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Building a set of BFM Omni 10.5's

P

pjwalsh

May 3, 2006
139
3
Elkton, Md
#1
I started building a set of Bill Fitzmaurice designed Omni 10.5's last weekend. These are a full range cabinet featuring a horn loaded 10" driver and piezo electric tweeters. They are reputed to be a good sounding bass cabinet SPL charts and other info at www.billfitzmaurice.com. Thought I would post the progress here for others to see.

I'll be using these cabs with a GK 700RB-II head and a Carvin LB70. I am after a reasonably loud rig that can still fit easily in my hatchback.

I am using the Eminence Beta 10 driver, GT 1016 piezo tweets, 1/2" arauco acx plywood and professional quality cabinet hardware. I am not exactly on a budget but am conscious of cost.

Here is the box o' stuff from parts express: about $260 in materials should finish these babies off nicely.

Omni10.5pics09212007021.jpg


edit: beautiful photography by my darling wife Kimberly
 
#2
If you are careful you can get the two omni 10.5's out of a single 4' x 8' sheet of plywood - I worked my cutlist out to scale on graph paper first and and only had to scrounge a couple of small bits. My pile of scrap is shown below. Also have to thank my buddy Paul for letting me use his nicely set up shop including an excellent tablesaw which made everything quite easy to cut.

th_Omni10.5pics09212007012.jpg


The plans from Bill Fitzmaurice are very easy to follow. I did not use his recommended sequence as I fancy myself an experienced woodworker and always think I know better anyway. In any case rather than use the cut and fit method described in the plans I chose to cut all the parts out in one shot and assemble in a second step. The pair of dry assembled kits is below:

Omni10.5pics09212007016.jpg


So far 4.5 hours of cutting - I'll finish the glue up this coming weekend and post some more pics.


edit: Beautiful photography by my darling wife Kimberly
 
#5
Funny you should mention the Avatar 1-12" - my first inclination was a pair of those or a 2-10" to match with this head. My brother (also a bass player) just got the Avatar neo 4-10" and loves it. From what I gather on the BFM site the Omni 10.5 pair should put out more sound than a 2-10", perhaps a bit less volume than a 2-12" or 4-10" but will have more even frequency response than any of these.
 
#6
So Glue up on Friday and Sunday evening. I used the PL premium glue that B. F. recommends - messy, messy, messy but seems very tough. Gloves are not optional. I dry fit the pieces together with drywall screws then disassembled, applied glue and refastened. It is a sloppy process. Once glued the screws are superfluous. If I ever build another cab I will use a brad nailer instead as now I must remove the drywall screws and fill the holes with bondo or some other quick setting, easily sandable schmutz.

Here is a pic of the glueing taken by Kimmo:

Omni10.5pics09212007005-1.jpg


On the second cab my assembly sequence improved - glued /screwed all the horn throat, tweeter baffle and sides etc then fasted to the top and bottom - much easier:

Omni10.5pics09232007001.jpg


I was not able to get the cleats needed to fasten the back out of the plywood so scrounged some nice cherry from Paul's scrap pile (Paul's shop I am working in - I need a shop!)

Omni10.5pics09232007005.jpg


Front view of glued/screwed cabs, note that I took this pic myself and that is why there is a finger in it:

Omni10.5pics09232007004.jpg


Another detail, the sled I used to make crosscuts on the plywood was a bit out of square and so there is some twist built into the cabs. I may be able to get rid of it when I put the backs on - advice for others: be careful to get truly square crosscuts - but then again this is a speaker cab not furniture.

Practical tip, the PL premium glue is very easy to scrape once cured, don't bother trying to clean it up while wet. Squoosh out is GOOD.
 
#8
Great job! You make it look easy but there's lots of woodworking skill there.

Is that carpet I see in your parts list? Carpet is still my favorite. Looks good to me, and easy to stick things to with velcro like fasteners. Or even stacking with a little velcro between.

On the glue, I'd highly recommend that people find a Fine Woodworking article, and follow up editorials on a glue test they did. Turns out that glue like Titebond III is a better glue all around. (strength, gap filling, flex, "safer", etc.) Controversial I know.
 
#10
Thanks for all the encouraging comments,

Seamonkey,

Yep, the finish will be carpet even though I don't really like the carpet on my present amp - the cats DO like it though. Really I'm using carpet because the duratex paint alternative costs $60 for a gallon and I'd only need a quart. 3 yards of carpet was something like $7. On the Titebond III, it is very good stuff but definitely will not fill gaps as well as the PL Premium - and despite the nice comments about my woodworking skill there are some gaps to fill - I did not point out the short baffle brace in the previous pictures but the PL is filling ~1/8" very nicely (see bottom cab brace in rear view pic of post #6).

Also glue up took about 3.5 hours so I am now at 8 hours of labor invested.

P.S.

BBMS, what does MY mean? I am terrible at internet lingo.

