How do I correctly measure my TL length?

[DiMarco]

Hello there, I'm new here.

As a beginning DIY speaker builder I put the hard task onto myself to start off building transmissionline designs.

So far I have learned a lot through many wonderful websites and some dutch forums.

I have designed a cabinet that *should* be tuned for an fs of 35hz (roughly 2.4 meters of tapered TL). The cabinet will be 93 x 26 x 35.5 cm and be driven by two TB W4-1337's in d'appolito with a Fostex FT17H supertweeter. The TL will be loaded with a Swan (HiVi) M8a midwoofer.

One thing though is still unclear to me is how EXACTLY to measure the length of a folded TL.

I have measured both ways of the line, added those up and divided the answer by two. Is this how to correctly measure the length?

Or should I use the approx middle of the line for my calculation?

Thanks for any help,

Marco

[GM]

Down the middle is close enough.

GM

[DiMarco]

*sigh of relief* Thanks GM, this means I probably won't have to go back to the drawing board.

This is the plan:



Any further comments / recommendations are welcome.

[planet10]

Originally posted by DiMarco
Any further comments / recommendations are welcome.

Is there a reason you aren't using a driver offset in your TL? My gut feeling id that the line may be a bit small in cross-section. Have you shortened the line to compensate for the taper and end correction? (i think your line is probably too long)

dave

[Scottmoose]

Looks to me like a 2:1 taper. In which case, Martin's tables suggest a length of 97in; throat of 68.25in^2 & terminus of 34.125in^2. Smaller will likely choke things. I'd probably be inclined to tune a little higher though.

[DiMarco]

I measured the line down the middle to be 2.4 meters, which should be right for the speaker's fs (35hz) as far as I know.

What do you mean by "driver offset"? I haven't heard that term before (english not being my native tongue). Would it improve the system?

I have just ordered 'ajhorn' to do simulations/adjustments with before I start building.

Please bare in mind that I have never built any cabinets before and this is my first ever go at a tline.

Thanks for your input.

[DiMarco]

I had to convert my measurements from millimeters to inches but here's the sizes of the undampened tline:

Length: 95.27" (from sound source till mouth)
Throat: 94" ^2
Terminus: 48.9" ^2

I know the line seems overly long from the drawing but the cabinet is rather small:

H: 36.61"
W: 10.24"
D: 13.78"

As the cabinet size is modest I plan to use 15mm MDF for the cabinet, 15mm hardwood multiplex for the front.

[Scottmoose]

Originally posted by DiMarco I measured the line down the middle to be 2.4 meters, which should be right for the speaker's fs (35hz) as far as I know.

Fs on the M8a is 30Hz. You don't necessarily want to be tuning to, (or below) that frequency though. Usually not, in fact, for best performance.

What do you mean by "river offset"? I haven't heard that term before (english not being my native tongue). Would it improve the system?

It's a typo. Dave meant 'Driver Offset' -for minimal passband ripple, there's always an optimum. By placing the driver at the end / throat of the line you excite all line resonances, fundamental & harmonics, equally.

I have just ordered 'ajhorn' to do simulations/adjustments with before I start building.

Wise. It's not great for TLs, but it's probably the best of a rotten bunch since Martin stopped access to his MathCAD worksheets. AkABAK is likely the most capable software around, but without a degree in electrical / mechanical engineering, you've had it.

I had to convert my measurements from millimeters to inches but here's the sizes of the undampened tline:

Length: 95.27" (from sound source till mouth)
Throat: 94" ^2
Terminus: 48.9" ^2

Not bad. It's still tuned too low though (IMO).

As the cabinet size is modest I plan to use 15mm MDF for the cabinet, 15mm hardwood multiplex for the front.

That's just begging for trouble. MDF isn't great at the best of times, but 15mm isn't going to be up to the drill. 3/4in minimum, & think about doubling the largest panels.

[DiMarco]

Okay, thanks for your comments Scottmoose. It will all be put to good use over here...

It seems I have made a miscalculation converting square millimiters to inches. the throat and terminus are too small for that 8.5" midwoofer. This means I will probably have to increase the depth of my cabinet... I will first run some simulations though using AJHorn once the disc gets delivered here.

Hi-Vi state that the fs of the woofer is 33hz, but they added it should be tuned for 35hz and has -3dB at 36hz (although I don't know if this is in a test cabinet or infinite baffle..). Info can be viewed here: http://www.swanspeaker.com/product/htm/view.asp?id=80

I will try tuning to higher (40 and 50hz) frequencies through simulation before radically changing the cabinet design as I am clueless at this moment about what it will do to the actual response.... Tuning the TL at a higher frequency could mean I can fold it three way instead of four and gain some volume at the end of the line which would probably be good.

Yes it is a shame Martin had to stop access to his mathcad worksheets. Understandable though, I think I would have done the same in his position... They would come in really handy for me at the moment but hey that's life.

Also, once I'm done simulating I will redesign the cabinet using 18mm (3/4") plywood. I reckoned MDF would be good enough as the SEAS Thor kit uses that as well...

Once again Scott, thanks for your info.

Oh and Carl, just because everyone else also goes through this mod process does not mean I have to like it. It's unconvenient that's all. It is also something I have never seen at any other forums before. You guys been having troublemakers on the forum? My posts are appearing out of sync because of this moderation thing which means I would have to use a lot of quotes to keep making sense when responding to multiple posts.

Edit: Blimey the moderation thing has vanished! Thank you so much.

[Scottmoose]

Note your comment has gone too. It's frustrating, but that's life. No point in complaining about it.

That's in a vented box, as noted on their spec. sheet. A TL doesn't work like a BR, nor does it have the same roll-off slope. FWIW, look at a 47Hz tuning, which should put F3 at around 39Hz.

MDF is vile stuff. Kills you too. The Seas Thor kit as-is is rubbish, although the cabinet material is the least of its problems (great drivers, mediocre XO, a shamefully poor implementation of the cabinet)