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Discussion Starter · #1 · (Edited)
8" Transmission Line Sub

So I had insomnia the other night so I started reading up on transmission line speakers. When used for subwoofers supposedly they sound very tight and musical. This sounds like it would go well in my music-only living room mated with my 30 watt 4" full range speakers.

I got an idea to make one out of a 8" sub driver I had lying around. Luckily, the T-S parameters are mosly in the "accepted" range for TL drivers. By my quick calculations, I'd need a pipe about 8 feet long. So, now I have some questions;

1. Where is a good place to get 8" PVC pipe? Home Depot and Lowes only carry 6". Since I want to fold my pipe I don't want to use 8" sonotube. I'd like to avoid making a box if I can help it.

2. Does anyone have experience to share about making TL subs?
 

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Discussion Starter · #4 ·
Now that I decided to go with a box, here is my rough layout. Some other folks helped me with the design for a tapered quarter length transmission line. This will be about 28" tall, 10" wide and 17" deep (minus amp). I have to buy some more MDF before I can continue.
 

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Discussion Starter · #7 · (Edited)
Hey Boom, How's it going?

I've never built a TL, but I thought the individual chambers/lines/passages are "constantly increasing", with no parallelism between them? I could be wrong though....:huh:

Bob
They can be parallel, increasing or decreasing. It's my understanding that having them decreasing allows you to use a shorter tube for the same tune and allows less high frequency to get out of the open end. Do a google for "Martin King transmission line" or go to http://www.quarter-wave.com to get a ton of information on the subject.

Here's another build pic (with some friends).
 

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Nice builds Boom!
If it's "your undertanding", then you're probably more correct than I. I've not attempted a TL, but have a few forum buddies that have done some. Most that I've seen (that I can recollect) have had an increasing cavity size from the driver chamber to the terminus. But there again, it's not like I've been "keeping score" or anything, just casually browsing the picks. If they were'nt friends, I probably wouldn't have even been reading the threads, you know.
A local forum dude brought over a very small pair of TL's equiped with 4" Lowthers (IIRC) for the sole purpose of showing me how much bass can be had from such small drivers. His source was an off the shelf unmodded CDP and a 5wpc Almarro A205A tube amp. Purely amazing man.... He played a LF tone that blew me away. Got me thinking about the capabilities of a couple purpose build monster sized 15" TL's for HT LFE's. :yes: YEA BABY!!
Regarding Martin, I've chatted with him about OB before. That dude is waaaay over my head. Helluva nice guy, but he's so smart, when he sneezes he looses more information about acoustics than I even know! I've checked out his site but my eyes start glazing over by the time I'm halfway down the page. I may be a bit bold in saying this, but he might just be the "Nikola Tesla" of audio.:T

Bob
 

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Greets!

Looking good! FYI, for sub duty there's no acoustic advantage to adding the diagonal corners to ~preserve the taper through the bends since the WLs are so large.

WRT pipe mains or subs where WAF/size isn't a major issue, consider a simple pipe horn:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=66173
http://www.zillaspeak.com/bib.asp

.............and for a slightly more compact sub duty only pipe horn where both the front and rear driver radiation is utilized:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=97674

GM
 

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Discussion Starter · #10 ·
Greets!

Looking good! FYI, for sub duty there's no acoustic advantage to adding the diagonal corners to ~preserve the taper through the bends since the WLs are so large.

Cool. Actually, the corners made it easier to keep the inner walls glued at a 90 degree angle to the sides but I'll keep that in mind for the future.
 

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Greets!

Maybe with MDF, which has a low MOE. TLs, horns are normally built with at least 19 mm void free 13 ply plywood, so structural rigidity isn't an issue, though larger passages are normally braced to divide up the area. As for helping with resonances, we want higher order harmonics above the sub's passband so they won't be excited and adding more mass with little rigidity gains is moving in the wrong direction when using MDF for subs since it's increasing the amplitude of lower order ones.

Some good info here: http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping

GM
 

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Discussion Starter · #13 ·
Update: I have everything put together. I had good weather today so I spent it painting. I ran out of bronze paint, but I got an okay first coat on. I also built a box for the amp to go in (it is seperate from the sub).

As far as me using MDF - its what I got. Since this is my first chop at a TL sub I wasn't going to go all out.
 

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Discussion Starter · #19 ·
Still not ready for a FR but I figured I'd give an update. Initially I was unhappy as this TL sub sounded very muddy. I tried adding more stuffing, then taking away stuffing and nothing seemed to work. I was letting it break in as I contemplated how I was going to chop up the enclosure and rueing all the time I spent making the folded path. Luckily after about four hours of techno the driver really cleared up as it broke in. The difference was really night and day - I've never had breaking in change the sound of a driver so much.

Anyway, now I'm happy. It sounds tight like a sealed sub with just a bit more extension down low. I had built a passive sub similar to the Triska sub that used the same driver and did an A/B test using same sub amp. There is no question - I like this TL better. It is tighter and has lower extension. Good thing, considering is weighs four times as much!

When I get more motivated I will do a FR.
 

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Discussion Starter · #20 · (Edited)
I'm sure this won't blow anyone's socks off, but here is an FR. The second graph is the predicted performance (made by MJL21193 on the DIYaudio forum). Seems to be a pretty close matchup to me but it is hard to tell with the room modes in my office.
 

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