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Old 8th January 2006, 06:33 PM   #101
MJK is offline MJK  United States
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Scott,

Quote:
By the way -do you have any thoughts or views regarding lining, rather than stuffing the cabinet? (oh boy, I've started at it now!)
Yes, a few years back SpeakerBuilder had an article where somebody had compared lining to stuffing a TL. What I concluded from reading the article, and thinking about the plotted data, was that lining can also be very effective in controlling the quarter wave modes. The wavefront profile will be flat across the cross-section so the lining will help attenuate the sound wave, there will be air velocity traveling in the lining. The air velocity cannot vary across the cross-section, no shear stresses allowed in the air. You get a similar effect as with stuffing. What you loose is any attenuation of the transverse standing waves, the velocity will be minimum at the walls and these waves are better damped by fiber on the centerline of the TL.
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Old 8th January 2006, 07:38 PM   #102
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Got it. Thanks for that Martin. I might experiment a bit with lining the next enclosure (think I might go for an MLTL with PM6Cs) and comparing to a stuffed one.
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Old 8th January 2006, 08:12 PM   #103
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Greets!

FWIW, virtually everyone that followed even the minimal stuffing density I use to recommend based on MJK's work found it to be up to ~3x too much, so have quit listing a recommendation. Ditto with the few that commented on RS's Alpha TL and Quarter Wave Reflex based on Augspurger's work I posted. Typically they described it as a muffled one note bass.

Models not accurate enough or folks want more harmonic distortion than they think they do? My SWAG is it's a combination of the two, with the latter dominating.

That said, using the well proven lining of one wall, back, and top originally used by Bell Labs/W.E./Altec with damping sheeting such as low density felt, 1" acoustic or R13-R19 fiberglass insulation, has worked well whether sealed, reflex, TL, or even certain horn alignments. For sure I prefer it to the 'sound' of stuffing regardless of the materials I've tried.

GM
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Old 9th January 2006, 02:55 AM   #104
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Hi Martin,

These are Ramon Cancel's and Todd Jenkins measures. You may recall us discussing this on the DIY list in 2003 (as part of a discussion surrounding Balaghs characterization of long fiber wool), and then in private email.

Bullock's T-line model was published in '86, and Todd and Ramon's work occurred in 93 to 95. Todd and Ramon stated the Bullock model worked very well with no stufffing. I have no reason to suspect issues with line geometry, but can't corroborate the method used. Todd had repeated the stuffing results numerous times with various materials over the course of the two years we were corresponding. Sorry that I don't have any more data than this. I believe I had sent you the measured results graphs several years back but please email me if you'd like them again.

Your similar results for wool and poly are consistent with Todd's, where he measured nearly identical attenuation profiles for them (though velocity showed differences) assuming audio grade poly from Madisound. Dacron poly was only 50% as effective, illustrating the differences in material types. If the fiber diameter and drag are similar, results would be similar.

Todd went on to work at EV 10 years ago, and I lost touch with Ramon. After a Google search I was sorry to read that Dr Bullock passed away in 2004.

Scott, the yellow compressed fiberglass is the fiber board I mentioned for my tests. If Bob is using this, then perhaps he's gunning for an attenuation tailored over frequency and location. Most of these materials have very high densities, and as my measurements show, will partially reflect if the angle isn't normal. The LDC corroborates this, with 4lb/ft3 fiberglass causing fo and f3 to increase vs 2lb/ft3. Todd found similar results.

I'm also surprised that different materials are not expected to provide differring results. I've measured wool vs poly vs glass q differences near field. Ken Kantor's on record numerous times articulating differences and preferences realized at NHT through experimentation.

Dave
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Old 9th January 2006, 11:16 AM   #105
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Hi Dave,

Quote:
You may recall us discussing this on the DIY list in 2003 (as part of a discussion surrounding Balaghs characterization of long fiber wool), and then in private email.
Honestly, I don't rmember the discussions. I looked in my folder where I keep other people's measurements and cannot find anything from you on this topic. Not sure why.

Quote:
Sorry that I don't have any more data than this. I believe I had sent you the measured results graphs several years back but please email me if you'd like them again.
Sure, I'll take a look.

Quote:
The LDC corroborates this, with 4lb/ft3 fiberglass causing fo and f3 to increase vs 2lb/ft3.
Those are extremely high densities, if somebody is using this much fiber then they are trying to brute force a TL solution, it would be better to adjust the geometry. I have never used more then 0.5 lb/ft^3 and have recently only been using half of that amount. I don't have much faith in the LDC when it comes to TL design, it is great source for closed and vented box design information but not anything too far out of the mainstream.

Quote:
I'm also surprised that different materials are not expected to provide differring results.
I have measured polyester (cheap stuff not "audio grade") and long fober wool and achieved almost identical results. The materials had very different diameters and lengths. I have also been sent a sereis of measurments of 4 or 5 different materials and the results also are all fairly close. I don't see any magic fiber material. At this point I don't believe type of fiber is a significant variable and have stopped chasing fiber differences. I personally like cheap and readily available fibers and concentrate on better geometry solutions.
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Old 9th January 2006, 10:11 PM   #106
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Moving away from the stuffing debate, I mentioned I'd present a bit more on in-room matters. So...

Here's a selection of responses from my Ariel MLTL thoretical design mentioned a couple of pages ago. They all assume a wooden floor, with the listener 3m away on-axis with the drivers. All I have done is move the speaker progressively from being rammed right up against a rear wall, further out into the room. Here's the first response: front baffle 15" from the rear wall. Bit of a suck-out in the mid to upper bss and lower midrange. Interesting double-dip -I imagine the second of these is floor-bounce, as it remains in basically the same place in all the graphs, but I'd be lying if I said I knew for certain -if anyone can put me right on this I'd be grateful.
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File Type: jpg ariel mltl 15in.jpg (60.0 KB, 198 views)
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Old 9th January 2006, 10:12 PM   #107
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Now 25" out. Note the mid to uper bass is starting to improve. The first dip has vanished. Seems smoother generally.
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File Type: jpg ariel mltl 25in.jpg (60.4 KB, 187 views)
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Old 9th January 2006, 10:14 PM   #108
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Yep, you've guessed it: 35" out. That double-dip has popued up again. Bit more ragged this one -i think it's better at 25" myself. See what you think.
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Old 9th January 2006, 10:16 PM   #109
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Now 45" (oh look, they're playing our song... ;-)

In turms of the overall response curve, this is my favourite. It's a little ragged in small increments, but the overall shape is smoothest, to my eyes.
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Old 9th January 2006, 10:18 PM   #110
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55" out. (Last one of this series this). To my eyes, this suggests that everything should be in moderation. Note that the double-dip is deeper than at 45" out, and though the bss is probably the best of the lot, it's even more ragged in detail above 500Hz.
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