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Old 26th June 2006, 12:20 AM   #1
Lucius is offline Lucius  United States
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Default Alpha Line

Hello All

I'm doing a knock-off of the "Alpha Line" that was in A.E. a couple years back.
It's a straight line T/L. Inside its 7"x7" W to just shy 0f 32"(17.78 X 17.78 x 81 cm)
Right now I'm putting the base coats of shellac on before I mix in some sparkle blue
pearl-ex powder. Jordan 92s drivers are waiting to go in. I hope the pic uploads.
I'm about to order 2" spikes (5cm). Anyone know if thats about right? Also I'm
getting fiberglass fill as the design allows a shorter line than if polyfill were used.
Has anyone else measured a difference between various fills?

Regards

Lucius
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Old 26th June 2006, 08:24 AM   #2
Nanook is offline Nanook  Canada
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Default fibreglass V plolyester...

in the Loudspeaker design Cookbook, Vance Dickenson lists a comparison (although not specifically for any loudspeaker type.

the key is to use the appropriate amount of "stuffing", and the appropriate type. Speed of sound in long fibre wool is approximately 405'/sec, Dacron and Fibreglass are less good.resulting in higher speeds and less dampening. Wool is more effective and should allow for an even shorter path (that behaves like a longer one) than fibreglass and Dacron.

I am a little out of my element here, but perhaps some of the TL gurus around here can give a better or more rigourous explaination.
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Old 26th June 2006, 11:27 AM   #3
Lucius is offline Lucius  United States
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Nanook

Later reports are that wool & say accusta stuff, pillow stuff do the same. In the
"Alpha Line" the author reports using fibreglass will allow the same amount of Bass
in a shorter line. I've heard that Wool can attract insects. I don't mind yellow snow
but those damn Canadian black flies drive me crazy.

Lucius
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Old 26th June 2006, 12:07 PM   #4
MJK is offline MJK  United States
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Quote:
Speed of sound in long fibre wool is approximately 405'/sec
405 ft/sec = 123 m/sec which is almost a reduction of a factor of 3 over the speed of sound in air! Where did you get that number? My experience is that the speed of sound in wool, fiber-glass, or Dacron can never get lower than 290 m/sec which represents an adiabatic process. A better estimate would be greater than that value, say 300 - 325 m/sec for theses materials. The damping provided by each would be different and a function of stuffing density.
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Old 26th June 2006, 05:44 PM   #5
Nanook is offline Nanook  Canada
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Default Black flies and the speed of sound..

Quote:
I am a little out of my element here, but perhaps some of the TL gurus around here can give a better or more rigourous explaination.
I think the last line in my previous post says it all.

The numbers I got for long fibre wool were found within the LDCB , 5th Ed (table 4.2, pg76).

I'll have to dig up the numbers for fibreglass, I know they're in there somewhere (I didn't just pull em outta my a**). The quality of the dampening and amount of stuffing may vary depending on what is being used... unless Accousta-stuff, etc and other "propriatary" materials do not perform as suggested.

As far as being an adiabatic process, Yes, but that assumption (of an adiabatic process) is based on no external influences--a closed system, and usually an ideal gas . Unless you consider venting and the atmosphere making up the closed system, the system should be closed. As far as practical experience, I do not doubt that MJK and others have vastly more to bring to the table, and any advice offered by them (MJK , Bob Brines, etc), should be heeded and followed. Their contributions to the DIY community cannot be over-stated (there are many others too, sorry if I seem to exclude any others)

I am really not trying to re-invent the wheel here. I was intending on just reporting the numbers that I recall reading to answer the question posed.

Never met a black fly that couldn't be tamed and kept as a pet , to be unleashed upon the hordes of 'mericans disturbing the yellow snow or piles of doggie doo doo. ''
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Old 26th June 2006, 05:51 PM   #6
MJK is offline MJK  United States
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Quote:
The numbers I got for long fibre wool were found within the LDCB , 5th Ed (table 4.2, pg76).
Those values were based on Bradbury's AES paper on fiber damping in TLs. Bradbury is the one who proposed the moving fiber theory which was mathematically very elegant but unfortunately incorrect. Personally, I believe Bradbury missed the tapered geometry influence on the standing wave frequency when he proposed his theory. The more modern approach to TLs no longer includes dramatic reductions in the speed of sound in a fiber filled pipe.
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Old 27th June 2006, 01:18 AM   #7
Lucius is offline Lucius  United States
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He was fooled by a swarm of Canadian black flies in his woolly fibrous tangle.

"Adiabatic" I think I caught it. The fiber makes the pipe look bigger & like
water in a hose, if its a narrow hose, the water flows fast, a fat hose, the water
flows slow but both the same amount of water.
So originally I should have said..Does using fibreglass in place of wool &
pillow stuffing make the pipe look bigger & therefore you can use a shorter
pipe?

Thanks for the insights,

Lucius
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Old 27th June 2006, 04:25 AM   #8
GM is online now GM  United States
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Greets!

In the Alpha TL article, two different materials are designed around, polyfil and fiberglass insulation, with the fiberglass designs being considerably shorter with less stuffing density, but with a larger cross sectional area (CSA), though net Vb is still close since it's what determines the unstuffed amount of acoustic gain. Not many 'free lunches' in audio.

GM
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Old 27th June 2006, 09:33 AM   #9
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Greg, Martin, is is me, or is there something of an analogy here between centrifugal and axiel flow compressors?
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Old 27th June 2006, 09:58 AM   #10
Colin is offline Colin  United Kingdom
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Augsperger's May 2000 AES article gives stuffing densitiies for various materials based on his TL alignments (which are also 'short' TLs). He says there isn't a lot of difference between the end results (and Acoustuff is similar to wool) but fibre glass is unpleasant to work with.

In case it's of use, the table is below.

I'm just completing a pair of JX92S Augsperger lines, so I'll be interested to read how the Alpas turn out.
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