Plywood for BLH horns

[kenev]

I have decided to build Ron's Dallas II horns with FE206E. I have a question regarding which type of plywood to use.
In particular, after some search, I managed to find two types of (good quality) 18 mm (0.700") plywood: the first is made of birch (no Baltic I assume, the seller told me that it is Russian) and the second one is made of okoume ( a tropical wood).

Is there a difference on the type of wood or anyone could do the job? I asked both sellers insistently and they both asuured me that the plywoods they supply are void free.

Any help on making my decision is welcomed.

Evangelos

[youyoung21147]

Tropical wood is harder, and therefore more rigid and reflects high frequencies better.

Birch is "shallower" and I assume it damps vibrations and highs better, at the expense of less rigidity.

I don't know which is the best for horns, I never built any.

May I suggest 18mm is a little weak for a 20cm driver ? I'd rather use 22 or 25mm.

My favorite material is MDF because it is easy to finish, and because of its ultra-fine particle structure, it allows a smooth cutting and assembling with no micro-holes that could affect sealing. It is not ultra rigid though, but very heavy so I assume it damps vibrations well. I think its very smooth surface doesn't helps damping highs, and I'm aware it isn't the best choice for speakers.
Using it in strong thicknesses is recommended, and this material is quite cheap.


Just my 0.00000000002€

[one1speed]

I've read many posts that recommend staying away from MDF, expecially with horns.

The plywoods you're looking at are a much, much better choice. However, I'm not sure which is better. Hopefully some others will chime in. I recall reading that the Russian plys are pretty good, but not positive on that one.

Also, many recommend using plys that have many layers. Often the better plywoods use 9 or more layers and are void free for the most part. It seems that the plys that used to be void free are no longer guaranteed.

A little more on MDF, one other reason many are staying away from it is that the dust is not only hard to clean up, it's also quite toxic. (Just a side note.)

[kenev]

youyoung21147,

Thanks for your reply.

The Dallas II cabinet is designed for 18 mm material. Therefore, it's not an issue of choosing a thicker material.

Regarding MDF vs. plywood, much debate has been taken place in this and other fora and there seems to be a general consensus that, for a BLH, the material of choice is plywood. I had been thinking of it for some time and ended up going for plywood.

Regards,
Evangelos

[kenev]

Originally posted by one1speed
Also, many recommend using plys that have many layers. Often the better plywoods use 9 or more layers and are void free for the most part. It seems that the plys that used to be void free are no longer guaranteed.

The seller of the birch plywood told me that it is made of 1.5 mm layers, that is 12 layers for 18 mm thickness.

[robertG]

Both plywood types you mention should be infinitely better than MDF.

In Canada birch plywood is sold as Russian plywood. It is very good for horns, quite dense, no void and a real pleasur to work with.

I built two horns with MDF and two identical horns with plywood and the plywood horns a much much better (sound is more focussed, more lively).

[youyoung21147]

Everybody seems to like plywood better.

I don't build my cabinets myself, a friend who learns carpenting does it for me and he likes working with MDF more, though I'd prefer him to use plywood, at least to give it a try.


I'm quite curious to know why plywood is better, I never found any explanation to that? Do you guys have any ?

[robertG]

MDF stores energy (in the form of damping), some is transformed in heat and some is transferred back to air as vibration.

Plywood stores less energy (it is stiffer)

Knock on MDF and knock on plywood of same thickness, you will get a higher pitch sound on plywood, indicating a higher res. frequency.

[Scottmoose]

I seem to recall that MDF resonates circa 1000Hz, whilst plys vary with type -average I believe is around 2400Hz, but don't take those as gospel! I tend to use MDF myself, a) for cost reasons, and b) because of the greater consistancy possible with MDF -no voids etc. A good plywood of equal size is around 3 times stiffer in compression and tension than MDF, so probably a combination of the two materials, a la Lynn Olson's Ariel would work best for regular cabinets. For a horn, i'd go with the ply.

Solid hardwoods (and some softwoods like pine when properly dried out) are also extremely good, but you have to be a real craftsman to know what you're doing with those -it's not just a matter of selecting the correct wood. It has to be aged properly, has to have the correct grain etc etc etc., otherwise, you're going to get the sonic signiture of the cabinet material, which for most of us, is not a good thing, unless you're designing a speaker in the same way as you'd design a new instrument. If you're not a violin maker or have a similar level of knowledge to Terry Cain, best avoid I reckon because, unless you're very lucky, the results you get will be inferior to those if you'd taken the easier (and much cheaper route.) I don't pretend to have that knowledge or ability, hense my liking for MDF. Oh, and chipboard by the way. Not to be sniffed at -it's much better than people believe, and it's spectacualrly cheap, which is nice.
Just to add my own warning to that mentioned above: MDF dust is carcinogenic, so make sure you've got a very good filter-mask on when working with it.
Best
Scott

[squalor]

You should build with the Okoume because then you would be the only Okoume Dallas in the world.
I built mine with three sheets of .500" void free russian Baltic Birch 5'X5' sheething. $21.00usd a sheet and all I could find in my area. I did alot of math to keep the sound path the same size so if you can get .700" do it.
I cut four 17.7" X 48" panels and drew the cart/chord points on the wood. then I measured the drawing to make a cut list. Then I nested the pieces to 49" X 11.040" panels. Would you like me to post the cut list ?