing. I have seen some highend speakers that revealed a plywood edge but the ply was exotic woods and it looked nice the way it was done...they were the Wilson Watts if I remember correctly.

A cabinet with non parallel sides is a good idea, but it's also a good idea to stick with a shape you can pull off. Maybe a simpler slanted baffle would be easier. Then the finishing would be alot simpler too.

This comment goes beyond the scope of finishing. I know the Thiel's are a nice speaker...I don't know what the crossover frequency between the tweeter and midbass driver is, but I've always been a stickler for mounting the centers of those two drivers no farther apart than the wavelength of that crossover frequency or it can cause phase shift and really goof up the sound. It's possible that they've compensated for that distance in the crossover network somehow. I just wanted to suggest that you not attempt what they've down...in fact, put those two drivers as close together as physically possible...they'll act and sound more like one driver.

Nitrocellulose lacquer is a nice clear coat finish, but you may find poly is easier. Good luck and keep us posted on your progress!

[Tom Miller]

How about using MDF on the front, and shape it however you want. Then, use some kind of product like bondo to smooth out and seal the surface prior to painting it.

You could get the same look as your example by painting the front, but applying veneer to the sides.

Regards,
Tom

[cgallery]

Plywood has a higher resonant frequency than MDF. In all other regards, high-quality (void-free) plywood is a superior enclosure material. It is stronger, less flexible, holds screws better, etc., than MDF. For ultra-low frequency, ultra-high SPL enclosures, plywood is superior. Once you introduce any drivers operating above a few hundred cycles, MDF is preferred.

If you want to hide the plys of the plywood, you are probably limited to sealing and painting the baffle.

[JimD]

I think we've had the plywood versus MDF discussions before. I do not think many minds were changed, I know mine was not. MDF is denser but not stronger than plywood. Looking at the sagulator for a shelf application should convince you that plywood is much stronger. Density is a good thing for a speaker, however, and MDF is better there. I think making the enclosure air-tight is more important than what it's made of. I also use fiberglass or one of the newer materials on at least one of two parallel sides to avoid standing waves. If you do that, I do not think you need to worry about resonant frequencies at least as much. I have not tested my 2 ways to see if I can find any peaks in frequency response that would be indicative of a resonant issue. I have a frequency generator and a sound pressure meter but I have not used them in awhile. I used to spend more time on speakers but I gravatated to the position that if they sound good to me and look go to me, that is enough.

Jim

[JonW]

Hey Guys,

Thanks for all the thoughts!

JimD-
Interesting that you laminated “real” wood to the BB ply. I thought about doing that as well- it could be made to look really nice. But was told it’s a bad idea because the ply and real wood will expand/contract differently. And with changes in heat and humidity over the years, it will come apart. Not your experience, eh? Hmmm…

LCHIEN and BT3 WOODOG-
In the speaker building crowd there are MDF people and BB ply people. I fall into the latter group. I think the BB ply has much less dust, isn’t so much a problem if it gets wet, and takes screws better. But there are reasons to favor MDF as well.


Uncle Hook-
Yeah, a nice lacquer for a top coat could be good. But what to put below the top coat?
I definitely like the idea of a hard wood laminate. But I’ve heard it’s not so good an idea to put hard wood over ply, due to expansion/contraction differences with heat and humidty changes. Is it maybe not such a big problem?

And maybe you’re right about only a clear finish over the BB ply. Here are some shelves I made. Finished only with 2 coats of oil-based polyurethane. They came out prettier than I expected. Although I’d think a good hard wood laminate, veneer, or the right paint could be even better.

[JonW]

Dustmight-
Even if I were to use MDF, I still have the issue of an odd #D shape, so veneering would be pretty tricky. Yeah, a simpler, slanted baffle would be easier, but it seems to be an acoustic compromise. I’m now designing my first speaker, so I’m trying to take into account everything I can to make it sound as good as possible. The finishing work is secondary to the exercise. But I still would like it to look nice.

Tom-
I see what you’re saying. Do you have any paints that are particularly nice? I think I should have no problem getting the proper 3D shape I’m after. But what type of paint would look real nice? I keep thinking of car paints and clear coats. No idea how well that works with wood/MDF.

cgallery-
Right. That’s the way I’ve been looking at it. Is there any type of paint and finish combo that you like?

JimD-
Yup, that’s about the size of it. And good attitude- if they sound good to you, you’re done.

[BigguyZ]

Another vote for MDF. That's what I used for my speakers, and it worked out well.

You don't have to veneer MDF, however. If you use a decent sandable primer you can put a painted finish on. You can even do a gloss piano black finish. I've used automotive spray paint to get a mirror finish. Start at 400 grit wet sanding (make sure not to get any water on the raw surface of the MDF), and work your way up to 2000. Finish with some 3M rubbing compound. If you want more details, I can try to dig up some links...

[JonW]

I've used automotive spray paint to get a mirror finish. Start at 400 grit wet sanding (make sure not to get any water on the raw surface of the MDF), and work your way up to 2000. Finish with some 3M rubbing compound. If you want more details, I can try to dig up some links...

Interesting. So you really can get a good automotive-like finish on wood? Yes please, if you have any good links or info that would be super.

[Tom Miller]

Tom-
I see what you’re saying. Do you have any paints that are particularly nice? I think I should have no problem getting the proper 3D shape I’m after. But what type of paint would look real nice? I keep thinking of car paints and clear coats. No idea how well that works with wood/MDF.

Well, keep in mind that I was kinda pulling this idea out of my...uhhh, out of thin air. But, once you coat with bondo and smooth that out, you're at the same point as someone applying a car finish. You should be able to spray an acrylic or enamel paint, and then wet sand to as smooth a finish as you want.

Regards,
Tom

[maxparot]

For speakers the best material is High Density Fiberboard (HDF) and the best way to deal with finishing is to veneer.