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Old 27th August 2011, 11:48 PM   #1
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Default Taking the Air Cavity/pre-chamber to the next level...

When Chris & i built (chris did the actual building) the 1st test set of Woden Maeshowe the Air Cavity behind the driver was purposely enlarged so that we could play with it. Sucess with the FE126 lead to Valiant, an adaption for FE126En.

All along we knew we'd have to fill it up/reduce its volume back to spec. I had the idea of using the "blocks" i often mention.

This is meant as an illustration of what i mean by that. I will note here that what follows was/is very successful.

Click the image to open in full size.

Blue is approximately where we put the divider, red is where it should have been.

1st i figured out how deep a block i would need to fill to the depth of the specified rear dividing panel (distance from blue ro red). I took this depth and multiplied & divided by the golden ratio.

I had to install thru the driver cutout so i figured a grid of blocks 4 high by 10 wide (9 blocks 15mm wide + 1 18mm). Chris made up a 1/3 each of the 15mm blocks in short, medium and long. The 18mm blocks were all made medium. Installed this gives a net air cavity volume as specified.

I arranged the blocks so that each row had the same number of s/m/l and then arranged them so that no 2 blocks with the same height were adjacent. (and mirror image for each speaker). Then using a dab of glue i installed into the cavity to create a "bumpy" rear wall. A layer of 1/2" cotton felt was placed over the top and the driver installed.

EL70eN was installed and the "issue" we were having that we were using as a benchmark had dissapeared. The new Alpair 7.3 was installed (sans extra magnet as that was quicker than cutting a hole in the damping to accomodate it) and worked superbly.

One can take this further by making a more complex grid of blocks. For instance, more of them, more lengths, cutting the outside end at an angle, using pyramids instead of rectangular prisms.

A precursor to this is the air cavity in Mikasa.

dave
Attached Images
File Type: jpg rawSpace.jpg (29.2 KB, 683 views)
File Type: jpg RawBlocks.jpg (81.1 KB, 687 views)
File Type: jpg blocksIn.jpg (75.3 KB, 679 views)
File Type: gif Maeshowe-airCavity.gif (18.5 KB, 1382 views)
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Old 28th August 2011, 06:07 AM   #2
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Slick, but when did bean bags fall out of style?

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Old 28th August 2011, 06:20 AM   #3
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I was trying to fake it with cans of salmon, but it wasn't quite cutting it

It seems forever that i have said that the air cavity volume could be adjusted with blocks, thot it was time for me to "eat my own dogfood" .

I think the diffusion device character doesn't hurt at all either.

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Old 28th August 2011, 06:46 AM   #4
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Yeah, glued in scrap wood makes good filler when you need it to be portable.

Agreed, though doing a proper skyline would be the theoretical ideal: http://www.avsforum.com/avs-vb/showthread.php?t=1312919

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Old 28th August 2011, 07:29 AM   #5
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I wasn't prepared to go as complex as that. I (mostly -- didn't quite get it finished) wrote software at the start of the decade for a specific calculation of that to check to see if an AES paper had made some errors -- it was IIRC at least a couple orders higher than th ones linked off that thread -- thanx for posting that.

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Old 28th August 2011, 01:06 PM   #6
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Default FYI

Just to get this in the database. this site has a wealth of information on Quadrature Residue Diffusors (QRDs): QRD diffusers: Technical Overview
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Old 28th August 2011, 01:32 PM   #7
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Quote:
Originally Posted by planet10 View Post
I wasn't prepared to go as complex as that. -- thanx for posting that.
You're welcome!

Understood, life's too short to spend much time on such a complex way to do such a simple diffusion.

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Old 28th August 2011, 10:37 PM   #8
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mmmmmmm... salmon

oh yea, good links.

edit: I've been reading too many pdf's... it's an addiction. well here's another. http://downloads.bbc.co.uk/rd/pubs/reports/1990-15.pdf

Last edited by revboden; 28th August 2011 at 10:44 PM.
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Old 28th August 2011, 10:44 PM   #9
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Quote:
Originally Posted by revboden View Post
mmmmmmm... salmon
We did have 2 BBQed sockeye as part of dinner on the day of the diyFEST we tested this out

How did you know?

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Old 28th August 2011, 11:18 PM   #10
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nobody BBq's a salmon without me knowing

Vancouver Island DIY-fest, and no one told me... grumble grumble grumble.
do these things get posted somewhere?
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