Transmission line build advice needed

[Flamin_D_Audio 10+ year member]

Ive been doing home / car audio for a long time now and have lots of experience with ported and sealed enclosures, and as a project want to build a t-line for two 8" drivers, with the aim of getting them to hit as hard and low as possible (30hz).

Ive researched and seen loads of variations, like some that are just folded lines, some that have a chamber before the line, some where the like gets wider, etc.

I know my line needs to be 112.5" long to get the 30hz tuning i want, as im doing 1/4 wavelength. (Let me know if doing 1/2 wavelength will be considerably better).

Just a few things to help me out:

- How wide should the line be? Ive got a feeling its to do with the drivers Q** values?

- Should it be a simple line, or have a primary chamber before the line?

- Should the line widen?

- Is there a 'best side' that the line should exit?

Any help with this would be great, cheers for reading!

Sam

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[Immacomputer 10+ year member]

The drivers' Qts, Bl, Re, Sd, and the shape function all take part in determining what your line area should be. Take a look at this spreadsheet to help you along with the design:

http://www.quarter-wave.com/TLs/Alignment_Tables_Calculator_3_3_09.xls

Having a compression chamber can help bring out the very bottom end and add a good bit of back-pressure for the driver. Do note that tuning a transmission line to 30hz means you will probably have substantial output below 30hz after room/cabin gain; they're not like ported enclosures that roll off quickly below tuning.

Line widening is called tapering or expanding and is used in a ratio form comparing the line opening (SL) to the closed end of the line (S0). That ratio is SL/S0. Expanding the line will mean you will have to increase the overall length of the line to get the same 1/4 wave frequency and tapering the line allows you to make it smaller than needed. The more taper, the less your very bottom end will be but the smoother the upper bass frequencies will be. The more you expand the line, the more it becomes like a horn and the greater your efficiency becomes.

Where you place your line opening should be based on where the enclosure is being located relative to the observer and the environment.

 

[Jroo 10+ year member]

A couple of questions using the supplied spreadsheet. The amount of taper on the sheet is defaulted to 1, should you leave it there? I understand that if I wanted to shorten the length and lose some low end, I would decrease the taper. So on the graph, would I make the taper 0? Where it shows SD at 220, my driver lists an SD of 0.034636. Is something off or do I need to do some calculation to get mine to match?

 

[Jroo 10+ year member]

Two more questions. I have 2 8ohm subs that I want to run in parrallel, how do I figure out for 2 drivers using the graph? Last one, can you also invert your subs and run a T line?

 

[Flamin_D_Audio 10+ year member]

alright cheers

i read today that the 'easy' way to taper is start the line off with a width at twice Sd, and finish it with the area equal to Sd. does this have the effect of canceling out having to make the line longer / shorter to maintain the original 30hz tuning?

also ive got no idea how to even start planning the fold of the line to build..

 

[jerseymike68 10+ year member]

Nice information