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Old 25th April 2006, 06:49 PM   #1
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Default am i on the right track??

i have these two 18" woofers that i i got years ago through a trade with my brother and i have never used them for anything and i was wanting to use them in a sub enclosure for my HT setup - the specs dont make them suitable for anything but a large ported box...and ive done some models on WinIsd....and im not really pleased with the results im getting so i started thinking about a TL with them - my only knowledge of TL's is what ive read on this board so i was wondering if #1- are these drivers even suitable and #2 - if im on the right track with my design -- the driver specs that i have are as follows (they havent made these things for like 10 yrs now):

Radio Shack Cat, No. 40-1309 (18" Subwoofer)

Nominal Impedance........................... 8 ohms
DC Resistance (RE)............................ 6.83 ohms
Free Air Response............................. 16.5 hz
Equivalent Air Volume(VAS)............... 41 Ft3
Moving Mass (Mms)........................... 192.6 Grams
Mechanical Q (QMS)........................... 1.986
Electrical Q (QES)............................... 0.225
Total Q (QTS)..................................... 0.20
Peak Power Handling........................ 200 Watts
Peak-to-Peak Linear Excursion(XD)... 6.1 mm
Gap Flux Density(BG)......................... 7850 Gauss (+/- 200 Gauss)

i have an 15' ceiling at the peak of my living room (right in the center) and was thinking i would stand this up with the open end firing into the peak with about 3' of clearance it would be placed right behind the LCD and center channel

i was hoping anyone with expierience with these designs could help me out....thank you

Click the image to open in full size.

i dont know if the picture will be big enough so i'll attach the file....my paint skillz suck!!...lol
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Old 26th April 2006, 12:23 AM   #2
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Isobarik placement of woofers has absolutely no benefit whatsoever in a transmission line enclosure, you'll probably get better results with only one woofer.

I recommend placing them in parallel.
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Old 26th April 2006, 12:35 AM   #3
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ok...i was wondering about that....i was worried about the low xmax of the drivers so i was thinking it might help things - im truly a begginer when it comes to understanding the mechanics of the speakers - i also thought that by running a speaker isobaric it cuts box volume in half and with the high Vas of these i thought it might help...but i have the room to build two of them...thanx for your input....does anyone know if my line length measures up or am i way off on that??
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Old 26th April 2006, 01:09 AM   #4
lndm is offline lndm  Australia
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There are a few ways to do this. This is one fairly typical one.

For a single driver, the frequency of your pipe should be approx 27Hz. This suggests a total line length of 10.5'. Your response would be down 3dB at 60Hz. The volume of your pipe should be about 20'^3, and the driver should be mounted about 20% of the way along the pipe for best results.

I cannot be certain about two drivers, but as you have them configured, I would guess that would halve the necessary pipe volume, and not affect the f3.

The reason for the highish f3 here is the low Qt. There is nothing wrong with your driver. Transmission line designs do seem to come out with better numbers when you use a driver with a higher Q. The suggestion above should give good sounding results as is.

Tweaking the design for lower f3 would be an experimental process as you may get unexpected results. Perhaps you could look to ways to increase the Q of the driver, or look at the Alpha/Omega transmission line designs. IIRC, these are like a TL mixed with a vented box.

Wiring the two drivers in series seems like a way to increase Q but I am not sure how you would go with this.
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Old 26th April 2006, 01:54 AM   #5
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ok...because of the low Qt of the driver i need to target the tune higher....i thought you were supposed to tune to the fs of the driver so i was going 16.5 - but i need a higher target - if im understanding this right then it would be better for me anyway....i could fit the line in the room without folding it - and then two seperate lines would be alot easier

i hope im understaanding correctly...lol
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Old 26th April 2006, 02:35 AM   #6
lndm is offline lndm  Australia
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I think you are. Tuning higher does compensate for the low Qt, I suspect this is a more complex relationship that it first appears. There is the classic view on building TL's where noone seemed really sure on anything. It was as recently as 2000 that Augspurger el al came up with something concrete. So, TL's ought to be built to a formula now.

http://www.t-linespeakers.org/ is one possible starting place.

I encourage you to investigate alignments (formulas for predictable known results adjustable to fit any driver). There are also line variants like the tapered line and the fitting of the driver away from the end etc. These help reduce unwanted resonances and can improve extension.
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Old 26th April 2006, 06:44 AM   #7
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ok...ty - after reading the ML for dummies im kinda getting what your saying although some of it still sounds foreign to me....i will play around with the mathcad tommorrow and study a little more to see if things are making sense to me or not...its obviously not as easy as some of the box design software but i guess thats because these designs are less predictable - bottom line....i need to research it more.....i guess its not as easy as hooking up some sonotube like others make it out to be - thanx for all of your help guys...it is appreciated
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Old 26th April 2006, 08:36 AM   #8
DorinD is offline DorinD  Romania
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dawgbite,
Maybe you could use your drivers into a Transflex enclosure. See here:

http://www.hi-fi.ro/fhifi/viewtopic.php?p=85033#85033
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Old 26th April 2006, 08:40 AM   #9
DorinD is offline DorinD  Romania
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dawgbite,
Maybe you could use your drivers into a Transflex enclosure. See here:

http://www.hi-fi.ro/fhifi/uacp.php?u...bdc4584eb8be38
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Old 26th April 2006, 08:46 AM   #10
lndm is offline lndm  Australia
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On the contrary, TL design now seems very predictable. If you can get your hands on Speaker Builder two, three and four: 2000, its only slightly more complex than say, a vented enclosure. These articles are a good read, and really spell it out while still going in to detail.

Thing is, knowledge of TL's is less common, so fewer people have written programs to simulate TL's, and those who do tend to have a highly mathematical approach that turns many away. (This is in no way a negative comment, I'm just trying to sympathise with your situation).
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