Discussion on what materials to build speakers out of - Page 53 - diyAudio
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Old 22nd June 2007, 11:11 PM   #521
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Quote:
Originally posted by liasom
John, my questions. - Mike

You stated there are two separate chambers in the 33 liter enclosure, correct? What is your estimate of the limit for the maximum height of the wedges in each? (this has to do with getting them through the hole and in glued in place more than anything else)

What are the basic specs for the 3-way crossover (xover points, slope)?

So I can understand more of what you hear. Your ear is telling you that there is a "hollowness" in the low-midrange bandwidth, if you were going to try to solve that with "traditional" methods what would you do first?

Hi Mike,
The lower (woofer) chamber is 33 litres. The upper is about 6 litres. Neither is very deep - about 3 inches behind the mid and 5 inches behind the woofer. The hole for the midrange is about 3.5 inches wide.

The crossover points will (eventually) be 310Hz and 3100Hz, a 24dB/octave active filter. My first listen was with a cobbled together series/parallel passive, so too much low mid might have been getting to the woofer.

What I hear...maybe hollow is not the right word. There is a peaked quality, where (I assume) certain frequencies are being reinforced by standing waves and re-emitted through the cone.
It's the way every empty box I've ever stuck the driver in prematurely sounds.
I'm sure that if I had bothered to run a sweep, I could have isolated the "resonance". Drivers have been removed to finish the paint job on the baffles. All of this said, things will be different with the active filter (once again I assume).

Traditionally I would first line the box with commercial rubber back carpet tiles (salvaged from a Toronto library). This would usually prove to be the cure, but if not I would add a small amount of polyfil stuffing - well away from the driver.
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Old 23rd June 2007, 11:30 AM   #522
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This may be of some value in determining the height & arrangement of wedges:

http://www.pmerecords.com/Diffusor.cfm

and another with some frequency ~ relationship:

http://www.mhsoft.nl/DiffusorCalculator.html

don't overlook the link to the BBC paper in the latter
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Old 23rd June 2007, 02:51 PM   #523
liasom is offline liasom  United States
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Thanks Ed for a great contribution. I saw the BBC paper or had seen the Schroeder design sometime in the past and I've used a room before in a studio with that type of diffuser. To my ear they seemed to work well. Someday I'm going to build a few panels for my house. That calculator you found is great!

Anyhow that idea of the square shapes is in the mix. If you think of anything else post it.

Another paper I've found is from RPG Diffusor Systems, Inc.

http://www.rpginc.com/news/library/tyndall_paper.pdf.

My brain is getting a workout!
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Old 23rd June 2007, 08:28 PM   #524
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Good one Ed, but my old lady would pitch a fit if she had to dust many of those!
Attached Images
File Type: jpg diffusor03.jpg (37.4 KB, 591 views)
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Old 24th June 2007, 12:50 PM   #525
SteveT2 is offline SteveT2  United Kingdom
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Just my two pennies worth again.

Have you settled on the material you plan to make the wedges or stems out of yet? Or has that already been aired and I have missed it?
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Old 24th June 2007, 05:38 PM   #526
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MJL21193

My wife would object as well.
I was thinking of it's use inside the box. However it may not be effective with the frequencies that need damping due to size limitations.
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Old 24th June 2007, 09:01 PM   #527
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I was thinking of its use inside the enclosure as well. My reason for asking was that cutting up hundreds of pieces of mdf would be tedious in the extreme and the benefits not worth the effort, perhaps. However...............flooring insulation comes in 50mm dense sheets. Model airplanes are made of it. Blue foam by Sheffield Insulation in the UK are the largest suppliers I believe and there will be an equivalent in your neck of the woods. This dense foam cuts with a hot wire very quickly and with a good surface. Admittedly it is not lossy, but it does have lots of very tiny "bubbles". It is light and will glue to itself or wood with regular PVA glue. It would be relatively easy and quick to make up some panels to insert inside the enclosure. Just a thought.
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Old 24th June 2007, 11:45 PM   #528
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Quote:
Originally posted by SteveT2
....flooring insulation comes in 50mm dense sheets. Model airplanes are made of it. Blue foam by Sheffield Insulation in the UK are the largest suppliers I believe and there will be an equivalent in your neck of the woods...
Extruded polystyrene insulation = Dow "blueboard"
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Old 25th June 2007, 01:25 AM   #529
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Quote:
Originally posted by redi


Extruded polystyrene insulation = Dow "blueboard"
I'd think that would work pretty well... and you'd still be able lift the box after you were done...

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Old 25th June 2007, 01:30 AM   #530
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Quote:
Originally posted by SteveT2

Blue foam by Sheffield Insulation in the UK are the largest suppliers I believe and there will be an equivalent in your neck of the woods.
Yes, Styrofoam. This is a definite possibility. Might work better than MDF, and certainly easier to cut (I'd still cut it on the table saw). One drawback is that it will add nothing to the overall rigidity of the panels, unlike a solid like MDF.
I used about $1000.00 worth of that stuff to insulate my basement walls. About $22 for a 2' x 8' sheet of 2" thick.
I can often find scraps of it kicking around at work that would be big enough to try out.

My plan is to dive in the first chance I get, with the reasoning that if it doesn't work, I can always either rip it out or add stuffing.
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