Stuffing my first TL's - help needed!

[Spasticteapot]

I'm building my first pair of speakers (aside from a few I built out of crappy computer speakers), and so far, it's going great. They're transmission lines based around the (decidedly awesome-sounding) RadioShack 40-1041 drivers, and sound awesome mounted simply in a 4" PVC pipe.

I have a length of PVC tuned to a bit under the 80hz spec'd for the drivers, and tapered pieces of wood to reduce the diameter to 60% of the normal diameter.

How should I stuff these? Should I staple a form of batting (foam?) to the wood? Or perhaps something else?

[Jonathan Bright]

Few things arouse controversy line TL stuffing. The proponents of TL's in the 60's led by Bailey in the UK advocated a natural "long fibre wool" (which in my neck of the woods was known as roller lapping) Others have used glass fibre as in roofing batts (insulation), some such as Fris (I think) were keen on polyester or terylene(?) fibre from the filters in fish tanks. Bonded Ascetate Fibre (BAF) has been widely used also. Then is then a debate about stuffing rates. I.E. weight per unit of volume. In your situation, if thre tube is vertical, you ought to try and ensure that the fibre in the tube doesn't 'settle' on the lower end over time. People have teased the fibre out through a mesh support used to keep the fibre uniformerly distributed. Others have deliberately not gone for uniformity as they have tried to cluster fibre near resonant nodes in the line. Again it depends on what you want and even what principle you belive TL's function on as not everyone agrees on that question!!! Dave at Planet 10 is most likely to be able to help as I can't locate my list of stuffing rates at the moment. Check out his very good site through his biog'. There is some good work being done over the last few years in generating the maths for 'lines'. Have you done a search in this forum? I recall some extensive threads. Good luck.

[Spasticteapot]

I did search; I must have missed them.

I've heard that a thin layer of foam or carpet stapled along the wood piece should work. It's what was done in the design I based this off of.

http://www.t-linespeakers.org/projec...son/index.html

I live near Madisound, but Acoustastuff is pricey, and I'd prefer something that was'nt liable to attract allergens. I've seen rubberized pads used, too - these would be ideal.

[Geek]

Isn't Acoustastuff just hideously overpriced hollowfill?

[Shoog]

You have to decide what you want from your TL.
At one extreme you can stuff in order to kill all the back wave, which allows the driver to dominate. At the other extreme you can not stuff at all, but you will have a very uneven response in the bass because of cancellation and reinforcement. A compromise is to try to damp all frequencies but the fundamental, which will require an amount somewhere between the two others.
Decide on this and you will have a better idea on how to stuff.

Shoog

[Scottmoose]

Agreed with Shoog.

Just to expand on this a little more; it depends what you call a Transmission Line, which is an electrical rather than an acoustic term. Bailey et al were effectively the first to soin it for a cabinet type, which, as Shoog points out, was heavily damped with the objective of damping the entire driver back-wave and flattening the impedence curve out. The fact that they were assumed to be 1/4 the wavelength of the tuning frequency long actually wasn't overly significant. In fact, 90% of the time, length was notionally incorrect for this anyway, because it wasn't appreciated that Fc (tuning frequency) / 4 = line length only works for straight, untapered lines. In reality, the tuning frequency for a line is a function of both length and taper. Expanding lines need to be significantly longer, while negative taper (narrowing) lines can be made shorter.

Quarter Wave Resonators, which as speaker cabinets came decades before TLs, were usually empty, or relatively lightly stuffed. There the object is very different: unlike TLs, which are effectively non-resonant cabinets, the QWR is highly resonant, with the objective of using the fundamental resonance (cabinet 1st mode) to boost the bass SPLs, which most FR drivers struggle to produce. What damping is in the cabinet is used to supress unwanted higher line harmonic resonances. This can be more or less effective depending upon how well the cabinet has been designed of course.

Most 'TL' designs we see are in reality hybrids between these two types of resonant and non-resonant enclosures -I tend to call they hybrid TLs because of this. They're usually fairly heavily stuffed, but not quite as heavily as a 'proper' TL in order to get some reinforcement of the bass frequencies from the line while keeping impedence fairly flat and damping the rest of the back wave. This is what most people appear to want from a TL, and a lot of the time, they try to juggle stuffing densities to get the balance right. There's an easier way though. Stuffing is at it's most effective where air-velocity is highest in the line. The point of maximum air velocity is dependant upon frequency. The higher the frequency, the closer maximum air velocity for that particular frequency is to the closed end, the lower, the closer to the open end. So, if you want a line which falls in the the definition of a hybrid outlined above, simply stuff most of the cabinet with a nice, uniform density -not too much, about 0.5lbs ft^3 will do nicely, but leave the last few inches of the line empty of stuffing. This will help prevent the absorbion of the fundamental resonance, and keep the bass up while damping all the rest. I regularly model these sorts of lines in MathCad for different uses.

Hope some of this is of use.
Scott

Scott

[Geoff H]

Well said Scott. Do you ever get time to work on your own projects? I am sure your contibutions help many.

I am currently "tuning" my plastic tls. I have reduced it to all but a piece of tontine (I think that is similar to your hollowfill) 4x8", starting behind the driver, and rolled into the centre tube. I definitley need more.

I am going to try dropping sections in the centre tube, with out compressing it, then compare the effect with the other tube.
And carry out my bonk test as I go, it may not apply to TLs

In my situ, I want to suppress all the midrange from the port, and reinforce the bass.

Spasticteapot, you could try starting with a length of hollowfill the length and width of the tube, compare that with the untreated pipe, then try 2 lengths in the 2nd pipe, and compare with the single length.

Good to see you have a pair under way. I am sure you will be pleased with the results. BTW, see my latest post on my plastic tls, I have found a big pair.

[Scottmoose]

Sometimes. Winter is not my building time as a rule though. The unheated, leaking garage is the problem. I might do one BIB project, but I don't need to think much for that as I alrady know the dims, and I've built plenty of them in the past.

These plastic pipes are an excellent way of creating a TL subwoofer with minimal cabinet work. Nelson Pass's El Pipe-o is the monster example of course, but a smaller version with 8in or 12in drivers will make a good showing of themselves. Plenty of examples on Dave's site: http://www.t-linespeakers.org/projects/index.html Might build a pair at some point for fun.

[Spasticteapot]

Speak of the devil - my design is based on one found on that very page.

http://www.t-linespeakers.org/projec...son/index.html

Sadly, I did'nt see anything on stuffing them.

[Geoff H]

Quoting Rob's Pipes.

" I also, now, attach the stuffing to the 2x4 with staples before insertion into the pipe. This seems to prevent wadding and clumping in the narrow pipe; it also keeps very light stuffings from settling down the line. Wool blanket fabric and/or bonded polyester quilt batting stapled to the 2x4 seem to give good resonance control if one wishes to not stuff heavily."

I had a late night stuffing the tubes. The inner pipe is lightly stuffed full length. The outer pipe is lightly stuffed about 1/3 the length. The click- bonk- boing test confirms the musical quality, which still surprises me at times.

I hope you get as much joy out of yours.

edit: don't be afraid to experiment.