Cheers,

Pete
 
#11
seamonkey said:
Turns out that glue like Titebond III is a better glue all around. (strength, gap filling, flex, "safer", etc.) Controversial I know.
Click to expand...
I recommend PL Premium or similar urethane adhesives. Since the slightest air leak can severely compromise performance traditional woodworking glues should not be used when building my designs.
 
#12
PL Premium is the glue of choice for speaker cabinets. It seals a thousand sins, and guarantees no air leakage. It will span 1/4", but if your work is that sloppy you deserve leaks...

:D

Mineral spirits will remove wet PL quite nicely. It takes PL off the hands, and off the work piece. I use it to clean squeez-out on joints that will be seen under stain. The mineral spirits does not appear to weaken my joint.

If you follow the plans, step by step, you will have a great pair of cabs when you are done.
 
#13
billfitzmaurice said:
I recommend PL Premium or similar urethane adhesives. Since the slightest air leak can severely compromise performance traditional woodworking glues should not be used when building my designs.
Click to expand...

Bill - I'm wondering if you could help me understand how small leaks along the seams are an issue in ported designs? I can visualize how they would come into play for a fully sealed cabinet, but it would seem for a ported cabinet, the relative area of the port opening would drastically limit air pressure against the seams, rendering leaks largely inconsequential. But this isn't so?
 
#15
Hello Bill,

Thanks for the great plans, so far this has been an elegantly simple process. On the PL Premium adhesive - it is a very good choice for this application. The gap filling properties are important and the toughness of the cured adhesive is excellent.

Also, glad you have had a look at my work - truly full service acoustical engineering from BFM Loudspeaker Design!
 
#16
tonebrulee said:
Bill - I'm wondering if you could help me understand how small leaks along the seams are an issue in ported designs? I can visualize how they would come into play for a fully sealed cabinet, but it would seem for a ported cabinet, the relative area of the port opening would drastically limit air pressure against the seams, rendering leaks largely inconsequential. But this isn't so?
Click to expand...

Good question. Though it's a bit OT, I will answer for ported systems. At frequencies much above the port tuning, the port is effectively inactive, so the system behaves like a sealed box, with the same air pressure inside. This is the pressure that makes leaks whistle.

And from an empirical standpoint, I have experienced noise from leaks in ported boxes, when I was sloppy with the gasketing around a driver.
 
#17
billfitzmaurice said:
I recommend PL Premium or similar urethane adhesives. Since the slightest air leak can severely compromise performance traditional woodworking glues should not be used when building my designs.
Click to expand...

They're your plans and you can recommend whatever you want.

Just going by the new testing and the current mode of thinking I read in woodworking forums. Polyurethane glues are not very good for home woodworker type use. Not very safe. Expensive. Not as strong as once thought. Foaming can push joints apart unless properly clamped. Foam in a gap has no strength, and over time flexing of the foam will cause it crumble. Glue and sawdust mixed makes a good gap filler. People building other cabinets should research glues and may want to stay away from Polyurethane glues.
 
#18
In my day job I am a materials scientist and I work with adhesive issues frequently so I am going to go a little off the topic of bass cabinets and address Seamonkeys very valid concerns about reactive polyurethane adhesives like PL Premium.

PL Premium contains about 10% of a material called MDI or diphenyl methane diisocyante which is a very reactive and toxic compound. At levels considered harmful MDI has no odor and long term exposure will eventually cause an allergic reaction and scarring of your lung tissue and mucus membranes. On the plus side PL is 100% solids and contains no solvents (VOC's).

The big advantage of reactive urethane adhesives is that they have good strength, will adhere strongly to a wide range of substrates and remain flexible and tough after curing. The foaming is a result of the crosslinking reaction in which carbon dioxide is evolved and it does reduce the strength and will push joints apart if they are not well secured. In a well clamped joint this characteristic becomes an advantageous gap filling property.
Urethane adhesives are generally much stronger than polyvinyl acetate based materials like Titebond III (which is also a good adhesive but is not a good gap filler and is not waterproof, only water resistant)

In spite of the health risk I would judge PL Premium to be a good choice for this application because the good performance outweighs the health risk for a hobbyist who will not be exposed for long durations on a frequent basis. The health risk can be minimized by simply keeping the adhesive off your skin and working in a well ventilated environment. A professional woodworker who will be working with these adhesives on a daily basis should either wear a respirator, have an outside supply of air or work in a ventilated hood.
 
#19
pjwalsh said:
Urethane adhesives are generally much stronger than polyvinyl acetate based materials like Titebond III (which is also a good adhesive but is not a good gap filler and is not waterproof, only water resistant)
Click to expand...
Urethanes are tenacious when used in joints as they expand into the wood, but OTOH they don't stick all that well on exposed surfaces, so they're very easy to scrape off when cured, and they sand easily without gumming the way aliphatics do.
 

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