Coaxial Wedge Collaboration

Re: Coaxial Wedge Collaboration

A week ago I promised I'd post my first design. Here it is.
View attachment Smithers_Bms12CN680Monitor_v0.pdf

Constructive feedback welcome. This is version 0, so not yet final. (Please go easy on me.) The last page shows how parts for 3 can be cut from a standard 2400x1200mm sheet of ply.

A lot of time has gone into meeting the BMS recommended internal volume (not including ports) and port tuning. I realise it's probably heavier than some would like, and the curves are larger than the largest router bit anyone sells (38mm radius roundover). I'm still looking into options here.

Cheers,
Michael
 
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Re: Coaxial Wedge Collaboration

Michael Smithers said:
I looked at angles on various commercial designs and decided (based on my needs) to go with ~45 degrees... (At the moment I use mostly JBL MRX512M's and QSC K10's).
Click to expand...

I don't like the 45 degree compromise. It's going to be firing into people's waists when they're close, and over their heads when they're far. The 60/30 angles offer both a good 'close performer' angle and a good 'far performer' angle.
 
Re: Coaxial Wedge Collaboration

Michael Smithers said:
I looked at angles on various commercial designs and decided (based on my needs) to go with ~45 degrees... (At the moment I use mostly JBL MRX512M's and QSC K10's).
Click to expand...
At 45 degrees they need to be out front 5+ feet from where you are standing - great for large deep stages. Not-so-great for smaller ones. Perhaps a 45/30 degree dual angle design? The only advantage that 60/30 has over that is pack space - 45 seems more usable than 60 on all but the deepest stages (unless you are a typical guitarist who - as we all know - have their ears placed on their knees ;)).
 
Re: Coaxial Wedge Collaboration

Michael Smithers said:
I looked at angles on various commercial designs and decided (based on my needs) to go with ~45 degrees... (At the moment I use mostly JBL MRX512M's and QSC K10's).
Click to expand...

If you aren't interested (for aesthetic reasons or otherwise) in a dual angle design I would suggest erring on the side of catering to a close performer. You can always stuff a block under the back if you need longer throw.

In the DIY wedge we were starting to discuss/create in this thread, one of the design goals I had mentioned was being arranged by default for a close performer, but to have an integrated mechanism to increase the angle (via built-in adjustable legs or similar). Perhaps you might want to consider that option as well. Look at the QSC CSM wedges as an example of this.
 
Re: Coaxial Wedge Collaboration

Since I have all the design calculations and spreadsheets from the last cabinet, I could CAD up a simple 60/30 design and cut sheet for the BMS 12CN680. Aesthetics would be so, so. Does this sound ok? It might take a few weeks - my "day" job is crazy right now.
 
Re: Coaxial Wedge Collaboration

For anybody interested here are the sealed 10" coaxial 60/30's I had made in action:

RRMWRC.jpg
 
Re: Coaxial Wedge Collaboration

Here's a PDF CAD design for another version. (BMS 12CN680, ~24L internal volume [after subtracting the driver, supports and the port volumes], ~57 Hz tuning.)

It has a 30 degree face and dual ports near the ground. As I CAD'd it up, I realised it's probably easier to build than the previous one, but I can only get 2 from a 1200x2400mm plywood panel.

Michael
 

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Re: Coaxial Wedge Collaboration

I see there's been 39 downloads on v1. Since no one's posted comments, I presume it's somewhat useful. If I'm off base then someone please let me know.

I just built the first one. (I'm yet to route for handles and dishes, and paint.)
6122315325_f31ee8d77f.jpg

6122317441_8799cb0927.jpg

http://www.flickr.com/photos/michael_smithers/6122317441
Here next to a JBL MRX512M.

There're some minor problems with the drawings. Not the least of which is that 18mm ply isn't always 18mm. I should have based the drawings around mainly the internal dimensions. The Penn Elcom handle code is also incorrect. I'll try posting updated drawings shortly.

The cabinet is solid and heavy. It's already fallen off the table saw - about 3 feet - and survived fine. It's made from marine plywood - not as good as I expected with 9 thick laminates and some voids. For anyone in Australia, I've since found a supplier for good, fine laminate, Russian Birch Plywood - Plyco in Melbourne. They claim to ship Australia wide.

Does anyone know how to confirm the port tuning? Can I just run an impedance trace on the low driver?

