# transmission lines / quarter waves



## nadaclue (Nov 8, 2006)

Can anyone shed some light on how to figure out the proper waveguide size/length for transmission line subs? 

I've read through http://www.quarter-wave.com and for whatever reason can't get my head to wrap around what he's talking about. 

I've got 4 DD 1008 (8" 4ohm) subs to play with, along with the 1000 watt sub amp from parts express. Well, if it ever comes in off back order. I want this to be a learning experience, and while I could just put them into a big ass ported box and be done with it, I'd like to try my hand at some new box building skill.

Something like this looks like fun, but I'm guessing my fiancee would be less than pleased to find them sitting in the living room:

http://www.teresaudio.com/haven/subs/subs.html

Or maybe I'll just buy a DIYMA sub and be done with it.


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## Oliver (Jun 25, 2007)

Perhaps Pedro leite's picture will do the trick for you, in the link you posted [ 1/4 wavelength ], look thru the designs that were built  

http://www.diysubwoofers.org/tls/

Maybe a horn would work for you ? 

subwoofers DIY :
http://www.diysubwoofers.org/


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## 60ndown (Feb 8, 2007)

as long as you build to plans there is no problem as all the math has been done for you,  

but if you want to design your own you might spend a week building and toss em because they sound horrible. 

i always build a speaker that has got great reviews for years.

to plans.

http://www.decware.com/newsite/mainmenu.htm


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## pontiacbird (Dec 29, 2006)

there was a dude on sounddomain who went by the name of Hexibase, he was the t-line king awhile back....i'm not sure if he's still around, but i'm sure if he's somewhere he could give you some advice


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## mghunt (Jun 26, 2007)

nadaclue,

I had good luck with my t-line designs. They were for the main speakers though, not a sub. Don't see why it would be any different.

I recommend that you follow the "classic" instructions if you don't want to mess with Martin's software.

Many people say to simply make the line length equal to the 1/4 the wavelength of Fs. This tunes the line way too low and makes the enclosure massive. I have a set I built this way and they don't have enough midbass. I don't recommend doing this.

I built a set for my friend that sound much nicer. I tuned the line to 1/4 wavelength of the woofer Fs in the t-line volume. This is an iterative process, meaning you probably want to create a spreadsheet. This is essentially what you want to do. 

1) Pick a starting and ending area for your transmission line. I typically choose 1.5 Sd for the start and 1 Sd for the end. This gives an average area of 1.25 Sd across the entire line.
2) Make a note of the woofer's Vas, Qts and Fs. 
3) In the spreadsheet, create 6 columns for calculations. (length, Vbox, Vr, Qr, Fbox, 1/4wavelength)
4) create about 60 rows with the length variable in column A. Go from about 6ft to 12 ft in 0.1 ft increments.
5) Column B (Fbox) = columnA * 1.25 Sd (cuft)
6) Column C (Vr) = Vas / columnB.
7) Column D (Qr) = sqrt (columnC + 1)
8) Column E (Fbox) = columnD * Fs
8) Column F (1/4 wavelength) = 1224 / column E / 4

The trick you ask? Match up column A to column F. The point where they match is where the line length matches the 1/4 wavelength of Fbox.

I just did this with a Vifa 8" woofer and ended up with a line length of about 8.6 ft. This could be easily built into a one fold transmission line about 5 feet tall with an enclosure base of less than 1 square foot. If you wanted your sub to be shorter, you could go with a 2 fold and add about 50% to the enclosure footprint.

-Mark


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## Oliver (Jun 25, 2007)

Nadaclue,

what did you think of "Pedro Leite's" bottom pic ?


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## Jroo (May 24, 2006)

That guy pete from Hexibase is still around. I have talked to him several times about plans for boxes and specifically t-lines. He is supposed to be a very well know box maker and people die to get him to make plans for them. Google hexibase forums and his new website should come up. He is also very good with email reply to questions you send him


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## nadaclue (Nov 8, 2006)

Hic said:


> Nadaclue,
> 
> what did you think of "Pedro Leite's" bottom pic ?


It's a nice illustrative example of a TL. I understand the general concept, it was the math that I was fuzzy on and while I know I don't "need" to understand the math to get the concept to work, I'm one of those guys that hates to do things just because that's how I was told to do it.

I spent part of my christmas holiday re-reading Martin's website and going over some of the mathcad examples. I have a better understanding of what's going on, and the mathcad sheets seem simple enough to use. However I've run into a new problem in that Digital Designs won't supply T/S parameters for their drivers for whatever reason. I may end up scrapping (selling actually) the DD drivers and buy something more common and diy friendly since I don't feel like setting up the needed parts to measure the T/S parameters or spending $100 on the parts express woofer tester.

It's a shame, with the DD known for high spl levels combined with the ability of a TL to generate low bass I thought I might come up with an interesting product.


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## nadaclue (Nov 8, 2006)

Mark,

Thanks for all the useful information, it's greatly appreciated. This is to be my entry point into making TL's and I hope to be able and play with them using a full range driver in the future. The size is a little off-putting to me, but the ability to use a single driver outweighs the aesthetic concerns that I have for now. Fostex drivers aren't too pricey, and saving on the x-over (money and time) offsets the higher price of fullrange drivers.

Chad


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## pontiacbird (Dec 29, 2006)

nadaclue said:


> Mark,
> 
> Thanks for all the useful information, it's greatly appreciated. This is to be my entry point into making TL's and I hope to be able and play with them using a full range driver in the future. The size is a little off-putting to me, but the ability to use a single driver outweighs the aesthetic concerns that I have for now. Fostex drivers aren't too pricey, and saving on the x-over (money and time) offsets the higher price of fullrange drivers.
> 
> Chad


it definitely sounds like an interesting and fun project to take on, you should post your progress if you do decide to undertake this project


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