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Discussion Starter #1
I am building a couple of transmission line subs. They are based on Nelson Pass' El-Pipe-O. Does anyone know how to calculate the effect of a specific amount of stuffing per cubic foot on the volume.

If I have 6 foot long 12" dia. tubes, how much stuffing per cubic foot will increase the effective length by 6 inches. This is just an example. I'm hoping there is some kind of formula.
 

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Hi

Unfortunately I don't have a simple answer to that. As far as I know stuffing a transmission line is not a trivial task (stuffing actually never is).

I don't know the plan you're using but technicalliy it goes like this:
The transmission line will give you better bass up to a specific frequency. At this frequency the output of the cone and the output of the line will be out of phase resulting in a big suckout and a very uneven frequency response above that. Stuffing the line at the right points will lessen this effect by lessening the lines output at certain frequencies.
Some go for a different approach and construct a tuned resonator (basically a small bass reflex cabinet) within the transmission line cabinet wich will stop the line from working at that problematic frequency. Later a bit of stuffing is applied to the line to linearize the frequency response and the resonator is fitted with some stuffing as well to broaden its effect a bit.
This whole process needs severaly tries and many measurements and listening tests in between them...

BUT you might be ok without any stufffing. If you cross over your subwoofer well below the phase problem, stuffing is not needed.

So all the advice I can give you is to try... not what you were looking for I suppose... :sad2:
 

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Discussion Starter #3
These are not the first TL subs from tubes for me. These are third generation. I can fine tune the amount of stuffing with a signal generator and a mic. I'm just looking for some kind of formula for calculating how much a given amount (let's say polyfil in pounds) will increase the effective tube length.

IE:
12" ID tube 6ft long, How much stuffing to make the effective length 6" longer.

The basic design for these TL subs came from the Pass DIY site. I'm using the active eq circuit and getting response down to 18hz. He suggests loosely stuffing the tubes. That's it.
I've stuffed and measured and tweaked. I'm satisfied with my results. I just want an easier way to determine the amount of stuffing.

Thanks for responding. I guess I'm in unfamiliar territory. No one else seems to have any ideas.
 

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Well in this case I suppose it's best to just go for the trial and error approach. Since you have the equipment and the experience, you might actually be a prime candidate to come up with a formula :T

In all seriousness though - I don't think that it's actually that easy to find a simple formular for that.
 

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Also note that Poly fill is not really suitable for stuffing TL's.
For mine I used long fibre Merino wool teased and then filled the entire length.
You need to re-tease the wool every two years as it settles over time.
Spot on Robbo..

From my experience with folded TL systems, the filling has to be very open in texture and I also used a teased synthetic wool..Polyfill is definitely not suitable..

With the folded TL system the first chamber was the most critical part of the back loading process and all fine tuning was done there..The rest of the line was just lined with acoustic foam..

With a straight line TL, (and they were very popular many years ago) they just VERY loosely filled the whole length.

I don't think it's possible to actually do a mathematical calculation because there are so many variables..

Things like the type of material..density, porosity, flow resistance, compactness etc. etc..and then theres all the different speaker parameters..

All of the systems I and others made, were done by trial and error and some measurements..
 

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Discussion Starter #8
Hey guys, no folded lines here. Just a straight sonotube. I was asking because I've never been able to find any information regarding my initial question. There has to be some kind of relationship between a given material and its effect on tube length.
 

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Aha!..If it's a Sonotube, then the filling requirements are different than a TL..

A TL system has a reducing chamber area through the length of the line...Hence the reason for a critical criteria for the filling material..

There should be some kind of mathematical formula for the filling material for that type of system..but I have no experience in that area..
 
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