Does anyone know where I can get the black epoxy paint used on commercial pro cabinets? I know it's overkill....

Cheers,
 
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Re: Coaxial Wedge Collaboration

Michael Smithers said:
I just built the first one. (I'm yet to route for handles and dishes, and paint.)
6122315325_f31ee8d77f.jpg



Does anyone know how to confirm the port tuning? Can I just run an impedance trace on the low driver?

Does anyone know where I can get the black epoxy paint used on commercial pro cabinets? I know it's overkill....

Cheers,
Click to expand...

You should see an impedance magnitude dip near the port tuning frequency, and you should see the impedance phase cross 0 degrees at this point. You will likely find some variation from what you modeled.

I would recommend Duratex for cabinet finishing, it is a popular DIY choice, easy to work with, and relatively inexpensive. I have used it on a few projects with good success. You can create a textured look with a roller that looks very pro. Not sure if it is available in Australia though.

Nice looking build by the way.
 
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Re: Coaxial Wedge Collaboration

Michael Smithers said:
Does anyone know where I can get the black epoxy paint used on commercial pro cabinets? I know it's overkill....
Click to expand...

Looks good, Michael.
I may try the Duratex that Jeff mentions at some point. The paint that many manufacturers use here in the US is Sherwin Williams Polane 700T. Last I checked the smallest quantity that could be purchased was 5 gallons for about $280.
Grant
 
Re: Coaxial Wedge Collaboration

Thanks very much Jeff, Grant.

Attached is an update to the CAD drawings. This should enable ply thicknesses other than 18mm. Bare in mind the port tuning is not verified, but it should be very easy to adjust before installing the driver. BTW: The round on the base front and back didn't require a router after all: just a few passes on the table saw at different angles then sanding.

Here's the first coat of paint - for now a satin black oil based enamel on oil primer. I've also routed both sides for the Penn Elcom dishes and handles. (Helps too to suspend it for painting.)

6134224638_aa58af0255.jpg
 

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Re: Coaxial Wedge Collaboration

Whilst waiting for the BMS driver, I thought I'd drop in an old 8 ohm 12" woofer and try some impedance sweeps. (After installing handles, dishes and some polyester filling.)

Can anyone help me interpret what I'm seeing from the sweeps? (Peak, Trough, Peak)
  • A (as per v1.1 CAD design): 23.5 Hz, 46.6 Hz, 116 Hz
  • B (internal volume reduced by about 1.5 litres, to match the larger displacement of the BMS): 23.5Hz, 46 Hz, 116.8 Hz
  • C (A with port area reduced by about 23%): 21.3 Hz, 45.7 Hz, 113 Hz
  • D (A with port length reduced by 50mm): 25 Hz, 47 Hz, 115 Hz
(The impedance phase crosses 0 deg at these frequencies.)

I presume the 46 to 47 Hz is the port tuning?

I'd like to get it closer to 57 Hz and it did rise a little in D but this is still within my margin of error. Also, the original port length in A is consistent with a number of online bass reflex calculators.

Any ideas where I should go from here? Is this old driver inappropriate for this testing and should I just waiting until I have the BMS?

Many thanks,
 
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Re: Coaxial Wedge Collaboration

I rechecked the design and the completed cabinets - I made a 2nd - and I miscalculated the internal volume. After accounting for everything inside (including the ports), the volume is 11% larger than what I was working to.

The actual volume is 26.6L (0.94 ft3) and the current ports should yield 48.5 Hz - close to my impedance measurements. Shortening the ports by almost half should increase this to 57 Hz. Will post more tomorrow.

One upside is that I can make the design even smaller. Reducing the internal width from 535mm to ~465mm (and changing the ports lengths accordingly) will get the volume down to 24L (0.85 ft3). I could even reduce the height by a few mm.
 
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Re: Coaxial Wedge Collaboration

This morning I removed the internal port tunnels. Only ~43mm of tunnel from the front support and front plywood remained.

The impedance trace showed a dip (and 0 degrees phase) at 57 Hz - finally.... (Updated CAD drawing attached.)

However bass reflex calculators all say that the port length should be closer to 92mm.

Could there be an effective lengthening of the port on the inside caused by two sides of the port continuing on? I.e. the cabinet floor and side-wall?

Thanks,
 

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